Class 11 Chemistry

CBSE Class 11 Chemistry Notes For Chapter 3 Periodic Trends In Properties Of The Elements

The properties of elements can be divided into two categories:

1. Properties of individual atoms: The properties such as atomic and ionic radii, ionisation energy, electron affinity, electro¬negativity and valency are the properties of the individual atoms & are directly related to their electronic configurations.
2. Properties of groups of atoms: The properties such as melting
   point, boiling point, heat of fusion, heat of vaporisation, atomic volume, density etc. are the bulk properties i.e., the properties ofa collection of atoms and are related to their electronic configurations indirectly.

All these properties which are directly or indirectly related to the electronic configurations of the elements are called atomic properties. Since electronic configurations of the elements are periodic functions of their atomic numbers, these atomic properties are also periodic functions of atomic numbers of the elements. Thus, atomic properties are also called periodic properties.

Remember that, when we descendin a group, the chemical properties ofthe elements remain more or less the same due to same valence shell configuration, but there is a gradual change in physical properties due to gradual change in the size ofthe atoms owing to addition ofnewelectronic shells.

Atomic size or atomic radius

If an atom is assumed to be a sphere, the atomic size is given by the radius ofthe sphere, called the atomic radius.

Atomic size or atomic radius Definition:

The distance from the centre of the nucleus to the outermost shell containing the electrons is called the atomic radius

Difficulties in precise measurement of atomic radius:

• It is not possible to isolate a single atom for the measurement of its radius.
• The electron cloud surrounding the nucleus does not have a sharp boundary as the probability of finding an electron can neverbe zero even at alarge distance from the nucleus.
• The magnitude ofatomic radius changes from one bonded state to another.

CBSE Class 11 Chemistry Notes Chapter 3 Periodic Trends

Types of atomic radius:

As already mentioned, the size of an atom varies from one environment to another. Therefore, several kinds of atomic radii have been defined.

These are

1. Covalent radius
2. Metallic radius
3. Van der
4. Waals radius.

1. Covalent radius:

It is defined as one-half of the distance between the centres of two atoms of the same element bonded by a single covalent bond

Thus for homonuclear diatomicmolecules, covalent radius (r) = ½ internuclear distance.

Example: In H₂ molecule, the internuclear distance is = 0.74 A

= 74pm. So, the covalent radius of hydrogen atom = ½  × 0.74

= 0.37 Å= 37pm.

1Å = 10^-10m, 1 pm = 10^-12m,

2. Metallic radius:

It is defined as one-half of the internuclear distance between two adjacent atoms in a metallic lattice.

Example:

The distance between two adjacent copper atoms in solid copper is 2.56 Å(determined by X-ray diffraction).

Hence, the metallic radius of copper is 1.28 Å. Similarly, the metallic radii of sodium and potassium have been determined as 1.86 Å and 2.31 Å respectively.

Note: Covalentradii ofNa and K are 1.54 Å and 2.03 Å

A metallic radius is always greater than a covalent radius.

3. Van der Waals radius:

It is defined as one-half of the distance between the nuclei of two non-bonded neighbouring atoms belonging to two adjacent molecules of an element in the solid state.

Periodic Trends in Properties of Elements Class 11 Notes

Example:

The distance between nuclei of two adjacent Cl -atoms of two adjacent chlorine molecules in solid states is 3.6  Å. So, the van der Waals radius of Cl-atom is

⇒ \(\frac{3.6}{2}\)= 1.8 A.

Since the van der Waals force of attraction is weak even in the solid state, the internuclear distances between the atoms of two adjacent molecules held by van der Waals forces are much larger than those between covalently bonded atoms (which involve mutual overlap of atomic orbitals).

Van der Waals radii are always greater than covalent radii. For example, the van der Waals radius of chlorine is 1.80 Å while its covalent radius is 0.99 Å

Sequenceofthree types of atomic radii: van der Waals radius >Metallic radius > Covalentradius

Variation of atomic radii or sizes in the periodic table

1. Variation of atomic radii or sizes across a period:

While moving from left to right across a period in the periodic table, atomic radii or atomic sizes progressively decrease.

Atomic radii across a period  Explanation:

The principal quantum shell remains unchanged in the same period. So, the differentiating electrons enter the same shellbutdue to an increase in the number of protons, the positive charge of the nucleus also increases. So, attractive force of the nucleus for electrons in the outermost shell also increases. Hence, the atomic sizes, rather than the atomic radii of elements in the same period, gradually decrease with an increase in atomic number. In any period, atomic size of the element of group-1A is maximum and that of the halogen of group VILAisminimum.

Variation of atomic [covalent] radii of the elements of the third period (n = 3]:

2. Variation of atomic radii or size down a group:

On moving down along any group of the periodic table, the atomic sizes rather than the atomic radii of the elements increase remarkably.

Atomic radii or size down a group Explanation:

On moving down a group, a new electronic shell is added to each succeeding element, though the number of electrons in the outermost shell remains the same. This tends to increase the atomic size. At the same time, there is an increase in nuclear charge with an increase in atomic number. This tends to decrease the size.

However, the effect of the increased nuclear charge is partly reduced by the shielding effect of the inner electronic shells.In practice,it is found that the effect of the addition of a new electronic shell is so large that it outweighs the contractive effect of the increased nuclear charge. Hence, there occurs a gradual increase in atomic radii on moving down a group in the periodic table.

3. Variationofsizein agroupforheavierelements:

On moving down a group, the relative rate of increase of atomic radio is slow for heavier elements. So, the differences in size for heavier species such as Cs (6s¹) and Fr(7s¹) of group-1 or Ba(6s²) and Ra(7s²) of group-2 are very small. This is due to the presence of electrons in the inner d and f-orbitals having a poor screening effect. D and f-electrons do not screen the outer electrons effectively from the pull of the nucleus. There is only a small increase in size in spite of the addition of a new electronic shell.

Example: Na(1.54 Å), K(2.03Å), Rb(2.16 Å), Cs(2.35Å) etc

In the case of transition elements, such a decrease in the atomic size is relatively less. Here the differentiating electron instead of entering the outermost orbit, goes to the penultimate (n- f )d -subshell which is closer to the nucleus. However, due to the poor shielding effect of the additional d -electrons, there is a small increase in effective nuclear charge and hence, the decrease in the size with an increase in the atomic number is relatively small.

In the case of lanthanide elements, the differentiating electrons enter the (n-2)f-orbital having a very poor shielding effect. So, in such cases, the increase in the effective nuclear charge is greater than that in the case of transition elements. Thus, the decrease in atomic sizes of the lanthanides is much more regular and distinct as compared to the transition elements. So, the difference in the extent of the decreased atomic sizes of these elements is due to their relative inability to screen the outermost electrons from attraction of the nucleus.

Periodic Trends in Properties of Elements Class 11 NCERT Solutions

Covalent radii (pm] of representative element:

 Variation of atomic radii with atomic numbers across the second period:

Variation of atomic radii with atomic numbers for group-1 metals:

Screening effect or shielding effect:

In multi-electron atoms, the electrons present in the inner shells shield the electrons in the valence shell from the pull of the nucleus. It means that the electrons of the inner shells act as a screen between the nucleus and the electrons in the valence shell. This is known as the screening effect shielding effect.

Screening effect or shielding Definition:

The ability of the inner electronic shells to shield the outer electrons from the attraction of the nucleus is called the screening effect or shielding effect.

Magnitude of the screening effect of electrons belonging to different subshells follows the sequence: s>p> d>f.

Periodic Trends Chapter 3 Class 11 NCERT Notes

Important points regarding atomic (covalent) radius:

• The alkali metals occupying positions at the extreme left side ofthe periodic table have the largest size in period
• The halogens occupying positions at the right side of the periodic table have the smallest size in each period.
• The noble gases present at the extreme right of the periodic table have larger atomic radii than those of the preceding halogens because van der Waals radii (but not covalent radii) are taken into consideration for noble gases.
• In transition series {d -block elements), there is only a small decrease in size with successive increases in atomic number because the differentiating electrons enter into (n-1)d subshell, which partially shields the increased nuclear charge acting on the valence electrons. This is known as d contraction.
• For inner-transition series ( /-block elements), the decrease in atomic radii wide increase in atomic number is relatively greater and more regular as compared to the transition series elements. This is so because the differentiating electrons enter into (n- 2)/ -subshell having a very poor shielding effect.

In a group of representative elements, there is a continuous increase in atomic radii with an increase in atomic number.

On going down a group of transition elements, there is an increase in size from first member to second member as expected, but for higher members, the increase in size is quite small. This is due to lanthanide contraction. For example, Cu(1.28 Å), Ag(1.44 Å), Au (1.44 Å)etc

CBSE Class 11 Chemistry Notes For Chapter 3 Classification Of Elements And Periodicity In Properties Introduction

At present, 118 elements are known to us. It is Almost an impossible task to remember the individual properties of these elements And A larger number of compounds derived from them. Several attempts were made by former scientists to arrange the elements in a coherent and orderly manner. After Dalton’s Atomic theory, attempts were made to establish a correlation between the atomic masses of various elements and their properties.

But until a method for the estimation of correct atomic masses of elements was innovated, the work on the proper classification of elements could not make any significant progress. However, after the atomic masses of elements were correctly determined, the attempts for the classification of elements received particular attention. The way of arranging similar elements together and separating them from dissimilar elements is called the classification of elements.

Historical Background Of The Classification Of Elements Based On Atomic Weight

Dobereiner’s Law Of Triads

In 11117, German scientist Doberelnor stated that in a group of three chemically similar elements, called a triad, the atomic weight of (the middle element of each triad Is very close to the arithmetic mean of those of the other two elements.

CBSE Class 11 Chemistry Notes Chapter 3 Classification of Elements

This was called Oohereiner’s law of triads. Some familiar triads, based on lids law, are shown below:

From the table, it is observed that the atomic weight of sodium (Na) is the average of the atomic weights of lithium (Li) and potassium (K)

⇒ \(\left[\frac{7+39}{2}=23\right]\)

This relationship is only applicable to a limited number of elements and hence fails to classify all the known elements.

However, it cannot be denied that it indicated the existence of an inter-relationship between the properties and atomic weights of elements.

Law of Telluric Screw

In 1862, Chancourtois attempted to classify the elements based on atomic mass. He took a vertical cylinder with 16 equidistant lines drawn on its surface (lines are parallel to the axis of the cylinder). He drew a spiral line or helix on the surface making an angle of 45° to the axis of the cylinder.

The atomic weights were plotted vertically along the spiral line. He arranged the elements on the helix in order of their increasing atomic weights. It was observed that in the telluric screw, the elements that differed from each other in atomic weight by 16 or multiples of 16 fell on the same vertical line. The elements lying on the same vertical line showed nearly the same chemical properties. However, this concept did not attract much attention.

Newlands’ Law Of Octaves

Arranging the known elements in the ascending order of their atomic weights, Newlands, observed (1865) that properties of the eighth element, starting from a given one, is a kind of repetition of the first, like the eighth note in an octave of music. He called this regularity the law of octaves.

Starting from Li, the eighth element is Na and the eighth element following Na is K. There exists a striking resemblance in properties among these elements. Similarly, F shows similarity with the eighth element Cl following it in properties. The law of octaves was found to be satisfactory in the case of lighter elements from hydrogen (H) to calcium (Ca). However in the case of heavier elements beyond calcium, it lost its validity and hence, the law was discarded.

Lothar Meyer Arragngement

In 1869, Lothar Meyer, a German scientist, studied the different physical properties of the known elements and plotted a graph of atomic volume (atomic weight divided by density) against the atomic weight of various elements. He noticed that the elements with similar properties occupied similar positions on the curve. Based on this observation, Lothar Meyer concluded that the physical properties of the elements are a periodic function of their atomic weights.

Classification of Elements and Periodicity in Properties Class 11 Notes

Periodic Law

In 1869, Russian chemist, Dmitri Mendeleev, examined the relationship between the atomic weights of the elements and their physical and chemical properties. From his studies, Mendeleev pointed out that the physical and chemical properties of elements are periodic functions of their atomic weights. This generalisation is called Mendeleev-Lothar Meyer Periodic Law or simply Mendeleev’s Periodic Law.

Mendeleev’s Periodic law:

The physical and chemical properties of elements are a periodic function of their atomic weights. This law implied that if the elements are arranged in the order of increasing atomic weights, the physical & chemical properties of the elements change regularly from one member to another and get repeated after a definite interval. This recurrence of properties ofthe elements at definite intervals is called the periodicity of elements.

Periodic classification and periodic properties:

Based on the periodic law, the classification of elements according to the increasing atomic weight is called periodic classification. The properties of the elements which are directly or indirectly related to their electronic configurations and show a regular gradation when we descend in a group or move across a period in the periodic table are called periodic properties.

For example:

The size of atoms or atomic radii, ionic radii, atomic volume, metallic character, ionisation enthalpy, electron affinity, electronegativity, melting point, boiling point, valency etc. Radioactivity is not a periodic property of elements: Radioactivity is neither directly nor indirectly related to the electronic configuration of atoms. It depends on the ratio between the number of neutrons and protons present in the atom.

Classification Of Elements Based On Outer Electronic Configurations

Based on electronic configurations of the ultimate and penultimate shell of the atoms, Bohr divided the elements into four classes viz., gas elements,

1. Representative elements,
2. Transition elements and
3. Inner-transition elements.

Inert gas elements

S and p -subshells of the outermost shell of the elements of this class are filled.

Except He (electronic configuration: Is²)

• All other inert gas elements have the valence shell electronic configuration: ns²np⁶.
• All these elements are stable and chemically inert as their outermost shells contain octets of electrons.
• They do not normally participate in chemical reactions because the gain or loss of electrons by their atoms would disturb their stability. So, they are called inert gas elements.
• Their valency being zero, they find a place in group ‘0’ or ’18 These elements act as a bridge between highly electropositive alkali metals and strongly electronegative halogens.

Representative Elements

Elements present in s – and p -blocks (except group-1) of the periodic table are known as representative elements. The electronic configuration of the outermost shell of these elements varies from ns1 to ns²np⁵. These consist of some metals, all non-metals and metalloids.

The name ‘representative’ has been assigned to the elements because of their frequent occurrence nature and because they typify the properties of all other members of the group to which they belong.

All the elements of groups 1A, 2A and from 3A to 4LA are included in this class.

These elements are very reactive Chemical reactivity of these elements can be ascribed to the ability of their atoms to attain inert gas electronic configuration (ns²np⁶ or Is²) either by gaining or losing electron(s) or by sharing one or more electron pairs with other atoms. These elements are also known as typical elements.

Transition Elements

Elements of this class are characterised by the presence of atoms in which the inner d-subshell is not filled. According to the modified definition, the elements in which atoms in their ground state or any stable oxidation state contain incompletely filled d -subshell are known as transition elements. Atoms of the elements in this class have the general electronic configuration: (n-1)d^1-10ns^1-2

Cu, Ag and Au, despite having filled d orbitals, are regarded as transition elements. This is because at least in one stable oxidation state of these elements, d subshell remains incompletely filled.

There are four transition series corresponding to the filling of 3d,4d,5d and 6d orbitals These four series belong to the 4th. 5th. 6th and the period of the periodic table.

Each series begins with a member of the group-3 and ends with a member of the group-12.

CBSE Class 11 Chemistry Classification of Elements Notes

Characteristics:

• All transition elements are metallic.
• They have more than one oxidation state or valency.
• Their ions are coloured.
• They form complex compounds.
• Elements of group-12 (11B) (Zn, Cd, Hg) are not considered transition elements because they have no partially filled d -d-orbitals in any of their oxidation states.
• Moreover, they do not form stable complexes and do not show characteristic colour and paramagnetism.
• However, their tendency to form complex is much greater than that of the representative elements.
• They exhibit properties of both transition representative elements.

Differences between typical and transition elements:

• During the building up of an atom of a typical element by the filling of electrons in its various orbitals, the last electron goes to s -or p -orbital of the outermost shell (n).
• However, in the case of transition elements, the last electron enters the inner d -d-orbital of(n- 1) th shell.
• For the representative elements, atomic volume or radius decreases but ionisation enthalpy and electro negativity go on increasing with the increase in atomic number across a period.
• In the case of the transition elements, as the last electron enters the inner (n- 1)d -orbital, the extent of change is relatively small.
• Most of the representative elements exhibit only one valency. Some elements, of course, show more than one valency.
• But transition elements show 2 or more valencies through the participation of inner d -d-orbital electrons.
• In the case of representative elements, the tendency to form complex compounds is almost negligible while transition elements are found to show a strong tendency to produce complex compounds due to the presence of incompletely filled d -d-orbital.
• Compounds formed by representative elements are, in general, colourless but the compounds of transition elements are mostly coloured.
• Due to the absence of odd electron(s), compounds formed by representative elements are diamagnetic while transition metal compounds, because of the presence of odd electrons, are paramagnetic.
• Many of the transition metals and their compounds act as catalysts in chemical reactions. Such a tendency is seldom observed in the case of representative elements.

Inner-transition elements

Elements of this class are also transition elements, although they may be distinguished from the regular transition series by their electronic configurations.

• Atoms of these elements not only contain incompletely filled d-subshell [(n-1)d] but also contain incompletely filled f-subshell [(n- 2)f].
• These elements comprise a transition series within a transition series and hence, they are called Inner-transition elements.
• The two series of inner-transition elements are O lanthanoids (rare earth elements) and actinoids.

In the case of 14 elements i.e., cerium (Cel to lutetium (₇₁Lu) following lanthanum (₅₇La), 4f- and 5d subshells remain incompletely filled. These are called lanthanoids.

Their general electronic configuration is:

4f^1-14 5d^0-16s² With the increase in atomic number (58-71). the differentiating electrons of these elements enter the 4f- subshell, despite the presence of a partially filled 5d -subshell. The total electron-accommodating capacity of the f-subshell Is 14.

So the number of lanthanoids is also 14. Likewise, 14 elements after actinium (₈₉Ac), from thorium (₉₀Th) to lawrencium (₁₀₃Lr) are called actinoids. Their general electronic configuration is 5f1’14 6d01 7s2. With the increase in atomic number (90-103), the differentiating electrons enter the 5f-subshell, despite the presence of an incompletely filled 6d -6d-subshell. Hence, like the lanthanoids, the number of actinoids Is also 14.

Lanthanoid contraction

In the case of lanthanoids (₅₈Ce – ₇₁Lu), it is observed that with an increase in atomic numbers, atomic and ionic size (M³⁺) go on decreasing, although the decrease in Ionic radii is much more regular than that of atomic radii.

This decrease in atomic and ionic radii with an increase in atomic number in the case of lanthanoids, is known as lanthanoid contraction.

Cause of lanthanoid contraction:

• The general electronic configuration of lanthanoids is 4f^1-14 5d^0-16s². The differentiating electrons of these elements enter the 4f-subshell.
• Now due to their diffused shape, f-orbitals have a very poor shielding effect.
• Thus with the gradual addition of the f- electrons, the atomic number increases by one unit while the shielding effect does not increase appreciably; i.c., there is a gradual increase in the effective nuclear charge acting on the outermost electrons.
• Consequently, the attraction of the nucleus for the electrons in the outermost shell increases, causing the electron cloud to shrink although its magnitude is small.
• Thus, there is a gradual shrinkage in the atomic and ionic radii with an increase in atomic number.
• Precisely speaking, f-orbitals are too diffused to screen the outermost electrons effectively against the attractive force of the nucleus. Thus, there is a slow contraction in atomic and ionic radii (lanthanoid contraction).
• In the same way, the d -contraction due to the accommodation of die electrons in (n- 1) d -subshell in the transition series can be interpreted.
• But d -d-orbitals are more effective in screening compared to tyre f-orbitals.
• So this effect is less pronounced in the case of transition elements.

Classification of elements as metals, non-metals and metalloids

All the known elements can be divided into three classes— metals, non-metals and metalloids based on their properties.

Metals

About 78% of the known elements are metals. They appear mainly on the left side and central portion of the long form ofthe periodic table.

Examples are:

1. Alkali metals,
2. Alkaline earth metals,
3. D -block elements,
4. F-block elements,
5. Higher members of p -block elements.

Metals have the following characteristics:

• They are solids at room temperature. Mercury is an exception, which is a liquid at ordinary temperature.
• Gallium (melting point 30°C) and caesium (melting point 29°C) are also liquids above 30°C.
• They usually have high melting and boiling points.
• They are good conductors of heat and electricity.
• They are malleable (can be flattened into thin sheets) and ductile (can be drawn out into wires).

Non-metals

There are only about 20 non-metals discovered so far. They are located towards the top right-hand side of the periodic table. Hydrogen and some p-block elements are non-metals.

• Six of the non-metals (C, B, P, S, Se and I) are solid.
• Bromine is the only liquid non-metal.
• The remaining non-metals (N, O, F, Cl, H and inert gases) are gases.
• Non-metals have low melting and boiling points (boron and carbon are exceptions).
• They are poor conductors of heat and electricity (graphite is a good conductor of electricity).
• Nonmetallic solids are usually brittle and are neither malleable nor ductile.

Metalloids

• There are some elements which have certain characteristics common to both metals and non-metals.
• These are called semimetals or metalloids. Examples are—silicon (Si), germanium (Ge), arsenic (As), antimony (Sb) and tellurium (Te).
• In most of their properties (both physical and chemical), metalloids behave as non-metals. However, they somewhat resemble the metals in their electrical conductivity. They tend to behave as semiconductors.
• This property is found particularly in the case of silicon and germanium.
• These two metals are mainly responsible for the remarkable progress in the past five decades in the field of solid-state electronics.

NCERT Solutions Class 11 Chemistry Chapter 3 Classification of Elements

IUPAC Nomenclature Of Transuranic Elements (Atomic Number More Than 100)

The elements beyond fermium (100) are called transfermium elements. They have atomic numbers above 101.

Fermium (100), mendelevium (101), nobelium (102), and lawrencium (103) are named after eminent scientists. Some of the elements with atomic numbers higher than 103 were synthesized and reported simultaneously by scientists from the USA and the Soviet Union.

Each group proposed different names for die same element, e.g., an element with atomic number 104 was named Rutherfordium by USA scientists while Soviet scientists named it Kurchatovium.

To overcome such controversies, the IUPAC (1977) has recommended a new method of naming these elements. This is discussed here.

1. The digits expressing the atomic number of an element are represented serially (from left to right) by using the numerical roots given below.

2. The successive roots are written together and the name ends with ‘mum’ To avoid the repetition of some letters, the following procedure is adopted.

• If ‘enn’ occurs before ‘nil; the second ‘n’ of ‘enn’ is dropped.
• Similarly the letters ‘i’ of ‘bi’ and ‘tri’ are dropped when they occur before ium,bi+ium= bium, tri+ium= trium, enn+nil= ennil etc.

3. The symbol of an element is derived by writing successively the initial letters (z.e., abbreviations) of the numerical roots which constitute the name.

CBSE Class 11 Chemistry Notes For Chapter 13 Hydrocarbons Introduction

Organic compounds containing only carbon and hydrogen atoms are called hydrocarbons. Their major sources are petroleum, natural gas, and coal. Hydrocarbons are considered to be the parent organic compounds. All other compounds are considered to be derived from hydrocarbons by replacement of one or more of their H-atoms by appropriate functional groups.

Hydrocarbons play a very important role in our daily life. Some of these are used as fuels. The largely used fuels are LPG (liquefied petroleum gas), LNG (liquefied natural gas), CNG (compressed natural gas), gasoline (petrol), diesel, kerosene etc.

These are a mixture of different hydrocarbons. The main constituent of natural gas is methane. Some hydrocarbons are used to manufacture polymers such as polythene, polypropene, polystyrene, nylon, terylene etc. Some hydrocarbons are used as solvents in the paint industry and as the starting material for the manufacturing of many dyes and drugs.

Petroleum Or Crude Oil Commercial Source Of Hydrocarbons

Petroleum is a dark viscous oily liquid that is a mixture of hydrocarbons containing different impurities and found within impenetrable rock structures deep below the earth’s crust In Latin, petroleum means rock oil [Latin: petra = rock, oleum = oil). As it is collected from underneath the earth, its alternative name is mineral oil.It is also called crude oil. Petroleum is also known as liquid gold because of its commercial importance. The colour of petroleum depends on its source and nature.

CBSE Class 11 Chemistry Notes Chapter 13 Hydrocarbons

Natural gas:

The gas mixture found above petroleum at various depths below the earth’s crust is referred to as natural gas. The main constituent of natural gas is methane (90%). It also contains ethane, propane, butane and very small amounts of pentane and hexane vapors.

Composition Of petroleum:

Petroleum mainly consists of three types of hydrocarbons. These are chiefly alkanes
(C₁-C₄₀),  small amount of cycloalkanes

For example: Methylcyclopentane, cyclohexane, methylcyclohexane) and a very small amount of aromatic hydrocarbons (benzene, toluene, xylene, etc.).

Besides hydrocarbons, it also contains certain organic compounds containing oxygen, sulfur, and nitrogen

Refining of petroleum:

The process of separating crude petroleum into different useful fractions having different boiling ranges with the simultaneous elimination of undesirable impurities is called refining of petroleum. Crude petroleum (mainly a mixture of hydrocarbons with carbon atoms ranging from(C₁-C₄₀) is separated into different fractions by fractional distillation. According to the demand and the necessity of different industries, each fraction, obtained by distillation under different boiling ranges is collected.

In addition to the low boiling volatile hydrocarbons, the four main fractions obtained by distillation of crude petroleum are:

1. Crude naphtha
2. Kerosene or paraffin oil
3. Fuel oil or diesel and
4. Residual oil.
5. Crude naphtha and the residual oil are further fractionated to get fractions within still narrow boiling ranges, suitable for different uses.

Different fractions obtained by fractional distillation and their uses:

Cracking And Reforming

1. Cracking Definition

The process in which high-boiling long-chain hydrocarbons are decomposed to give a mixture of low-boiling smaller hydrocarbons by the action of heat alone or heat in the presence of a catalyst is called cracking.

Cracking involves the breaking of carbon-carbon and carbon-hydrogen bonds. Thermal decomposition of organic com¬pounds is known as pyrolysis and when it is applied to alkanes it is known as cracking. The hydrocarbons that will be formed by cracking depends on the conditions applied for the process

Example:

Thermal cracking:

The type of cracking that involves the conversion of high-boiling long-chain hydrocarbons into a mixture of low-boiling smaller hydrocarbons by the action of heat alone is called thermal cracking.

Due to the random dissociation of C—C bonds in thermal cracking, a complex mixture of a large number of hydrocarbons (both saturated and unsaturated) is obtained. The components may be separated from the mixture so obtained.

Catalytic cracking:

Cracking carried out at a relatively lower temperature (330-380°C) in the presence of a catalyst is called catalytic cracking. The most commonly used catalyst is a 4: 1 mixture of silica (SiO₂) and alumina (Al₂O₃). About 85% of the world’s total production of gasoline is obtained through this method. Kerosene is converted into gasoline applying this process

Oceano number:

All gasoline are not equally effective as fuel. n-heptane ranks as a fuel of inferior quality because its combustion takes place rapidly, producing a knocking in the internal combustion engine. On the other hand, 2,2,4- trimethylpentane or isooctane which burns smoothly, does not produce any significant knocking in the engine. Hence, 2, 2, and 4-trimethylpentane having higher antiknock properties (rated as 100) and n-heptane, with lower antiknock properties (its fuel rating taken to be 0) have been introduced as standards for rating fuel.

Based on these two extreme cases, different fuels are standardized with respect to octane numbers. So, octane number indicates the relative antiknock tendency of a gasoline sample.

• Octane number is defined as the parts by percent of isooctane that must be added to a sample of n-heptane to produce the same fuel efficiency of the fuel whose standardization is to be made.
• The octane number of a fuel is 35’—which means that the efficiency of the fuel is identical to a mixture of 35% isooctane and 65% n-heptane.
• The higher the octane number better is the fuel efficiency.
• The knocking tendency of n -n-isomers is much greater than that of branched-chain alkanes.
• When tetraethyllead [(C₂H₅)₄Pb] is added in small quantities to gasoline, it converts the n -n-alkanes to branched-chain isomers, consequently decreasing the knocking tendency i.e, the octane number of gasoline is increased
• Gasoline obtained by catalytic cracking is more effective for internal combustion engines than that obtained by direct distillation of crude petroleum.
• Because this gasoline contains a large amount of unsaturated hydrocarbons & has a higher octane number

2. Reforming Definition

Reforming It is the method of changing low-grade gasoline to high-grade quality gasoline by changing the structures of constituent hydrocarbons by isomerization and aromatization

Isomerization:

In isomerization, a straight-chain hydrocarbon is heated with Al₂Cl₃ orPt to give a branched one.

Example:

Aromatisation:

In aromatization, a straight-chain hydro¬ carbon is converted into a cycloalkane by cyclization, which is converted into an aromatic hydrocarbon by dehydrogenation. The process is carried out by heating the alkane in the presence of a catalyst (Pt, Pd or Ni) at 400- 600°C.

Example:

After aromatization, hexane is converted into cyclohexane and heptane into methylcyclohexane.

Platinum is the most effective catalyst used in the reforming process and so the process of reforming is also called platforming.

Importance of reforming:

• The octane number of a fuel can be improved by increasing the percentage of branched-chain alkanes and aromatic hydrocarbons which possess greater efficiency as a fuel.
• A mixture of benzene, toluene, and xylene obtained in the reforming (aromatization) process is known as BTX. Many benzene derivatives may be prepared from these compounds

Classification Of Hydrocarbons

Classification of hydrocarbons may be summarised as:

Based on (their structure, hydrocarbons can be broadly classified

Into two main classes:

1. Acyclic or open-chain hydrocarbons and
2. Cyclic or closed-chain hydrocarbons.

1. Acyclic or open-chain hydrocarbons

In the molecules of these compounds, the carbon atoms are attached to form open chains, which may be branched or unbranched. They are also called aliphatic hydrocarbons. Depending on the nature of carbon-carbon bonds, these are further classified into the following two categories, these are saturated hydrocarbons (or alkanes) and unsaturated hydrocarbons (alkenes and alkynes)

Saturated hydrocarbons or alkanes:

The hydrocarbons in which the carbon atoms are linked with each other by single covalent bonds are called saturated hydrocarbons.

Example:

1. CH₃—CH₃ (Etliane)

2. CH₃— CH₂—CH₃ (Propane)

3. CH₃—CH₂—CH₂— CH₃ (Butane)

Unsaturated hydrocarbons:

Hydrocarbons in which at least two adjacent carbon atoms are linked by a double bond or triple bond are called unsaturated hydrocarbons.

Compounds containing carbon-carbon double bonds (C=C) are called alkenes and those containing carbon-carbon triple bond (C=C) are called alkynes.

Example:  Some alkenes are

1. CH₂=CH₂ (Ethene)

2. CH₃—CH=CH₂ (Propene)

3. CH₃—CH₂—CH=CH₂ (But-l-ene) etc.

Example:  Some alkynes are

1. CH=CH (Ethyne);

2. CH₃—C=CH (Propyne)

3. CH₃—CH₂—C=CH (But-1-yne)

CBSE Class 11 Chemistry Notes Chapter 13 Hydrocarbons

2. Cuclic or closed-chain hydrocarbons

The hydrocarbons having closed chains or rings of carbon atoms in their molecules are called cyclic or closed-chain hydrocarbons. They are further divided into two classes, alicyclichydrocarbons, and aromatic hydrocarbons.

Alicyclic hydrocarbons:

The cyclic or closed-chain hydrocarbons which have properties similar to those of aliphatic hydrocarbons are called alicyclic hydrocarbons. They can be classified as saturated and unsaturated alicyclic hydrocarbons. Saturated hydrocarbons are cycloalkanes. Unsaturated hydrocarbons are further divided into cycloalkenes and cycloalkynes.

1. Cycloalkanes:

Alicyclic hydrocarbons in which all the ring-forming carbon atoms are joined by single covalent bonds are called cycloalkanes.

2. Cycloalkenes:

Alicyclic hydrocarbons containing one carbon-carbon double bond are called cycloalkenes.

Example:

3. Cycloalkynes:

Alicyclic hydrocarbons containing one carbon-carbon triple bond are called cycloalkynes. Lower cycloalkynes are highly strained and unstable, cyclooctyne is strained but somewhat stable while cyclone yne and higher members are unstrained and stable.

Example:

Aromatic hydrocarbons:

Aromatic hydrocarbons are of two types: benzenoid aromatic hydrocarbons and non benzenoid aromatic hydrocarbons.

1. Benzenoid aromatic hydrocarbons:

Hydrocarbons containing one or more benzene rings (either fused or isolated) are called benzenoid aromatic hydrocarbons.

They are also called arenes.

Example:

2. Non-benzenoid aromatic hydrocarbons:

Aromatic hydrocarbons containing no benzene ring are called non- benzenoid aromatic hydrocarbons.

Examples: Azulene, Pentafulvalenes etc..

CBSE Class 11 Chemistry Notes For Chapter 13 Hydrocarbons Aliphatic Hydrocarbons

Alkanes

Open Chain saturated hydrocarbons are referred to as alkanes. At ordinary temperature and pressure, they generally do not show any affinity towards most of the reagents such as acids, bases, oxidising and reducing agents and because of this inertness, they are called paraffins (Latin: parum = litde, affinis= affinity). Each C-atom presentin an alkane molecule is sp³ -hybridised.Four cr -bonds formed by each sp3 -hybridised carbon are directed towards the comers of a regular tetrahedron. Thus, alkanes have tetrahedral structure around each carbon atom. The molecular formula of alkanes is CnH₂n + 2 [wheren 1, 2, ]. Their general formula is RH (R: alkyl group).

Nomenclature of alkanes

The nomenclature of alkanes according to the IUPAC system has been thoroughly discussed. Here, only the trivial names ofthe isomers of butane and pentane and the IUPAC names of some higher alkanes are mentioned

Structure of alkanes

1. Alkanes contain only carbon-carbon and carbonhydrogen single bonds. They have the following structural characteristics:

2. Each C-atom is sp3 -hybridized. Four sp³ -hybrid orbitals are directed towards the comers of a regular tetrahedron. The carbon atom lies at the centre ofthe tetrahedron

3. All C—C and C—H bonds are strong sigma bonds. Each C —C cr -bond is formed as a result o axial overlapping of two sp3 orbitals, one from each carbon atom and each C—H bond is formed by the axial overlapping ofone sp3 orbital ofcarbon with the s -orbital of hydrogen.

C—C and C—H bond lengths are 1.54A & 1.12Arespectively.

4. All bond angles in alkanes (C —C —C, C —C —H and H—C—H) have a value of 109°28′ . Thus, alkanes possess tetrahedral structures

5. Carbon atomsin an alkane molecule having three ormore carbon atoms do not lie along a straight line. Instead they form a zig-zag pattern. Thisis because each carbon atomis sp³ -hybridised and naturally the C—C— C bond angle is 109°28′ instead of 180°. It becomes clear from the structure ofpropane shown

6. C—C and C—H bond dissociation enthalpies are,83kcal -mol-1 and 99 kcal-mol^-1 respectively.

Structural isomerism in alkanes

Alkanes (except methane, ethane and propane) exhibit chain isomerism, a type of structural isomerism. This type of isomerism arises due to the difference in the nature of carbon chain or the skeleton ofthe carbon atoms.

Example: 

1. The two chain isomers having molecular formula (C4H10) are n-butane and isobutane. If a 1° or 2° H atom of a propane molecule is replaced by a methyl group, then these two isomers are formed.
2. The two chain isomers having molecular formula (C4H10) are n-butane and isobutane. If a 1° or 2° H atom of a propane molecule is replaced by a methyl group, then these two isomers are formed

2. Three chain isomers of molecular formula C₅H₁₂ are n -pentane (CH₂CH₂CH₂CH₂CH₃), isopentane [(CH₃)₂CHCH₂CH₃] and neopentane [(CH₃)₄C]. These isomers are formed on replacement of different H -atoms ofn-butane and isobutane bymethyl group

There are five chain isomers having molecular formula C6H14 and these are obtained by replacement of different five chain isomers having molecular formula

Hydrocarbons Class 11 NCERT Notes

Conformational isomerism in alkanes Definition

Electron distribution of the sigma molecular orbital of a C—C bond is cylindrically symmetrical around the internuclear axis and as this is not disturbed due to rotation about its axis, free rotation about the C—C single bond is possible. Infinite number of spatial arrangements of atoms which result through rotation about a single bond are called conformations or conformational isomers or rotational isomers or simply conformers or rotamers and the phenomenon is called conformational isomerism.

The difference in potential energy between the most stable conformation and the conformation under consideration is called the conformational energy ofthe given conformation. It is to be noted that the rotation around a C—C single bond is not completely free.It is hindered by a very small energy barrier of 1-20kl-mol^-1 due to very weak repulsive interaction between the electron clouds of different cr -bonds. Such repulsive interaction is called torsional strain. Conformations are three-dimensional. These are generally represented in paper by three projection formulae: flying wedge formula, sawhorse projection formula and Newman projection formula

Conformations of ethane

A molecule of ethane (CH₃—CH₃) contains a carbon-carbon single bond (σ-bond) and each carbon atom is attached to three hydrogen atoms. The two —CH₂ groups can rotate freely around the C—C bond axis. Rotation of one carbon atom keeping the other fixed results into infinite number of spatial arrangements of hydrogen atoms attached to the rotating carbon atom with respect to the hydrogen atoms attached to fixed carbon atom.

These are called conformational isomers or conformations or conformers. Thus, there are infinite number of conformations of ethane. However, there are two extreme cases. The conformation in which the hydrogen atoms attached to two carbons are as close together as possible, /.a, in which the dihedral angle between two nearest C —H bonds of two — CH₃ groups is zero, is called the eclipsed conformation.

The conformation in which the hydrogen atoms are as far apart as possible, i.e., the dihedral angle between two C —H bonds is 60° is called the staggered conformation. The eclipsed conformation suffers from maximum torsional strain whereas in staggered conformation this strain is minimam.

So, the eclipsed conformation is much less stable than the staggered conformation. Any other intermediate conformation i.e., the conformation in which the dihedral angle is between 0-60°, is called the skew conformation. Its stability is in between the two extreme conformations. Therefore, the order of stability of these three conformations is: staggered > skew > eclipsed. It is to be noted that in all these conformations, the bond angles and the bond lengths remain the same.

Saturated hydrocarbons containing more than two carbon atoms have different conformations. However, as there is only one carbon atomin methane, it does not existin the above-mentioned conformations. The eclipsed and the staggered conformations of ethane can be represented by the flying wedge formula, sawhorse projection formula.

Newman projection formula is as follows

1. Flying wedge formula:

In this representation, the two bonds attached to a carbon atom are shownin the plane of the paper and of the other two, one is shown above the plane and another below the plane. The bonds which are in the plane are shown by normal lines (—) but the bond above the plane is shown by solid wedge ( —) and the bondbelowtheplane isshown by hashed wedge.

2. Sawhorse projection formula:

In this projection, molecule is viewed along the molecular axis. It is then projected on paper by drawing the central C —C bond as a somewhat elongated line. Upper end of the line is slightly tilted towards righthand side. The front carbon is shown at the lower end of the line, whereas the rear carbon is shown at the upper end. Each carbon has three lines attached toit corresponding to three H -atoms. The lines are inclined at 120° angle to each other

3. Newman projection formula:

In this projection, the molecule is viewed along the C —C bond. The C-atom nearer to the eye of the viewer (i.e., the front carbon) is represented by a point and the three H-atoms attached to the front C-atom are shown by the three lines drawn at an angle of 120° to each other. The C-atom situated farther from the eye of the viewer (i.e., the rear carbon) is represented by a circle and the three hydrogen atoms attached to it are represented by three shorter lines drawn at an angle of 120° to each other.

Eclipsed and staggered conformations of ethane in I H H terms of Newman projection formula (along with dihedral angles, 0) are shown below

Energy barrier between two extreme conformations is actually very small and so, rotation of two —CH₃ groups takes place extremely rapidly. Due to this, it is not possible to separate the conformations of ethane. However, at any moment, majority of ethane molecules exist in the staggered  conformation ofminimum energy {i.e., maximum stability).

The eclipsed conformation is least stable because hydrogens and bondingpairs ofelectrons of eclipsed C —H bonds involving adjacent C-atoms are very close to each other causing maximum repulsion. The staggered conformation is most stable because the hydrogens and bonding pairs of electrons of each pair of C —H bonds involving adjacent C-atoms are at a maximum distance. This causes minimum electronic as well as steric repulsion.The potential energy of the molecule is minimum for staggered conformation.

It increases with rotation and reaches a maximum at eclipsed conformation. Experimentally, it has been found that staggered conformation of ethane is 2.8 kcal-mol_1 more stable than eclipsed conformation. (E eclipsed ~ = 2 8 kcal-moH ). Therefore, rotation about C—C bond is not completely free. However, this energy barrier is not large enough to prevent rotation at room tempe¬ rature as collisions between the molecules supply sufficient kinetic energy to overcome this energy barrier.

Dihedral angle Φ

Dihedral angle (Φ) is the angle between the X—C—C and the C—C—Yplane of X-C-C-Y unit ,it is the angle between the H—¹C— ²C plane and ¹C—²C—H plane, i.e., it is the angle between the ¹C—H bond and the ²C—H bond in the Newman projection formula. It is also called the angle of torsion.

Conformations of propane (¹CH₃–²CH₂–³CH₃):

In propane molecule Both C₁ —C₂ & C₂ —C₃ bonds are equivalent in propane molecules. An infinite number of conformations of propane can be obtained as a result of rotation about the C₁ —C₂ (or C₂ —C₃ ) bond. The two extreme conformations are the eclipsed conformation (I) and the staggered conformation (II).

The staggered conformation is more stable than the eclipsed conformation by 3.4 kcal-mo^-1

Conformations of n-butane (CH₃-CH₂-CH₂-CH₃): nbutane contains two kinds of C —C bonds. So, conformations likely to be generated depend on that particular C —C bond around which C-atoms are made to rotate.

Rotation about the C₁—C₂ bond:

Keeping C₁ fixed, when C₂ is rotated around the C₁—C₂ bond axis, infinite numbers of conformations are obtained. Among these, twoprincipal conformations are eclipsed (I) and staggered (II) conformations. Their order of stability is: staggered > eclipsed, i.e., molecules of n-butane spend most of their time in staggered conformation (II).

 Rotation about the C2-C3 bond:

Infinite number of conformations are possible if C₃ is made to rotate around C₂—C₃ bond axis, keeping C₂ fixed. Among these, the four chief conformations are— antistaggered (1), gauche staggered (3), eclipsed (2) and fully eclipsed (IV). In an-staggered conformation, the two —CH₃ groups exist anti to each other, i.e., they are oriented at an angle of 180° (<p = 180°).

In the gauc/ie-staggered conformation, the two —CH₃ groups make an angle of 60° with each other (Φ = 60°). In the eclipsed conformation, the two pairs of —CH₃ and H and one pair of H -atoms are in direct opposition, while in the fully eclipsed conformation, the two pairs of H-atoms and one pair of CH₃ groups are in direct opposition.

The order of their stability is:

1 >3 >2 > 4, i.e., the molecules of n -butane pass most of their time in and-staggered conformation (1).

Their Newman projection formulae are shown below:

The most stable and least stable conformations ofn -butane are anti-staggered and fully eclipsed conformations respectively. The angular distance between two similar bonds in the anti-staggered conformation is maximum (180°). Thus, repulsion between electrons of such bond pair is minimum. Again, two —CH₃ groups are located farthest from each other and so, no sterlc hindrance or steric strain acts between them. On the other hand, the angular distance between two similar bonds in the fully eclipsed conformation is minimum (0°). Thus, repulsion between electrons ofeach bondpair is maximum.

Again, two —CH₃ groups are in direct opposition and hence there occurs severe steric strain involving these two CH₃ groups. For this reason, anti-staggered conformation is the most stable while fully eclipsed conformation is the least stable conformation of n -butane

General Methods Of Preparation Of  Alkanes

1. From compounds containing the same number of C-atoms

By hydrogenation of unsaturated hydrocarbons (alkenes or alkynes):

Alkanes may be prepared by the reduction of alkenes or alkynes by hydrogen in the presence of finely powdered nickel platinum or palladium catalyst. This process is called catalytic hydrogenation. The pressure and temperature of the reaction depend on the nature of the catalyst used.

When a mixture of the vapors of any unsaturated hydrocarbon and hydrogen is passed over a nickel catalyst heated at 200 – 300°C, alkanes containing the same number of carbon atoms are obtained. This process is known as Sabatier-Senderens reduction.

Reduction of alkenes or alkanes can be carried out at a lower temperature 25°C) by using highly 200°C active Raney nickel as a catalyst

• Each mole of alkene combines with 1 mole of hydrogen while each mole of alkyne combines with 2 moles of hydrogen to yield the corresponding alkane.
• Raney nickel: When an alloy containing equal amounts of Ni and Al is digested with sodium hydroxide solution, aluminum dissolves in alkali and finely divided nickel is obtained as residue. This is called Raney nickel. It is washed with water and stored under water or alcohol

• Hydrogen gas thus produced remains adsorbed and occluded in the finely divided nickel and for this reason, the efficiency of Raney nickel as a catalyst is very high.

Applications of hydrogenation reaction:

1. Hydrogenation reaction takes place quantitatively and the volume of hydrogen added can be easily estimated. Therefore, with the help of this reaction, the number of double bonds present in an unsaturated compound can be determined.
2. Vanaspati or vegetable ghee,
   • For example: Dalda, margarine, etc., (saturated glycerides)
3. May be prepared from edible vegetable oils,
   • For example: soybean oil, sunflower oil, cotton-seed oil, etc., (unsaturated glycerides) by catalytic hydrogenation.

Hydrocarbons Class 11 NCERT Notes

By reduction of alkyl halides:

Alkanes can be prepared by the reduction of alkyl halides with zinc/hydrochloric acid zinc/acetic acid, zinc/sodium hydroxide, zinc-copper couple/ethanol, aluminum amalgam/ethanol etc.

Mechanism

Zn→  Zn^2+- + 2e

R→X + e → R^• + X^–

^•R + e→ :R^–

^•R^– + H⁺→ R—H

or, :R + C₂H₅OH→ RH + C₂H₅O^–

Example:

Alkanes may also be obtained by the reduction of alkyl Red P/150°C halides with lithium aluminum hydride (LiAlH₄) sodium borohydride(NaBH₄) or hydrogen in the presence of palladium (Pd) catalyst.

1. LiAlH₄ is not suitable for the reduction of tertiary alkyl halides because in that case alkenes are obtained. However, if NaBH₄ is used, the corresponding alkane is  obtained

2. Primary, secondary, and tertiary alkyl halides may be reduced to the corresponding alkanes by triphenyltin hydride (Ph₃SnH or TPH ).

3. The order of reactivity of alkyl halides (RX) in reduction reaction is: RI > RBr > RC

By Clemmensen, the reduction of aldehydes and ketones:

When aldehydes and ketones are reduced with amalgamated zinc and concentrated hydrochloric acid, the corresponding alkanes are obtained. The reaction is so-called after the name of the discovery

Reduction of alcohol, alkyl iodide, aldehyde, ketone, and carboxylic acid by red P and HI:

When alcohol, alkyl iodide, aldehyde, ketone, and carboxylic acid are reduced by heating with concentrated aqueous solution of hydroiodic acid at 150°C in the presence of a small amount of red phosphorus, the corresponding alkanes are obtained. The reaction is conducted in a closed vessel

Examples:

Red phosphorus reacts with 12 to regenerate HI. Therefore, the backward reaction leading to the formation of the starting compounds does not take place.

3I₂ + 2P→2PI₃

PI₃ + 3H₂O→H₃PO₃ + 3H

By the hydrolysis of Grignard reagents:

When dry and pure metallic magnesium is dissolved in a dry ethereal solution of an alkyl halide, an alkylmagnesium halide (R—MgX) is obtained. This organometallic compound is known as Grignard reagent.

In this compound, the carbon atom is directly attached with the Mg-atom, and the C—Mg bond is a highly polar covalent bond. When Grignard reagents are treated with water or dilute acids, the corresponding alkanes are obtained in this reaction. The alkyl group (R) of the Grignard reagent takes up a proton to generate alkane (RH).

It is to be noted that Grignard reagents may also react with other compounds containing active hydrogen such as alcohols, ammonia, amines etc., to form alkanes

2. From compounds containing a greater number of C-atoms than the corresponding alkanes:

By decarboxylation of carboxylic acids:

When a mixture of anhydrous sodium or potassium salt of a carboxylic acid and soda lime (NaOH+CaO) is strong, a molecule of carbon dioxide is eliminated from the acid (decarboxylation) to produce an alkane

The alkane obtained has one carbon atom less than that of the corresponding carboxylic acid

3. From compounds containing less number of C-atoms than the corresponding alkanes:

1. By Wurtz reaction:

When a dry ethereal solution of an alkyl halide (preferably bromide or iodide) is treated with metallic sodium, the two alkyl groups of two alkyl halide molecules combine to form an alkane. This reaction for the preparation of an alkane is known as the Wurtz reaction. The resulting alkane contains twice the number of carbon atoms present in the molecule of alkyl halide.

Mechanism: Two different mechanisms have been suggested.

1. Through the formation of the organometallic compound as an intermediate

2Na → 2Na⁺ + 2e

R —X + 2e → R^– + X^–

2Na⁺ + 2X→ 2NaX

2. Through the formation of free radicals as intermediates

2Na → 2Na⁺ + 2e

2R—X + 2e → 2R^•+ 2X^–

R^•+ R^• → R—R

2Na⁺ +  2X^–→ 2NaX

Example:

Some important points related to Wurtz reaction:

• Metallic sodium acts as a reducing agent and ether acts as a solvent.1° and 2° alkyl halides participate in the Wurtz reaction while 3° alkyl halides do not participate in this reaction due to steric effect.
• Methane cannot be prepared by this reaction because this reaction always leads to the formation of alkanes containing more than one carbon atom.  This processis not suitable for the synthesis of unsymmetrical alkanes.
• This is because in that case, the reaction is to be carried out using an alkyl halide containing an even number of carbon atoms (RX) and an alkyl halide containing an odd number of carbon atoms (R’X).
• RX combines with R’X to yield the desired alkane, R—R’ but at the same time, two molecules of RX combine to form the alkane, R —R and two molecules of R’X combine to form the alkane, R’ —R’.  Therefore, a mixture of three alkanes are obtained.
• Although the desired alkane is obtained, its yield is low and it cannot be separated from the mixture easily as the boiling point of the formed alkanes are very close to each other.

Example:

When methyl bromide and ethyl bromide are made to react with each other for the preparation of propane; ethane and butane are also produced along with propane. This results in a very poor yield of propane and it cannot be easily separated from the mixture

Wurtz reaction is applicable for the preparation of symmetrical alkanes containing an even number of Catoms but not for the preparation of unsymmetrical alkanes containing an even or odd number of C-atoms.

Because a symmetrical alkane can be divided into two required equal parts and so two types of alkyl halides are not required for its preparation. However, an unsymmetrical alkane cannot be divided into two equal parts and so two different alkyl halides are required for their preparation.

• The symmetrical alkanes that require tertiary (3°) alkyl halides for their preparation cannot be synthesized by the Wurtz reaction.
• The order of reactivity of various alkyl halides: RI > RBr > RCl

Importance Of Wurtz reaction:

This reaction leads to the formation of C — C bond. The formation of C — C bond is very important in organic synthesis. Also, bicyclic compounds can be prepared by intramolecular Wurtz reaction.

For example:

By Kolbe’s electrolysis method:

When a cold and concentrated aqueous solution of sodium or potassium salt of a carboxylic acid is electrolyzed between platinum electrodes, hydrogen gas and NaOH or KOH are formed at the cathode and at the anode, alkane, and CO₂ are obtained.

When the mixture of CO₂ and alkane is allowed to pass through a caustic soda solution, CO₂ is absorbed and the alkane is obtained: This process for the preparation of alkanes is known as Kolbe’s electrolysis.

Mechanism:

RCOOK ⇌  RCOO- + K⁺ 2HO⇌  2H⁺ + 2OH^–

At anode: RCOO^– → RCOO^•+ e; RCOO^• →R^•+ CO

R^•+ R^•→ R —R

At cathode: 2H⁺+ 2e→ [2H^•] → H₂

Example:

Electrolysis of concentrated and cold aqueous solution of potassium acetate between platinum electrodes produces ethane at the anode

Some important points related to Kolbe’s electrolysis method:

In this method, alkanes with double the number of carbon atoms present in the alkyl group of the carboxylic acid is obtained. Thus,if n is the number of carbon atoms present in the salt of carboxylic acid, the alkane formed must contain 2{n- 1) carbon atoms.

Therefore, methane cannot be prepared by this method This is [fcWI because in this case a mixture of aqueous solution of sodium or potassium salts of two different carboxylic acids is to be subjected to electrolysis and as a result, two more alkanes in addition to the desired alkane will be produced. This reduces the yield of the desired unsymmetrical alkane and it cannot be easily separated from the mixture

Importance of Kolbe’s electrolytic method:

This reaction leads to the formation of C — C bond which is synthetically important. Also, alicyclic compounds can be prepared by intramolecularKolbe’s electrolytic method.

Corey-House synthesis:

An alkyl halide, RX is first treated with lithium metal in dry ether medium to form alkyl lithium (R—Li) which is then treated with iodide to formlithium dialkyl cuprate (R₂CuLi). Lithium dialkyl cuprate is finally’ treated with a suitable alkyl halide (R’X or RX) to form desired alkane (R—R’ or R—R)

The third step i.e., the final step is an S_N2 reaction, and therefore, no tertiary’ (3°) alkyl halide (R’X) can be used in this step.

Example:

Importance of Corey-House alkane synthesis: This reaction can be used to prepare both symmetrical and Therefore, methane cannot be prepared by this method. unsymmetrical alkanes in good yield.

NCERT Class 11 Chemistry Chapter 13 Hydrocarbons Notes

 4. Preparation of alkanes from inorganic compounds

From inorganic carbides:

Some inorganic carbides react with water to liberate saturated hydrocarbons.

For example:

When beryllium carbide and aluminum carbide are heated with water, they get hydrolyzed to form methane. This method gives pure methane

The carbide compounds which react with water to form methane are called methanides.

From alkyl boranes:

1. Alkanes may be prepared by treating trialkyl boranes, obtained by hydroboration of alkenes, with propanoic acid (protonolysis)

2. When a trialkyl borane is heated with a mixture of AgN03 and NaOH at 30-40°C, an alkane of high molecular mass is obtained.

For example: When tripentylborane is heated with a mixture of AgNO₃ and NaOH at 30-40°C, decane is formed as the product

General Properties Of Alkanes

1. Alkanes Physical properties

1. With increase In the number of carbon atoms, the physical s states of the alkanes change in the order: gas -> liquid ~> solid. At normal temperature and pressure, straight chain alkanes from C₁ to C₄ (i.e., methane, ethane, propane, and butane) are colorless gases, C₅ to C₁₇ (from pentane to heptadecane) are colorless liquids and from C18 onwards are colorless solids.

2. Alkanes are non-polar, lighter than water and almost insoluble in water but they are soluble in non-polar or less polar solvents like benzene, chloroform, ether, carbon tetrachloride etc

Boiling points of isomeric pentanes:

3. With an increase in molecular mass, boiling points, melting points, and viscosities of straight-chain alkanes increase regularly. Among the Isomeric alkanes, the boiling point decreases with an increase in branching, i.e., a branched chain alkane hasInvariablylower boiling point than the corresponding n -n-alkane,

For example:

In the case of isomeric pentanes, n-pentane has the highest boiling point while neo-pentane has the lowest boiling point.

4. It is also evident that the increase in melting point Is relatively more in moving from an alkane having an odd number of carbon atoms to a higher alkane while it Is relatively less in moving from an alkane with even number of carbon atoms to a higher alkane. As n -n-alkanes with an even number of carbon atoms are more symmetrical than those containing an odd number of carbon atoms, they pack more closely in the crystal lattice involving much stronger intermolecular forces of attraction

The reason behind decreasing boiling point:

Among nonpolar hydrocarbon molecules, the forces of attraction\which come into play are weak van der Waals forces. These which come into play are weak van der Waals forces. These Waals forces depend on the area of contact between Waals forces depends on the area of contact between the molecules. Branching reduces the area of contact because

A branched compound has a more compact, nearly spherical shape, and spheres touch only at a point. For this reason, branching reduces the van der Waals forces and so it reduces the boiling point. Since branching, for example, increases gradually on going from n-pentane to neopentane, the area of contact gradually decreases and consequently, the boiling point gradually decreases as van der Waals forces go on decreasing

Van der Waals forces of attraction: in non-polar molecules, the center of positive charge density coincides with the center of negative charge density. However, due to the random 300-movement of electrons around the nucleus, a momentary 280 distortion of their distribution may occur.

This results in both momentary loss of electrical symmetry and the formation of a momentary dipole in the molecule. This instantaneous dipole induces a dipole in a second molecule. These dipoles then attract each other which hold the molecules together. These attractive forces are known as van der Waals forces of attraction

2.  Alkanes Chemical Properties

Alkanes are generally inert substances. They do not easily react with acids, alkalis, oxidizing agents, and reducing agents. However, under suitable conditions, they form compounds by substitution reactions.

Reasons for chemical internees of alkanes:

The reasons for the chemical reactivity of any compound are polar bonds,

• The presence of one or more lone pairs of electrons,
• The presence of an atom with an expandable octet and

Presence of an atom with an incomplete octet. Alkanes possess none of the above characteristics.

For example:

The C —H and C —C bonds present in alkane molecules are non-polar.

So the polar reagents do not find any suitable site to attack the alkane molecules. There is no lone pair of electrons in alkane molecules. In alkane molecules, the octet of carbon is filled with electrons and hydrogen has also attained the stable electronic configuration of an inert helium atom. Again, due to the absence of any orbital, carbon cannot expand its octet. All these factors collectively contribute to the general inertness of alkanes

3. General reactions of alkanes

1. Oxidation reactions of alkanes:

Combustion:

Alkanes burn in the presence of excess oxygen or air to produce carbon dioxide and water along with the liberation of huge amounts of heat. For this reason, alkanes are used as fuels. Chief constituents of LPG used for household cooking are n-butane, propane and isobutane, and a small amount of ethyl mercaptan.

CH₄ + 2O₂ → CO₂ + 2H₂O + 213 kcal.mol^-1

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O + 368 kcal.mol^-1

The general equation for combustion of an alkane may be given as follows:

C_xH_y + (x+y/4) →x CO + y/2 H₂O + heat

When burnt in a limited supply of air or oxygen, alkanes produce different quantities of carbon (carbon black) and carbon monoxide, besides carbon dioxide and water.

Controlled oxidation:

Controlled oxidation of alkanes by oxygen at high temperature and pressure in the presence of metal or metallic oxide catalyst produces alcohols, ‘ aldehydes and carboxylic acids

Example:

1. The controlled oxidation of methane yields methyl alcohol and formaldehyde.

2.  Methane is oxidised by ozone to form formaldehyde

CH₄ + 2O₃→HCHO + H₂O + 2O₂

3. Alkanes containing tertiary hydrogen are oxidized to tertiary alcohol potassium permanganate

2. Substitution reactions:

The characteristic reaction of saturated hydrocarbons are substitution reactions. In this reaction, the hydrogen atom attached to the carbon atom is displaced by any monovalent atom or group.

1. Halogenation:

In the reaction of halogens with alkanes presence of light, heat (250 – 400°C) or catalyst, the hydrogen atoms of the alkanes are easily replaced by halogen atoms to give haloalkanes and hydrogen halide. This is called a halogenation reaction.

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Example:

1. When methane is allowed to react with chlorine in the presence of diffused sunlight at ordinary temperature, all the hydrogen atoms of methane are replaced by chlorine atoms successively to yield different substitution products

Mechanism:

The displacement of hydrogen atoms of alkane by chlorine atoms in the presence of diffused sunlight proceeds through the free radical mechanism.

It is a chain reaction which occurs through the following steps:

First step (chain initiation):

In the presence of light or heat, ClCBSE Class 11 Chemistry Notes For Chapter 13 Hydrocarbons First Step Chain Initiation molecules get excited and its covalent bond undergoes homolytic cleavage to form free radicals (Cl ).

Second step (chain propagation):

Two reactions occur in this step.

1. Chlorine-free radical abstracts a hydrogen atom from a methane molecule to form methyl radical

2. The formed methyl radical abstracts a Cl-atom from Cl₂ molecule to form CH₃Cl and another chlorine-free radical C

The two reactions (1) and (2) are repeated again and again and as a result, the chain gets propagated. Consequently, the amount of CH₃Cl in the reaction mixture gradually increases.

[The resulting CH₃CI reacts similarly with Cl to form CH₂Cl₂ which in turn leads to the formation of CHCl₃ which subsequently facilitates the formation of CCl₄ by similar mechanisms].

Third step (chain termination):

The two free radicals (same or different) combine to terminate the chain

From the above-mentioned mechanism, it is observed that, methyl free radical forms as an intermediate in the halogenation of methane. Two methyl free radicals (C^•H₃) formed in tills way combine to terminate the chain thereby forming an ethane molecule

Therefore, it can be said that a small amount of ethane may be formed during chlorination of methane

2. Bromine atoms also substitute hydrogen atoms of alkanes, but bromination reaction takes place at a much slower rate than chlorination reaction. This reaction also follows the free radical mechanism.

3. The iodination reaction is reversible because hydroiodic acid produced in this reaction, being a strong reducing agent, reduces the alkyl iodide back to alkane. In this case, an oxidizing agent capable of oxidizing HI such as HIO₃, HNO₃, HgO etc., is used and as a result, the reaction becomes irreversible

CH₄+ I₂ ⇌  CH₃I +HI

5HI + HIO₃ → 3I₂ + 3H₂O

4. Fluorination of alkanes with pure fluorine has very little practical use. It cannot be controlled under ordinary conditions as fluorination of alkanes is very vigorous. It causes extensive breaking of C —C and C —H bonds and a mixture of products is formed. However it is done by diluting fluorine with an inert gas such as nitrogen or argon. Suitable inorganic fluorides like AsF₃, SbF₃, AgF, HgF₂ etc., are heated with suitable chloroalkanes to get alkyl fluorides.

It is called the Swarts reaction.

The reactivity of halogen towards the alkanes follows the order: of fluorine > chlorine > bromine > iodine.

If any alkane contains two or more non-equivalent hydrogen atoms then all the isomeric monohalogenated derivatives are formed. However as the reactivity of different types of hydrogens differ, the monohalo derivatives are obtained in different quantities, The reaction follows free radical mechanism, and since the stability of free radicals

Follows the order:

3° > 2° > 1°, the reactivity of different hydrogens towards halogenation reaction follows the order: 3°H > 2°H > 1°H.

For example: 

2-chloropropane is obtained as the major product in chlorination of propane and 2-bromobutane is obtained as the major product in bromination ofbutane.

Nitration:

The reaction in which the hydrogen atom of an organic compound is replaced by a nitro (-NO₂) group is called nitration. When a mixture of an alkane and fuming HNO₃ vapours are heated at higher temperatures (400 – 475°C) under pressure, it undergoes nitration to yield a nitroalkane.

All types of hydrogen atoms of an alkane molecule may be replaced by —NO₂ group during a nitration reaction. Also, there is a possibility of C —C bond cleavage leading to the formation of a mixture of lower nitroalkanes.

Sulphonation:

The substitution reaction in which the hydrogen atom of an organic compound is replaced by a sulphonic acid group (-SO₃ H) is called sulphonation. When an alkane is heated with fuming sulphuric acid (H₂SO₄ + SO₃ ) or oleum (H₂S₂O₇ ) at a higher temperature, an H-atom of alkane is substituted by -SO₃ H group to form alkane sulphonic acid

3. Pyrolysis or Cracking:

The thermal decomposition of organic compounds is called pyrolysis and in case of alkanes, it is called cracking. When an alkane containing a large number of carbon atoms is heated at high temperatures (500 – 700°C) in the absence of air, it undergoes cracking to yield a mixture of lower alkane, lower alkene, and hydrogen

Example:

Isomerization:

The conversion of one isomer of a compound into another isomer is called isomerization. When an n -alkane is heated at high temperature (300°C) in the presence of a catalyst (anhydrous AlCl₃/HCl or AlBr₃/HBr ), it gets converted into abranched chain isomer.

Example:

Isobutane is obtained from n-butane by isomerization. Isobutane is the chain isomer (a type of structural isomer of n-butane

Isomerization is a very important reaction for the preparation of high-quality gasoline (higher octane n

4. Dehydrogenation and dramatization:

When an alkane is passed over a suitable catalyst (oxides of Cr, Mo, Al, etc.) heated at 500-750°C , one molecule of hydrogen is eliminated from a molecule of the alkane to liberate an alkene

When alkanes containing six or more carbon atoms are passed over Pt, Pd or Ni catalysts heated at higher temperatures, benzene or alkylbenzenes are obtained. This reaction is known as aromatization. Aliphatic hydrocarbons may be converted into aromatic hydrocarbons by using this reaction.

Example:

When n -n-hexane is heated at 400-600°C in the presence of Pt catalyst, benzene is obtained. Similarly, n -heptane gives toluene.

4. Use of hydrocarbons as fuel

• Natural gas: The gas found above petroleum deposits at various depths below the earth’s crust is referred to as natural gas. The main constituent of natural gas is methane (90%). This gas is supplied by pipelines for use as a fuel.
• CNG (Compressed Natural Gas): Natural gas kept in steel cylinders under high pressure is called CNG. It is used as an alternative fuel in different vehicles in many metropolitan cities.
• LNG (Liquefied Natural Gas): Liquefied natural gas kept in steel cylinders under high pressure is called LNG. It is also used as a fuel
• LPG (Liquefied Petroleum Gas): Liquefied petroleum gas kept in steel cylinders under high pressure is called LPG.It is mainly a mixture of n-butane and isobutane with a small amount of propane.It is mainly used as a pollution-free fuel in small-scale industries and for household purposes.

Methane

Methane is the simplest paraffin or alkane. Its molecular formula is CH₄. The general formula of the paraffin is (CnH_2n + 2). When n = 1, it gives the formula of the first member of the alkane series, methane (CH₄).

1. Preparation of methane

1. Laboratory preparation:

Methane  Principle:

Methane is prepared in the laboratory by heating a mixture of anhydrous sodium acetate (1 part) and sodalimde (3 parts).

Methane  Purification:

• Acetylene is eliminated by passing the evolved gas first through the ammoniacal cuprous chloride solution.
• Then ethylene and moisture are removed from the gas by passing it through fuming sulphuric acid.
• The methane gas obtained contains a small quantity of sulphuric acid vapors and hydrogen.
• Vapors of H₂SO₄ are then eliminated by passing the gas over solid potassium hydroxide and the gas thus obtained is collected by downward displacement of mercury. In this methane, a small quantity of hydrogen is present as an impurity.
• In order to remove hydrogen, this gas is passed through palladium heated to about 100°C. H₂ gas is adsorbed by palladium. Pure methane obtained is collected over mercury.

2. Preparation of methane at room temperature:

By hydrolysis of aluminum carbide: At ordinary temperature, methane is prepared by treating aluminum carbide with water. If dilute HC1 is used instead of water, there is less possibility of the formation of aluminum hydroxide layer on aluminum carbide

Al₄C₃ + 12H₂O → 3CH₄ + 4Al(OH)₃

Al₄C₃ + 12HCl → 3CH₄ + 4AlCl₃

Besides this, methane can also be prepared by the hydrolysis of beryllium carbide (Be₂C).

Be₂C + 4H₂O→ CH₄+ 2Be(OH)₂

By the reduction of methyl iodide:

Almost pure methane can be prepared by the reduction of methyl iodide with ethyl alcohol and Zn-Cu couple or aluminum amalgam. Methane thus obtained contains traces of hydrogen as an impurity

By the hydrolysis of methylmagnesium iodide (a Grignardreagent):

CH₃Mgl +H₂O →CH₄ + Mg(OH)I

By the hydrolysis of zinc dimethyl:

Zn(CH₃)₂ + 2H₂O → 2CH₄ + Zn(OH)₂

3. Synthetic methods:

Methane is obtained when a mixture of H₂ and CO or CO₂ is passed through overpowered nickel at 250-400°C.

Methane is obtained in small amounts when the electric spark is produced in H₂ gas with the help of carbon electrodes. electric spark Heat

2. Properties of methane

Physical properties:

• At normal temperatures, CH₄ is a colorless, odorless, tasteless & non-poisonous gas.
• It is lighter than air and sparingly soluble in water but is highly soluble in organic solvents (alcohol, acetone, and ether).
• On cooling, it turns into its liquid & solid state. Its boiling & melting points are -161.4°C & -183°C respectively.

Chemical properties:

• Methane, being a saturated hydrocarbon, is highly stable.
• At ordinary temperature, it is inert to acids, bases, oxidizing and reducing agents. However, methane participates in different substitution reactions

Will O The Wisp:

Methane is produced in marshy lands due to bacterial decomposition of organic matter and hence, it is called marsh gas. Again, due to the putrefaction of animal bodies, phosphine, (PH₃) and diphosphorus tetrahydride (P₂H₄) are also produced in marshylands. So methane gets contaminated with PH₃ and P₂H₄. P₂H₄ readily bums in the air.

So when the whole mixture comes in contact with air, P₂H₄ sets the gases on fire and the heat produced causes methene to bum with a blue flame. As a result, an intermittent source of light is produced, known as will-o’- the-wisp. Thus, light can be seen in stagnant swampy areas, especially graveyards. Will-o’-the-wisp is not a supernatural phenomenon

3. General reactions of methane

Combustion:

Methane does not support combustion but in the presence of air or oxygen, it bums with a non-luminous bluish flame with the formation of carbon dioxide & water.

CH₄ + 2O₂ →CO₂ + 2H₂O + 213 kcal-mol^-1

A mixture of methane and air or oxygen explodes when comes into contact with fire and this is a possible reason of explosions in coal mines.

Substitution reactions:

1. Reaction with chlorine:

Methane does not react with chlorine in the dark, but in the presence of direct sunlight, methane reacts with chlorine explosively to form carbon (in the form of soot) and hydrogen chloride

In presence of diffused sunlight, methane undergoes substitution reaction with chlorine. In this case, the hydrogen atoms of methane are successively replaced by chlorine atoms to form methyl chloride, methylene chloride, chloroform and carbon tetrachloride respectively

The substitution reaction of methane with chlorine proceeds via free radical mechanism In this reaction, a mixture of different chloro compounds is always obtained. The constituents of the mixture can be separated. The reaction can be restricted to the first step by using excess of methane and consequently, methyl chloride may be obtained as the predominant product.

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2. Reaction with bromine:

Like chlorine, bromine also reacts with methane. However, the reaction proceeds slowly because bromine is less reactive than chlorine.

3. Reaction with iodine:

The reaction of iodine with methane is extremely slow and the reaction is reversible. So, the reaction is carried out in the presence of an oxidizing agent like HIO₃ or HNO₃

CH₄ + I₂  ⇌ CH₂ HI, 5HI + HIO₃ +3I₂ ⇌  3H₂O

4. Nitration: Methane reacts with nitric acid vapour at about

Reaction with fluorine:

Fluorine reacts violently with methane with explosion to form carbon and hydrogen fluoride

CH₄ + 2F₂→ C + 4HF

Recently, it has been possible to prepare different fluoro compounds by reacting fluorine with methane in the presence of inert gas (dilution of the active reagent).

Reaction with ozone:

Methane undergoes oxidation by ozone to yield formaldehyde (HCHO).

CH₄ + 2O₃ → HCHO + 2O₂ + H₂O

Controlled oxidation:

Methane on controlled (partial) oxidation by oxygen at much higher temperature (1500°C) produces acetylene.

Oxidation:

• When a mixture of methane and oxygen solution of by volume) is passed through hot copper tube at 200°C under a pressure of 100 atmosphere, methanol is obtained.
• Methane is oxidised by air at 450°C, under pressure in the presence of molybdenum oxide as catalyst to yield formaldehyde

Reaction with steam:

When a mixture of methane and steam is passed over nickel (catalyst) kept on alumina heated at 800 – 900°C, a mixture of CO and H₂ is obtained

Thermal decomposition:

At 1000°C, methane decomposes to produce fine carbon powder.It isknown as carbonblack

4. Uses of methane 

• Carbon black is used for preparing printers’ ink, black paints and in rubber industry for making motor tyres
• Useful chemicals like methyl chloride, acetylene, formaldehyde, methanol etc. are produced from methane
• 3. The gas mixture (CO + H₂) obtained by the reaction of methane with steam is used for the commercial preparation of hydrogen.
• When steam is passed through a catalyst heated at 500°C, CO is converted into CO₂ and H₂ is obtained:
• CO + H₂O→CO₂ + H₂. Also, when the gas mixture containing CO (1 part) and H₂ (2 parts) is passed through a mixture consisting of CuO, ZnO, and Cr₂O₃ as catalyst under 200 atmosphere pressure, methanol is obtained: CO + 2H₂→ CH₃OH.
• Methane is used as fuel. Its calorific value:1000Btuper eft

5. Identification of methane

Methane reacts with ozone to yield formaldehyde. So the gas being tested is subjected to react with ozonised oxygen. Ifit is methane, it emits the characteristic pungent smell of formaldehyde. Water is added to the reaction mixture to prepare a dilute solution of formaldehyde and with this solution, Schryver’s colour test is performed.

Schryver’s colour test:

2mL of an aqueous solution of phenylhydrazine hydrochloride is added to lmL 5% aqueous solution of potassium ferricyanide and to this mixed solution, a small quantity of the above test solution is added followed by the addition of 5mL of concentrated HCl. If formaldehyde is present, the solution turns pinkred. It indicates that the gaseous sample is methane

Methane is a saturated hydrocarbon—Proof:

Methane is chemically inert. At ordinary conditions, it does not react with acids, bases and oxidising or reducing agents. When methane gas is passed through red bromine water or through alkaline KMnO₄ solution, colours of the reagents do not change, i.e., methane does not react with these reagents.

So methane is not an unsaturated compound. It undergoes a substitution reaction with chlorine to produce four chloro compounds (CH₃Cl, CH₂Cl₂, CHCI₃ and CCl₄) and HCl. It means that methane is a saturated compound.

5. Structure of methane molecule

The central C-atom of methane is sp³ -hybridised and it has a regular tetrahedral structure. Four C—H bonds of formaldehyde, methanol etc. are produced from methane. bond energy and the same bond length.

If any one of the H-atoms of methane is replaced by a monovalent atom or group (Z), only one type of derivative CH₂Z is obtained. This proves that four H-atoms of methane are equivalent.

Methane has a regular tetrahedral structure—Proof:

If two hydrogen atoms of methane are substituted by two similar atoms or groups (Z), only one disubstituted methane (CH₂Z₂) is obtained

For example:

Only one kind of methylene chloride (CH₂Cl₂) is known to exist which has no isomer. From the above observation and in the perspective of tetracovalency of carbon, it can be concluded that structurally methane molecule is a regular tetrahedron—it is not square planar, rectangular planar, pyramidal with square or rectangular base.

This is becauseit will not have any isomer only when it becomes tetrahedral but if it assumes any structure other than tetrahedral, then it will have more than one isomer.

Ethane (C₂H₆)

Ethane is found to exist along with methane in natural gas. Ethane is also obtained in small amount from coal gas.

1. Preparation of ethane

1. By heating sodium propanoate with soda lime:

Ethane thus obtained is impure and contains some amount of methane and hydrogen as impurities.

2. By the reduction of ethyl iodide:

Ethyl iodide, on reduction with Zn-Cu couple or aluminium-amalgam and ethyl alcohol, yields ethane

3. By Wurtz reaction: When metallic sodium reacts with methyl iodide dissolved in dry ether, ethane is obtained

4. By Kolbe’s electrolytic method :

When a concentrated aqueous solution of sodium or potassium acetate is electrolysed by using platinum electrodes, ethane, and CO₂ are evolved at the anode. At the cathode, hydrogen gas is evolved and sodium or potassium hydroxide is produced

5. By hydrolysis of ethyl magnesium iodide:

When ethylmagnesium iodide (CH₃CH₂MgI) is treated with water, pure ethane is obtained.

CH₃CH₂Mgl + H₂O→  CH₃ —CH₃ + Mg(OH)I

6. By hydrogenation of ethylene or acetylene:

2. Properties of ethane

Physical properties:

• At ordinary temperature, ethane is a colourless and odourless gas. Its boiling and melting points are -89°C and -172°C respectively.
• It is slightly soluble in water but highly soluble in organic solvents such as ether, alcohol etc.

Chemical properties:

• Ethane is a saturated hydrocarbon and hence,itis quite stable.
• Like methane,itis chemicallyinert to acids, bases, oxidising and reducing agents.
• The main reactions that ethane undergoes are substitution reactions similar to methane.

3. Reactions of ethane

1. Combustion:

Ethane burns in air or oxygen with a nonluminous flame, producing CO₂ , H₂O and considerable amount ofheat.

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O 368 kcal-mol^-1

2. Halogenation:

In diffused sunlight, ethane undergoes a stepwise substitution reaction with chlorine or bromine to yield different compounds. Ethane contains two methyl groups. So ethane produces two disubstituted, two trisubstituted, and two tetrasubstituted chloro or bromo derivatives in this substitution reaction,

Due to the substitution of H-atoms of C₂H₆ by P & Cl- atoms, chlorofluoroethanes are formed. Some chloroform-ethanes are known as freons,

For example: Cl₂FC—CCIF₂ (Freon 113), CH₃, (Freon-114) F₃C—CClF₂(Freon-11 5) etc.

3. Nitration:

Ethane reacts with nitric acid vapor sat 400°C to form nitroethane.

4. Pyrolysis:

When ethane is heated at 700°C in the absence of air, it decomposes to yield mainly ethylene

Uses of ethane:

Ethane is mainly used as fuel.

It is used to prepare ethyl chloride and C2H4 etc., C2H4 is an important raw material for the preparation of various organic compounds

Alkenes Olefins

Unsaturated hydrocarbons in which at least two adjacent carbon atoms are linked by a double bond are called alkenes. Alkenes are also called olefins (Greek: Olefiant = Olt forming) because the lower members (eq, ethene, propene) react with halogens cl or Br to form silly substances. They are represented by general formula C_nH_2n, where n = 2,3… etc

Nomenclature of alkenes

IUPAC nomenclature of aJJames has been only the IUPAC names of some higher alkenes mentioned here

Structure of the carbon-carbon double bond

The carbon-carbon double bond consists of one cr -bond and one σ-bond. The tr-bond is formed by head-on overlapping or axial overlapping of two sp² -hybridized orbitals of two C-atoms while the σ-bond is formed by lateral or sideways overlapping of the two unhybridized p-orbitals (let us assume, p_z -orbital) of the two C-atoms. The n -n-electron cloud remains distributed above and below the plane in which the two C-atoms and the atoms attached to them exist.

If now one of the C-atoms of the double bond is rotated along the axis of the cr -bond keeping the other C-atom fixed the (>. orbitals will no longer be parallel and there will be no overlap between them. As a result, the π -bond will break, However, the breaking of an n -bond requires 60 kcal mol^-1 of energy which Is not provided by the collision of the molecules at room temperature.

Consequently, free rotation of the doubly bonded carbon atoms Is not possible at room temperature. So the relative positions of the four groups (a, b, and a, b) attached to the two doubly bonded C-atoms remain fixed. The value of each of the three bond angles originated around the doubly bonded C-atoms (sp² -hybridized) is 120°

Isomerism of alkenes

Alkenes exhibit both structural isomerism and geometrical or ds-trans isomerism.

Structural isomerism:

Alkenes containing three or more carbon atoms can exhibit position, chain, and ring-chain isomerism.

1. Position isomerism:

This isomerism arises due to the difference in the position of the double bond in a particular carbon chain. For example, but-l-ene and but-2-ene are two position isomers

CH₃CH₂CH=CH₂ (But-l-ene)

CH₃CH =CHCH₃(But-2-ene)

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2. Chain isomerism:

This type of isomerism arises due to differences in the carbon skeleton. For example, but-l-ene and 2-methyl prop-l-ene are two chain isomers

Ring-chain isomerism:

This type of isomerism arises due to ring closure. For example, propene and cyclopropane are two ring-chain isomers

2. Geometrical or cis-trans isomerism:

Due to restricted rotation about die carbon-carbon double bond, alkenes exhibit geometric or ds-trans Isomerism.

For example The two geometric isomers of 2-butene are as follows

As each of the doubly bonded carbon atoms Is attached to two different groups, two types of spatial arrangements of groups are possible. The geometric isomers which have similar groups on the same side of the double bond are called c/s-isomers while the geometric isomers which have similar groups on the opposite sides of the double bond are called trans-isomers. Both the isomers have same structure but they have different configurations (arrangements of groups in space)

4. Relative stabilities of alkenes

Hydrogenation of alkenes leads to the formation of relatively more stable alkanes and the amount of heat evolved when 1 mol of an alkene is hydrogenated is cailed heat of hydrogenation. Higher the heat of hydrogenation of an alkene, less is its stablity. Therefore, the stabilities of alkenes can be predicted from the values of their heats of hydrogenation

Order of stabilities of alkenes based on their heat of hydrogenation values: 1 >2 >3 > 4 > 5  > 6.

Explanation of relative stabilities of alkenes:

Relative stabilities of alkenes can be explained based on hyperconjugation. The greater the number of a -H atoms i.e., the greater the number of hyperconjugative structures, higher the stability of the alkene.

A number of a-H atoms present in the alkenes I,  II, III, IV, V, and VI are 12, 9, 6, 6, 3 and 0 respectively. Therefore, the order of stability is: I >II >III = IV > V > VI. However, due to presence of two methyl (-CH₃) groups on the same side ofthe double bond in ds-2-butene (IV), it is somewhat less stable than the trans-isomer (III) due to steric hindrance. Hence, the correct order of stabilityis: I >II >III > IV > V > VI.

Generalalkenes Methods Of Preparation Of Alkenes

By elimination reactions

Reactions that involve the removal of two atoms or groups from two adjacent carbon atoms of an organic compound resulting in the formation of a double (or triple) bond in between those two carbon atoms are called elimination reactions.

By dehydration of alcohols:

• When alcohols are heated with molecule of water gets eliminated to form alkenes.
• In these reactions, —OH group is lost from the a -carbon atom while H atom is lost from the p -carbon atom.
• Therefore, for a dehydration reaction, the alcohol must contain a ft hydrogen atom. 0 Concentrated phosphoric acid can be used instead of concentrated sulphuric acid’

Mechanism of dehydration:

It is an El (Elimination unimolecular) reaction which proceeds via three steps.

These are

• Protonation of alcohol
• Removal of H₂O and formation of carbocation
• Loss of proton by the carbocation.
• The second step (slowest step) is the rate-determining step of the reaction.

Some important points related to the dehydration of alcohols:

• For dehydration of primary and secondary alcohols, concentrated H₂SO₄ and for dehydration of tertiary alcohols, dilute H₂SO₄ are effective.
• For different alcohols, ease of dehydration follows the one order: 3° alcohol > 2° alcohol > 1° alcohol.
• Alcohol vapors, when passed over phosphorous pentoxide (P₂O₅) or heated alumina(Al₂O₃) produce alkene with the elimination of one water molecule

• Rearrangement during dehydration of alcohols: During dehydration of alcohols, sometimes unexpected or rearranged alkenes are formed. These rearrangements happen due to 1, 2-hydride or 1, 2- methyl shift in order to  form a more stable carbocation intermediate.

Examples:

By dehydrohalogenation of alkyl halides:

When alkyl halides are heated with alcoholic caustic potash (KOH dissolved in ethanol) solution, alkenes are produced.

In this reaction, the halogen atom is lost from the a carbon atom while H-atom is lost from the β -carbon atom and therefore, it is also β -elimination reaction.

Mechanism:

Example:

Important points related to dehydrohalogenation of alkyl halides:

1. Due to the greater solubility of KOH than that of NaOH in ethanol, ethanolic potassium hydroxide is a more effective reagent. In this reaction, alcoholic solution of sodium or potassium alkoxide can also be used. ₃^-1

2. In that case, alkoxide ion (RO^–) acts as a base. For example, potassium ethoxide (C ₂H ₅O^–K⁺) dissolved in ethanol, potassium ferf-butoxide (Me ₃CO^–K⁺) dissolved in tertbutyl alcohol.

3. Alkyl halides undergo hydrolysis in the presence of KOH (or NaOH) dissolved in the more polar solvent water to give mainly alcohols through substitution reactions.

4. On the other hand, they undergo dehydrohalogenation reaction, i.e., elimination reaction in the presence of KOH (or NaOH) dissolved in the less polar solvent ethanol to produce alkenes as the major product

5. When an alkyl halide is heated in the presence of ethanolic KOH solution or potassium alkoxide dissolved in alcohol, ether is obtained as a side product along with alkene. If a primary alkyl halide is used, the possibility of the formation of ether becomes much higher because in that case reaction is more likely to proceed through S_N2 mechanism.

6. The order of reactivity of different types of alkyl halides towards de hydrohalogenation reaction is: alkyl iodide > alkyl bromide > alkyl chloride.  In case of alkyl groups, order of activity is: tertiary > secondary > primary.

By heating 4° ammonium hydroxide:

Alkene is obtained by heating 4°  ammonium hydroxide.

For example:  When tetraethylammonium hydroxide is heated, C₂H₄ is formed.

Saytzeff and Hofmann rules:

During the preparation of alkenes via elimination reaction (E2 mechanism), more than one alkene can be produced if two or more carbon atoms adjacent to the carbon atom containing the leavinggroup

For example: —X, —⁺NR₃, –⁺SR₂) have available H-atom.

1. According to the Saytzeff rule, if the leaving group be halide (except fluoride) or sulphonate (neutral substrate), the E2 reaction leads to the formation of a highly substituted alkene as the major product. This is called Saytzeff product. For example, when 2- bromobutane is heated with ethanolic KOH solution, 2-butene (80%) is obtained as the major product.

2. According to Hofmann rule, If the leaving group be a charged one

For  example: ⁺NR₃ or – ⁺SR₂),

Then the E2 R — C = C —R- frans-alkene (major) reaction leads to the formation of a less substituted alkene as the major product. This is called the Hofmann product. For example, when cetyltrimethylammonium hydroxide is heated, 1- butene (95%) is obtained as the major product

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By dehalogenation of vicinal dihalides:

Dihalogen derivatives of alkanes in which the two halogen atoms are attached to adjacent carbon atoms are called vicinal dihalides or 1,2- dihaloalkanes. When a methanolic solution of a vicinal dihalide is heated with Zn-dust, one molecule of halogen is eliminated from the dihalide molecule to produce an alkene.

By the reduction of alkynes

When alkynes are reduced with sodium in liquid ammonia, (rans-alkenes are obtained as the major product.

Reduction of alkynes by hydrogen in the presence of Lindlar’s catalyst [Pd-CaCO₃ partially poisoned with lead acetate, Pb(OAc)₂ ] gives cfs-alkene predominantly

By Kolbe’s electrolysis

When an aqueous solution of sodium or potassium salts of saturated dicarboxylic add are electrolyzed, alkenes are obtained.

By pyrolysis or cracking of alkanes

When alkanes are passed through a tube heated at 500-600°C in the absence of air, they undergo thermal decom¬ position to produce lower alkenes, alkanes and hydrogen.

By pyrolysis of some other compounds

Alkenes are obtained by the pyrolysis of esters, xanthates and N-oxides of tertiary amines

Cope elimination reaction:

Tertiary amine oxides on thermal decomposition produce alkenes and dialkylhydroxylamines. This reaction is known as Cope elimination reaction

General Properties Of Alkenes

1. Physical properties of Alkenes

• The first three members of alkene family, i.e., ethene, propene and butene are colourless gases, the next fourteen members (C₅-C₁₈) are liquids while the higher members are solids at room temperature. All are colourless.
• Except for ethene which has a pleasant odour, all other alkenes are odorless.
• All alkenes are lighter than water and insoluble in water. However, they are soluble in non-polar organic solvents like benzene, petroleum ether, carbon tetrachloride, alcohol, chloroform etc.
• Due to the presence of double bond (C=C), some alkenes exhibit geometrical isomerism.

Boiling points:

The boiling points of alkenes increase regularly with increase in molecular mass and for the addition of each CH₂ group to the chain, the boiling points increase by 20- 30.

In cls-alkene, the alkyl groups lie on the same side of the double bond but in trans-alkene, the alkyl groups lie on the opposite sides of the double bond. For this reason, the molecules of cis-isomer are polar but the molecules of trans-isomer are non-polar or less polar. Thus, in the liquid state, the intermolecular attractive forces are relatively stronger in the case of the cis-isomer and hence it has a higher boiling point than the trans-isomer.

Melting points:

In trans alkene, the molecules are more symmetrical, and therefore, they can pack more closely in the crystal lattice than the molecules of cis-alkene. Due to this, the intermolecular forces operating in frans-alkene are stronger and iso, a larger amount of heat is required to break the crystal lattice of trans-alkene than the corresponding lattice ofc/s-alkene. Thus, the melting point of trans-alkene is higher than that of c/s-alkene

2. Chemical properties  of Alkenes

The double bond present in alkenes consists of a strong cr-bond and a weaker 7r-bond. The 7T -electrons are loosely held and are easily polarisable. So the n-bond takes part in chemical reactions easily.Itis for this reason, alkenes are more reactive than alkanes. The typical reactions of alkenes are addition reactions in which the rr -bond breaks and two new (r-bonds are formed. Two monovalent atoms or groups become attached to the doubly bonded carbon atoms to form a saturated compound.

General Reactions Of Alkenes

Chemical reactions of alkenes are generally divided into five classes:

• Combustion reaction
• Addition reaction,
• Oxidation reaction
• Polymerization reaction and
• Substitution reaction.
• Combustion reactions

Alkenes are combustible substances. They bum in air or in O₂ with a luminous flame to yield CO₂ and H₂O with the evolution of heat. The percentage of carbon content in alkenes is higher than that in alkanes and so during combustion alkenes produce black smoke due to liberation of free carbon.

2C_nH_2n + 3nO₂→  2nCO₂+ 2nH₂O + heat

Example: CH₂=CH₂ + 3O₂ →2CO₂ + 2H₂O + heat

1. Addition reactions Definition 

The reactions in which two reactant molecules are added together to yield a single molecule of product are called addition reactions

Characteristics:

• The compounds obtained in addition reactions are called addition products.
• Alkenes give additional products by reacting with halogens (Cl₂, Br₂ or I₂), hydrogen halides (HX, X = Cl, Br, I), ozone etc.
• The molecule which gets attached to the unsaturated molecule in addition reaction is known as addendum.

Addition of hydrogen:

Presence of finely divided platinum, palladium or Raney nickel at ordinary temperature or finely divided nickel at 200-300°C hydrogen adds to the double bond of the alkene to give an alkane

Addition of halogen:

1. At ordinary temperature, halogens (Cl₂ or Br₂) participate in addition reactions with alkenes to produce vicinal dihalides. Generally, the alkenes are added to solutions of halogen dissolved in carbon tetrachloride (CCl₄).

2. In case of iodine, this reaction is reversible and takes place very slowly. © At ordinary temperature, fluorine does not from additional compounds with alkenes. However, at extremely low temperatures (-78°C) & under controlled conditions, the addition reaction may be carried out by using xenon fluoride as the reagent. In addition reactions, the order of reactivity of halogens is: CI₂ > Br₂ > I₂.

Mechanism of the reaction:

The reaction of halogen with alkene is an electrophilic addition reaction. Bromine molecule is non-polar. However, under the influence ofn -electrons of C=C bond, the displacement of cr -electrons of the bromine molecule takes place.

As a result, the Br-atom which is close to the double bond acquires partial positive charge while the other Br-atom gains partial negative charge.

The reaction of an alkene with this polarised bromine molecule takes place in two steps as follows:

First step:

The σ-electrons of carbon-carbon double bond attacks the bromine atom having partial positive charge. This results in heterolytic fission of Br—Br,  σ-bond and the positive bromine atom (electrophile) becomes attached to both the doubly bonded carbon atoms concertedly to form a cyclic bromonium ion. A bromide ion is obtained along with it. It is the slow or rate-determining step of the reaction.

Second step:

The bromoniumion undergoes nucleophilic attack (S_N2) by the Br^– ion and the cyclic ion opens up Markownikoff’s rule: In the addition reactions of producing 1,2-dibromoethane (addition compound). This step is fast

As the addition reaction is initiated by electrophilic end of the reagent, it is termed an electrophilic addition reaction.

If the electrophile contains no unshared pair of electrons or if one of the double bonded carbon contains an aromatic ring (as in the case of C₆H₅CH=CH₂), then a carbocation intermediate is formedin first step.

Addition of halogen hydracids:

Halogen hydracids HX (X = Cl, Br, I) undergo an addition reaction with alkenes to form alkyl halides. The reactivity of halogen hydracids towards addition reactions of alkenes follows the order: HI > HBr > HCl > HF

Example:

(Ethylene)CH₂=CH₂ + HBr → CH₃CH₂Br(Ethyl Bromide)

When a halogen hydracid reacts with an unsymmetrical alkene, there is a possibility of formation of two different alkyl halides.

Example:

Propene is an unsymmetrical alkene. Propene, when reacts with HBr, may give rise to both 1-bromopropane and 2-bromopropane. However, one of the two products is formed predominantly

In addition reactions of this type, i.e., in case ofaddition of an unsynunetrical addendum (as H—X) to an unsynunetrical alkene, out of the two addition compounds, the compound obtained as themajorproduct is governed by a rule known as Markownikoff’s rule.

Markownikoffs rule:

In the addition reactions of producing 1,2-dibromoethane (addition compound). unsymmetrical addendum (adding molecules) with unsymmetrical alkenes, the negative part of the addendum becomes attached mainly to that doubly bonded carbon atom which carries a lesser number of hydrogen atoms

Example:

In the addition reaction of HBr with propene, 2- bromopropane is obtained as the major product because C-2 of propene contains lesser number of hydrogen atoms than C-l and the negative part of her, i.e., bromine (Br) is added to C-2

Similarly, the addition of HI with 2-methylpropene takes place according to Markownikoff’s rule and 2-iodo- 2-methylpropane is obtained as the major product

Mechanism of the reaction:

In the first step of the following electrophilic addition reaction,

CH₂CH=CHCH₂+ HBr→  CH₃CH₂CHBrCH₃

the electrophilic end (H) of \(\stackrel{\delta+}{\mathrm{H}}-\stackrel{\delta-}{\mathrm{Br}}\) is attracted by the r -electron cloud of the double bond, i.e., the σ-bond attacks the H atom of HBr, thus forming a new C—H bond while breaking the H—Br bond. As the remaining carbon atom of the original double bond is left with six electrons, a carbocation is formed. This is the rate determining step of the reaction. In the second step, the nucleophile Br^– ion is added to C⁺ of the carbocation to form 2-bromobutane as the only additional compound.

Since the addition of the electrophile occurs in the determining step (the first step), the reaction is called an electrophilic addition reaction

Explanation:

When propane (CH₃CH=CH₂) reacts with HBr, the proton obtained from HBr can add to the number1 carbon atom (C- 1) to form an isopropyl cation (a secondary carbocation) or it can add to the number 2 carbon atom (C- 2) to form n -propyl cation (a primary carbocation).

The secondary carbocation is formed more easily and rapidly because it is more stable than the primary carbocation. Therefore, 2-bromopropane is expected to be formed as the major product in this reaction. In fact, 2-bromopropane is the predominant product obtained in this case

Exception:

The addition reaction of CH₂=CH —CF₃ with halogen hydracids

For example: HCl takes place contrary to Markownikoff rule.

1. In this case, the carbocation formed by the addition of H⁺ to C-l, in spite of being a secondary (2°) carbocation, becomes less stable than the primary (1°) carbocation formed by the addition of H⁺ to C-2 because of the presence of strong electron-attracting CF₃ group. So the reaction occurs mainly through the formation of primary carbocation thereby producing ClCH₂CH₂CF₃ as the only addition compound.

2. During addition of halogen acids to the alkene sometimes rearrangement occurs by 1, 2-hydride shift or 1, 2-methyl shift to form a more stable carbocation intermediate

Peroxide effect:

Addition of hydrogen bromide (HBr) to an unsymmetrical alkene in the presence of organic peroxides such as benzoyl peroxide (C₆H₅CO)₂O₂, differ-butyl peroxide (Me₃COOCMe₃) etc., or oxygen, takes place contrary to Markownikoff’s rule (antiMarkownikoff addition), i.e., Br” is added to that carbon atom of the double bond which contains greater number of H-atoms.

This abnormal addition in the presence of a peroxide is known is the peroxide effect or Kharash effect.

Example:

Addition reaction of propene with HBrin presence of an organic peroxide leads to the formation of 1- bromopropane as the major product.

Mechanism of the reaction:

The reaction of HBr with unsymmetrical alkene, e.g., propene (CH₃CH=CH₂), in the presence of an organic peroxide, follows free radical mechanism. The reaction takes place in four steps—

First step: Homolytic fission of benzoyl peroxide gives rise to phenyl radical

The reactive bromine radical (Br)thus produced uses itselfin the third step to continue the chain reaction.

1. The exception to Markownikoff’s rule in the presence of peroxide is observed only in the case of HBr.

Class 11 Hydrocarbons Important Topics

2. HF, HCI and HI do not exhibit peroxide effect:

Only if the two reactions of the third and fourth steps ofthe reaction mechanism described above are exothermic, then this additional reaction becomes possible. However, if any step becomes endothermic, the reaction will not occur.

It has been observed experimentally that onlyin the case of HBr both the steps are exothermic but in the case of HF, HCI or HI, one or the other of these steps is endothermic. For these reasons, HF, HCI, and HI do not respond to peroxide effect.

3. Alternative explanation:

The H—Cl bond is stronger than H—Br bond so the former is not cleaved by the free-radical. On the other hand, H—I bond is weaker than the H—Cl bond but iodine free radicals combine with each other rapidly to form iodine molecules instead of being added to carbon. Thus, HCI and HI do not exhibit a peroxide effect.

Addition of sulphuric acid:

Alkenes are absorbed by cold and concentrated sulphuric acid to form alkyl hydrogen sulfate (an inorganic ester)

(Alkene) RCH=CHR + H→OSO₃H(Sulphuric acid) → RCH₂CHR—OSO₃3H (Alkyl hydrogen sulphate)

Example:

H₂SO₄ is an unsymmetrical addendum. So the additional reaction of unsymmetrical alkene,

For example: Propene (CH₃CH=CH₂) with H₂SO₄ takes place according to

Markownikoff’s rule. Consequently, isopropyl hydrogen sulfate is formed as the predominant product

Alkyl hydrogen sulphates, when heated with water, undergo hydrolysis to give alcohols.

Addition of water (hydration):

Water adds to alkenes in the presence of acid catalysts to produce alcohols.

Example:

As H₂O is an unsymmetrical addendum, an unsymmetrical alkene like 2-methylpropene reacts with water to form fert-butyl alcohol following Markownikoff’s rule.

Addition of hypogeous acid:

The addition of hypohalous acid (HO^{δ -δ+}Cl or HO^{δ -δ+}Br ) to an alkene occurs when the alkene is treated with chlorine (Cl₂) or bromine (Br₂) in the presence of water. As a result, a vicinal halohydrin (a compound that contains both an —OH group and a halogen atom bonded to adjacent carbons) is produced. A small amount of vicinal dihalide is also obtained.

Hydroboration (Reaction with diborane):

When alkene reacts with diborane (B₂H₆), trialkylborane, an addition compound of BH₃, is produced.

When trialkyl borane is subjected to oxidative hydrolysis by treating with H₂O₂/NaOH, alcohol is produced. As a result of hydroboration and oxidative hydrolysis, antiMarkownikoff hydration of an unsymmetrical alkene

Oxymercuration-demarcation:

The reaction between an alkene and an aqueous solution of mercuric acetate leads to the formation of p -hydroxyalkylmercuric acetate (an addition compound). This reaction is known as oxymercuration. In this reaction, tetrahydrofuran is used as a solvent

When the compound produced is reduced with alkaline sodium borohydride, the —HgOCOCH₃ group is replaced by hydrogen to yield an alcohol. This is called demercuration.

Oxymercuration-demarcation is an alternative process for the addition ot water molecule to an alkene:

• In this method, the addition of water to unsymmetrical alkenes takes place by Markownikoff’s rule.
• As there is no possibility of a rearrangement reaction in this process of hydration, it is more effective than the conventional process of acid-catalysed hydration.

Ozonolysis:

The complete process of preparation of ozone by reacting with an alkane and then allowing ozone to decomposition of the resulting ozonide to give carbonyl compounds (aldehyde and/or ketone) Is called ozonolysis.

1. First step:

When ozone gas or a mixture of ozone and oxygen is passed through an alkene dissolved in a solvent with which ozone does not react

For example:

Chloroform, carbon tetrachloride or glacial acetic acid), ozone adds on to the alkene to form an addition compound, called alkene ozonide

2. Second step:

If alkene ozonide is heated with water or acetic acid in presence ofzinc dust, it decomposes (hydrolysis) to give aldehyde or ketone or a mixture of both depending on the structure of alkene. This decomposition of ozonide is referred to as reductive decomposition

Reason for using Zn-dust:

If the ozonide is decomposed by water alone (without zinc dust), the aldehyde formed is oxidised by H₂O₂ (another product of hydrolysis) to carboxylic acid. As zinc dust destroys (reduces) HO , the possibility of this oxidation reaction can be avoided and hence, aldehyde is obtained

RCHO + H₂O₂ → RCOOH + H₂O 

H₂O₂ + Zn → ZnO + H₂O

At present, ozonide is decomposed by dimethyl sulphide (Me₂S). As a result of this reaction, dimethyl sulphoxide (Me₂S= O) and carbonyl compound arc were produced. This reaction also provides no scope for the oxidation of aldehyde

Dissociation of C=C bond by ozonolysis:

In ozonolysis, the C=C bond of an alkene dissociates and as a result, two molecul of carbonyl compound are produced from one molecule of alkene.

Prediction about the nature compounds to be obtained on ozonolysis: if the structure of an alkene is known, the carbonyl compounds to be obtained on its ozonolysis can be easily determined.

After writing the structure of the alkene, the molecule is divided into two parts by causing fission of the double bond and by adding an oxygen atom to each carbon atom of the double bond. Thus, the nature of the carbonyl compounds to be obtained on ozonolysis can be ascertained

Different products of ozonolysis depending an the nature of alkene:

Determination of the structure of an alkene from the products obtained on its ozonolysis:

The structures of the two carbonyl compounds formed by ozonolysis are written side by side, their carbonyl groups facing each other. The two oxygen atoms are then removed and finally, the two carbonyl carbon atoms are joined by a double bond.

Example:

If ozonolysis of an alkene forms acetaldehyde and acetone as products, it may by concluded that the alkene is 2-methyI-2-butene.

Some important points related to ozonolysis:

1. If formaldehyde is obtained on ozonolysis, then the double bond of the alkene lies at the terminal position of the carbon chain i.e., the alkene must contain a terminal methylene group (=CH₂).

2. When ozonolysis gives rise to two molecules of the same carbonyl compound, the original alkene must be a symmetrical one but if two different carbonyl compounds are formed, then the alkene must be an unsymmetrical one.

3. If an alkene, on ozonolysis, leads to the formation of only one dicarbonyl compound, then the original alkene must be a cyclic alkene.

For example, cyclobutene on ozonolysis gives butanediol:

4. Any compound having two double bonds (an alkadiene) on ozonolysis produces one molecule of a dicarbonyl compound and two molecules of carbonyl compound.

For example:

1, 3 -pentadiene, when subjected to ozonolysis, gives one molecule of acetaldehyde, one molecule of glyoxal and one molecule of formaldehyde.

5. When two geometrical isomers are subjected to ozonolysis, they yield the same compounds. For example, both the cis-and trans-isomers of 2-pentene, when subjected to ozonolysis, give acetaldehyde and propanal.

6. If CO₂ is evolved on ozonolysis, then the original alkene must be an allene, i.e., an alkadiene in which the double bonds are adjacent to each other.

For example:

7. In the process of ozonolysis, carbon-carbon double bond undergoes fission to yield carbonyl compounds. So ozonolysis is an example of cleavage reaction of alkene

8. Ozonides on being reduced by LiAlH₄or NaBH₄ gives alcohols instead of carbonyl compounds.

For example:

9. When ozonides are decomposed by water in the absence of Zn, c-atom of the double bond, bonded to two h-atoms is ethylene converted into CO₂ whereas die doubly bonded c, bonded to one H-atom and one alkyl group, is converted to carboxylic acid. however, if any doubly bonded c atom bears two alkyl groups, it is converted to a ketone. this decomposition is referred to as oxidative decomposition

Ozonolysis of alkynes:

Alkynes on ozonolysis yields 1,2-dicarbonyl compounds.

For example:

Glyoxal is obtained on ozonolysis of acetylene. 2-oxopropanal is’ obtained on ozonolysis of propyne and butanedione is obtained on ozonolysis of but-2-yne

Class 11 Hydrocarbons Important Topics

2. Oxidation reaction

1. Oxidation by alkaline KMnO₄:

1. Cold and alkaline potassium permanganate solution oxidizes an alkene to yield a 1,2-diol or vicinal diol by attaching one hydroxyl group to each of the doubly bonded carbon atoms. This reaction is called hydroxylation of alkenes. In this case, cis-hydroxylation occurs because the addition of two OH groups takes place from the same side of the double bond.

Baeyers test:

Dilute alkaline solution (1-2%) of potassium permanganate is known as Baeyer’s reagent. The colour of the reagent is purple which disappears when it is allowed to react with any compound belonging to the class of alkenes or alkynes. So this reaction is used to test the presence of unsaturation, i.e., carbon-carbon double or triple bond in an organic compound. This is known as Baeyer’s test.

2. A hot and concentrated solution of alkaline KMnO₄ oxidises alkenes to give acid or ketone or a mixture of both. In this case, cleavage of the double bond occurs to yield the products.

• If =CH₂ group is present in the alkene, then it is oxidised to CO₂ and H₂O. If the alkene contains =CHR group, then it is oxidised to RCOOH
• If the alkene.contains =CR₂ group, then the group gives rise to R₂C=O by oxidation

By osmium tetroxide (0sO₄):

Osmium tetroxide (OsO₄) adds on to the double bond of an alkene to form osmic ester (a cyclic compound). When this cosmic ester is hydrolysed by the aqueous ethanolic solution of sodium bisulphite, 1,2- diol or vicinal diol is obtained.

Therefore, it is also a hydroxylation reaction. In this case, cis-hydroxylation preferentially because the two hydroxyl (—OH) an occurs groups add on to the doubly bonded carbon atoms from the same side of the double bond

2. Epoxidation (Formation of epoxide):

1. Alkene oxides or epoxides are formed when alkenes react with per acids

For example:

Perbenzoic acid, chloroperbenzoic acid etc.) This reaction is called Prileschaiev’s reaction

The resulting epoxide oh hydrolysis with dilute acid or alkali yields 1,2-diol. Epoxidation followed by hydrolysis causes the addition of two OH groups from the opposite sides of the double bond, i.e., in this case, trans-hydroxylation occurs.

Formation of carbonyl compounds (Wackcr process): when a mixture of alkene vapours and oxygen, at high pressure and 50°C, is passed through a solution of palladium chloride containing CuCl₂, the corresponding carbonyl compound is formed. Pd(II) ion is reduced to metallic Pd

Example:

3. Polymerisation

Polymerisation Definition:

The reaction in which two or more molecules of a simple compound unite together under the influence of suitable pressure, temperature and catalyst to produce a single large molecule, i.e., a macromolecule of high molecular mass (whose molecular mass is a multiple of the molecular mass of the simple compound) is called polymerization (Greek word poly = many, meros = unit, member, or part).

The product obtained by the reaction is called polymer and the simple small molecules from which the polymer is formed are called monomers. Alkenes possess a tendency to polymerize.

Polymerization of ethylene:

Depending on the reaction conditions and catalyst used,

Two types of polyethylene or polythene may be obtained.

1. Low-Density Polythene (LDPE):

It is used for making waterproof covers, toys, sheets used in construction work etc

2. High-Density Polythene (HDPE):

The tensile strength of this type of polythene is much higher. It is used in making pipes, bottles, water tanks, and as insulation for electric wires and cables.

Polymerisation of propylene:

Polymerisation of propene or propylene leads to the formation of polypropylene

Polypropene is much harder than polyethylene and its tensile strength is also higher. It is used for making ropes, buckets, automotive moulding, seat covers, containers for storing oils and gasoline, carpet fibers and packing.

Polymerization of substituted ethenes:

Teflon is inert towards acids, alkalis and other chemicals. It is used in making chemically resistant pipes and surgical tubes. Due to chemical inertness and high thermal stability, teflon is used for making non-stick utensils

5. Substitution reactions

Although addition reactions are the characteristic reactions of alkenes, yet alkenes also respond to substitution reactions under special condition, e.g., O when a mixture of ethylene and chlorine gas is heated at 350-400°C, vinyl chloride is formed through substitution reaction

6. Isomerisation

When alkenes are heated at 200 – 300°C in the presence of aluminium chloride as catalyst or at 500 – 700°C in absence of a catalyst, they yield isomeric compounds through change in the position ofthe double bond or alkyl groups. Example: When 1-butene is subjected to isomerisation, it gives 2-butene and 2-methyipropene

Ethylene Or Ethene (C₂H₄)

Ethylene (CH₂=CH₂) is the first member of the olefin or 1 alkene series. The general formula of alkenes is CnH₂n. The formula of ethylene can be obtained by putting n = 2.

1. Preparation of ethylene

Laboratory preparation:

Principle:

Ethylene is prepared in the laboratory by heating a mixture of ethyl alcohol and excess of concentrated sulphuric acid (about three times the volume of ethanol) at 165-170°C . Concentrated sulphuric acid being a strong dehydrating agent eliminates a molecule of water from ethanol to liberate ethylene. In this ethyl alcohol undergoes intramolecular dehydration.

Steps of the reaction:

The reaction takes place in two steps. In the first step ethyl hydrogen sulphate (an inorganic ester) and water are produced. In the second step, ethyl hydrogen sulphate decomposes at 165-170°C to yield ethylene and sulphuric acid.

CBSE Class 11 Chemistry Chapter 13 Hydrocarbons Overview

Collection:

• Concentrated sulphuric acid being a strong oxidising agent oxidises ethyl alcohol to CO₂ , itself being reduced to SO₂ So, CO₂ and SO₂ thus produced remain as impurities in ethylene.
• These acidic impurities are removed by passing the gas through NaOH or KOH solution.
• The gas is collected by the downward displacement of water in a gas jar.

Drying:

Ethylene thus collected contains a little water vapour which is removed by passing through a tower filled with fused CaCl₂ and the pure gas is collected over mercury

• In the preparation of ethylene, concentrated sulphuric acid (H₂SO₄) acts as an acid catalyst and water absorbent.
• In this process, if the quantity of cone. H₂SO₄ is not high or the quantity of alcohol taken is high and the temperature is less than 165 – 170°C, one molecule of water is removed from two molecules of alcohol, yielding diethyl ether (intermolecular dehydration).

• The use of syrupy phosphoric acid at 230°C instead of concentrated sulphuric acid improves the purity of ethylene (free from CO₂ and SO₂  ) formed.
• Ethylene cannot be dried by concentrated sulphuric acid because it absorbs ethylene to form ethyl hydrogen sulphate

CH₂=CH₂ + H₂SO₄  →CH₃— CH₂ —OSO₃H

Other methods of preparation of ethylene:

1. Ethylene Is prepared by heating ethyl alcohol In the presence ofstrong dehydrating agent like phosphorus pentoxide.

3C₂H₅OH + P₂O₅  → 3 [CH₂=CH₂] + 2H₃PO₄ ]

2. When ethyl alcohol vapours are passed over heated alumina at 350°C, ethylene Is produced (Industrial preparation of ethylene).

3. Ethylene gas is obtained when ethyl chloride, bromide or iodide is heated in the presence of a concentrated alcoholic solution of caustic potash

[X= Cl,Br,I]

4. When alcoholic solution of ethylene dichloride or dibromide is heated with zinc dust, ethylene is produced.

5. Pure ethylene may be prepared from impure ethylene by using Oxidation reactions using this reaction. The impure ethylene is first converted into ethylene dibromide by allowing it to react with bromine. It is then washed with Na₂ CO₃ solution, dried by anhydrous CaCl₂ and finally distilled to obtain pure ethylene dibromide. Pure ethylene dibromide thus obtained gives pure ethylene when heated with zinc dust

6. By partial hydrogenation of acetylene: When a mixture of acetylene and hydrogen is passed over palladium on silica gel support at 200°C, ethylene is produced.

By Kolbe’s electrolytic Process:

By thermal decomposition of petroleum: To meet industrial demand

For example:

Manufacture of polythene, production of ethyl alcohol on industrial scale etc.), ethylene is required in large quantities. Industrial requirements of ethylene are mainly fulfilled by cracking of petroleum

2. Properties of ethylene

Physical properties:

• Ethylene is a mild, sweet-smelling colourless gas.
• Its boiling point Is -105°C and its melting point Is -IG9.5°C.
• It Is almost as heavy as air. Its vapour density is 14.
• It is liquefied at 0°C and 44atm pressure. h is sparingly soluble in water but is highly soluble in alcohol and ether.
• It possesses anaesthetic properties.

Chemical properties:

In ethylene, the two C-atoms are linked by a double bond, one of which is a strong cr -bond and the other is a weak σ-bond. Hence, ethylene is easily oxidised and participates in an addition reaction. Due to the presence of π-bond, ethylene is more reactive than ethane. During the addition reaction,π -bond in ethylene is broken and two new σ -bonds are formed by which two monovalent atoms or radicals get attached to the C- atoms

General Reactions Of Ethylene

1. Oxidation reactions

Combustion:

Ethylene is a combustible gas but it is not a supporter of combustion. It bums with a sooty luminous flame with liberation of large amount of heat

C₂H₄ + 3O₂→2CO₂ + 2H₂O + 337 kcal-mol^-1

Due to the presence of unsaturation, the percentage of carbon in ethylene is more than that in ethane, having the same number of carbon atoms. So, as a result of its partial oxidation in air, some carbon particles are produced and the presence of hot carbon particles results in the emission of luminous flame. In excess of oxygen, ethylene undergoes complete combustion.

Reaction with potassium permanganate:

1. When ethylene gas is passed through cold and dilute (1-2%) alkaline KMnO₄ solution, the purple (reddish-violet) colour of permanganate is discharged. Ethylene gets oxidised to form colourless ethylene glycol. This proves that ethylene is an unsaturated compound

2. Ethylene is oxidised to CO₂ entered alkaline solution of KMnO₄ and H2° by hot an

Formation of ethylene oxide:

When a mixture of air and ethylene is passed under pressure over silver catalyst at 250-400°C, ethylene undergoes partial oxidation by O₂ to yield ethylene oxide.

Formation of acetaldehyde:

At 50°c, when a mixture of ethylene and O₂ under high pressure is passed through a solution of palladium chloride (PdCl₂) containing cupric chloride (CuCl₂), acetaldehyde is produced.

CH₂ = CH₂ + PdCl₂ +H₂O →CH₃CHOI = Pd +2HCl

2. Addition reactions

1. Addition of hydrogen:

At ordinary temperature and pressure in the presence of Pt or Pd or Raney nickel catalyst or at 200-300°C in the presence of Ni catalyst, hydrogen combines with ethylene to form the saturated hydrocarbon, ethane.

2. Addition of halogen:

1. Chlorine combines directly with ethylene in the presence of sunlight to yield additional compound 1,2-dichloroethane. It is an oily liquid which is commonly known as Dutch oil.

CH₂ = CH₂ + Cl →ClCH₂CH₂Cl

• When a mixture of ethylene and twice its volume of chlorine is ignited, ethylene bums with a red flame with the formation of carbon (soot) and hydrogen chloride. This reaction proves the presence of carbon in ethylene.

CH₂ = CH₂ + 2Cl₂ → 2C + 4HCl

•  At 350 – 450°C ethylene undergoes substitution reaction with chlorine to give vinyl chloride.

H₂=CH₂ + Cl₂ → CH₂ =CHCl (Vinyl chloride) + HCl

3.  When ethylene gas is passed through a red-brown solution of bromine in CCl₄ or CHCl₃, the solution becomes colourless due to the formation of colourless 1,2-dibromoethane. This reaction is used as a test for detecting ethylene and ethylenic unsaturation

3. The reaction of ethylene with iodine is very slow and in fact, the reaction is reversible.

4. Fluorine is highly reactive, so it does not participate in addition reaction. It decomposes ethylene to give hydrogen fluoride and carbon

3. Addition of halogen hydracids:

The addition reaction of halogen hydracids with ethylene produces ethyl halides.

CH₂=CH₂ + HX → CH₃CH₂X [X = I, Br, Cl, F]

Order of reactivity of halogen hydracids: HI > HBr > HC1 > HF

4. Formation of halohydrin (addition of HOX):

When ethylene reacts with chlorine water or bromine water, ethylene chlorohydrin or 2-chloroethanol and ethylene bromohydrin or 2-bromoethanol are respectively formed the reaction with chlorine water, some amount of 1,2-dichloroethane (ClCH₂CH₂Cl) and with bromine water, (BrCH₂CH₂Br) are produced.

In this reaction, the red colour of bromine water some amount of 1,2 dibromoethane is discharged. So it can be used as a test for the detection of ethylene or ethylenic unsaturation in an organic compound.

2. Addition of sulphuric acid:

When ethylene gas is passed through concentrated sulphuric acid (98%), it gets absorbed by sulphuric acid to form ethyl hydrogen sulphate.

Ethyl hydrogen sulphate, when heated with a dilute sulphuric acid solution, undergoes hydrolysis to give ethyl alcohol and H₂SO₄ alkaline solution, Baeyer’s reagent (a drop or two) is added. If the reddish-violet or purple colour of permanganate is discharged and brown-coloured MnO₂ is precipitated, then it indicates that the compound contains ethylenic unsaturation

1. It is to be remembered that in the test for unsaturation in an unknown organic compound, the disappearance of ; the reddish-violet or purple colour of cold dilute KMnO₄ solution does not necessarily confirm the presence of unsaturation in the compound. If the organic compound contains any group

For example:

—CHO ) which is easily oxidised by KMnO₄, the purple colour of KMnO₄ may be discharged in spite of the absence of unsaturation in the compound.

2. Again, while dissolving the test sample in an organic i solvent in place of water, it should be remembered that the solvent must not be oxidised by KMnO₄.

For example: 

In this experiment, acetone can be used as a solvent but not ethanol because the latter is readily  oxidised by KMnO₄

3. Test with bromine solution

Dilute solution of bromine (2%) in carbon tetrachloride or chloroform is added dropwise to the solution of the organic sample in the same solvent. If the reddish-brown colour of bromine solution is discharged and no HBr gas is evolved (moist blue litmus paper held over the mouth of the test tube does not turn red), then it indicates the presence of ethylenic
unsaturationin the organic compound under consideration

• The above reaction is an addition reaction. HBr is not liberated in this reaction. The colour of bromine may also be discharged by substitution reaction, but in that case, the evolution of HBr takes place.

For example:

• Again, when an organic compound contains a group (e.g., —CHO group) which can be oxidised by bromine, then the colour of bromine gets decolourised with the liberation of HBr.

For example, glucose discharges the colour of bromine water

• In fact, for the detection of ethylenic unsaturation, both Baeyer’s test and Br₂/CCl₄ test are performed. If both the tests give positive results, then it can be concluded that the given organic compound contains ethylenic by unsaturation.

Alkynes Or Acetylenes

The unsaturated hydrocarbon containing carbon-carbon triple bond (C= C) are called alkynes. Each family member has four hydrogen atoms less than the corresponding alkane. The general formula of alkynes is C_nH_2n-2  where, n = 2, 3, 4, ••• etc.

The first member of the series is acetylene and so the other members of this series are also collectively known as acetylene

1. Nomenclature of alkynes

In trivial or common system of nomenclature, the name of the simplest alkyne is acetylene. The names of the higher alkynes are given as substituted acetylenes.

The IUPAC names of alkynes are derived from the names of the corresponding alkanes by replacing the suffix -ane by -yne. IUPAC nomenclature of alkynes has been discussed thoroughly in

Only the IUPAC names of some higher alkynes are mentioned here

2. Structure of the carbon-carbon triple bond

1. The C = C bond present in the simplest alkyne acetylene consists of one sigma (cr) bond and two pi (a-) bonds. Each carbon atom of acetylene has sp-hybrid orbitals. One sp -hybrid orbital of each C undergoes head-on (axial) overlap with sp -hybrid orbital of another carbon to form a C —C σ-bond. The second sp-hybrid orbital of each carbon overlaps axially with the ls-orbital of each of the two hydrogen atoms to form two C —H sigma bonds.

2. Each carbon atom has two unhybridised p -orbitals (let us assume, py and pz) which are perpendicular to each other as well as to the internuclear axis. The two parallel py orbitals, one on each carbon, overlap sideways to form a n -n-bond. Similarly, the sideways overlapping of two parallel pz orbitals, one on each carbon, leads to the formation of a second σ-bond. These two; r -electron clouds are perpendicular to each other.

3. The value of H —C= C bond angle in acetylene is 180° and therefore, the shape of the molecule is linear

4. The C—H bond length is 1.06A and the C = C bond length is 1.20Å. The C = C bond length is shorter than the C=C bond (1.34Å) and C—C bond(1.54Å)

3. Isomerism in alkynes

Alkynes exhibit 4 types of structural isomerism:

1. Position isomerism:

Alkynes (except ethyne & propyne) exhibit position isomerism due to difference in position of the triple bond in the carbon chain. For example,

CH₃CH₂C = CH (But -1- yne) and CH₃C= CCH₂ ((But -2- yne)

CH₃CH₂CH₂C (Pent-1-yne)= CH and CH₃CH₂C = CCH₃ (Pent-2-yne)

2. Chain isomerism:

Alkynes having five or more carbon atoms show chain isomerism due to differences the type of carbon skeleton. For example,

CH₃CH₂CH₂C (Pent-1-yne)= CH and (CH₃)₂CH – C≡ CH

3. Functional group isomerism:

Dienes, i.e., compounds containing two double bonds, and alkynes are functional isomers of each other.

For example:

CH₃CH₂C = CH (But -1- yne) and CH= CH-CH = CH₂ (Buta – 1,3 – diene)

4. Ring-chain isomerism: Alkynes are ring-chain isomers of cycloalkenes.

For example:

4. Classification of alkynes

Alkynes are classified into two types

• Terminal alkynes and
• Non-terminal alkynes.

In terminal alkynes, the C = C bond is present at one end of the carbon chain and in non-terminal alkynes, the C= C bond is present in any position other than the terminal positions

Terminal alkynes:

CH₃CH₂ C ≡ CH (But -1- yne)

CH₃CH₂CH₂C ≡ CH(Pent -1- yne)

Non-terminal alkynes: 

CH₃C ≡ C CH₃ (But -2- yne)

CH₃C≡ C CH₂ CH₃ (Pent -2- yne)

CBSE Class 11 Chemistry Chapter 13 Hydrocarbons Overview

General Methods Of Preparation Of Alkynes

Like alkenes, alkynes can also be prepared by elimination

1. By dehydrohalogenation of vicinal dihalides:

1. When vicinal dihalides are refluxed with ethanolic KOH solution, alkynes are obtained due to the removal of two molecules of hydrogen halide.

2. Alkynes are formed at lower temperatures when sodamide (NaNH₂) dissolved in liquid NH₃ is used instead of alcoholic KOH

3. When vicinal dihalides are heated with ethanolic KOH solution, vinyl halides are obtained due to the removal of one hydrogen halide molecule. The resulting vinyl halides on treatment with NaNH₂/liq. NH₃ at lower temperatures lose another molecule of hydrogen halide to yield alkynes

Mechanism:

When NaNH₂/liq. NH₃ Is used as the reagent, the alkyne is obtained as a salt because acetylenic hydrogen is weakly acidic. This salt, on acidification, liberates the alkyne.

For example: 

In the above reaction, propyno, as soon as it is produced, reacts with NaNH₂ to form the corresponding salt. When dilute HCl is added to this salt, propyne is liberated.

Preparation of alkynes from alkenes:

Acetylene, for example, may be prepared from ethylene as follows:

2. By the dehydrohalogenation of gem-dihalides:

Dihalo compounds having two halogen atoms attached to the same carbon atom are called gem-dihalides.

1. When gem-dihalides are heated with ethanolic KOH solution, haloalkenes or vinyl halides are obtained. Alkynes are produced by treating the vinyl halides thus obtained with NaNH₂ /liq. NH₃

2. When gem-dihalides are refluxed with ethanolic KOH, alkynes are obtained

3. By dehalogenation of tetrahaloalkanes:

When tetrahaloalkanes (each of two adjacent carbon atoms is linked to two halogen atoms) dissolved in ethanol are heated with zinc dust or vapours of tetrahaloalkanes are passed over zinc dust, then alkynes are formed by the removal of four halogen atoms (as two molecules of ZnX₂).

In fact, Tetra haloalkanes are generally prepared from alkynes and hence, this method loses its importance for the preparation of alkynes in general.

Preparation of higher alkynes from alkynes containing acetylenic hydrogen atom:

When vapors of acetylene or terminal alkynes (RC = CH) are passed over heated metallic sodium or made to react with soda mide dissolved in iquid ammonia, their sodium salts (sodium alkynes) are produced. When these salts are allowed to react with primary alkyl halides

For example:

CH₃Br, CH₃ CHBr etc) higher alkynes are formed. In the case of acetylene, both the H-atoms can be replaced by alkyl groups

Examples:

Higher alkynes can also be prepared from lithium salts of terminal alkynes. These lithium salts are prepared by the action of lithium amide (LiNH₂) or n -n-butyllithium (n-BuLi) on terminal alkynes

General Properties And Chemical M Reactions Of Alkynes

1. General properties

The first three alkynes (ethyne, propyne, and butyne) are colorless gases at ordinary temperature and pressure, the next eight members (C₅-C₁₂) are liquids and higher alkynes are solids.

• Alkynes are sparingly soluble in water but highly soluble in organic solvents like alcohol, benzene, acetone etc.
• The melting and boiling points of alkynes are higher than those of the corresponding alkenes and alkanes.

Reason for higher melting and boiling points of alkynes:

Due to the presence of triple bonds, lower alkynes have linear structures. Hence, their molecules remain more closely packed in the crystal lattice as compared to those of the corresponding alkenes and alkanes.

Consequently, alkynes have higher melting and boiling points. Comparison of reactivities of alkenes and alkynes; Due to the smaller distance between the two triply bonded carbon atoms and better p -orbital overlap, the electrons in the triple bond are held more tightly. Therefore, they are poorly available to an electrophile as compared to that of a double bond.

Again, when electrophilic addition involves bridged-ion intermediates (a sin die case of Br₂ addidon), those arising from triple bonds are more strained [i.e., less stable) than those arising from double bonds.

Hence, although there is a higher concentration of electrons between the carbon atoms of a triple bond than in a double bond, alkynes are in general less reactive than alkenes towards electrophilic addition reactions

2. Chemical reactions

• The reactions of alkynes can be divided mainly into five classes, such as—
• Combustion reaction,
• Addition reaction,
• Oxidation reaction,
• Reactions due to acidic behaviour of acetylenic hydrogen and
• Polymerisation reactions.

Acetylene Or Ethyne (C₂H₂)

Acetylene (HC = CH) is the first member of the alkyne series. The general formula of alkynes is C_nH_2n-2. When n = 2, the general formula reduces to C₂H₂ which is the formula of acetylene. In 1865, Edmund Davy discovered this gas. It is not available in nature. Coal gas contains a very littie amount (0.06%) of acetylene.

1. Preparation of acetylene

Laboratory preparation:

Acetylene Principle:

At room temperature, acetylene is prepared by the action of water on calcium carbide

CaC₂ + 2H₂O→Ca(OH)₂ + C₂H₂↑

Acetylene Impurities:

Acetylene gas thus produced is not pure but contaminated with small amounts of phosphine (PH₃ arsine (AsH₃), hydrogen sulphide etc, Due to the presence of these gases, acetylene gas emits a foul smell.

Pure acetylene gas is sweet-smelling.

Acetylene Purification:

The impure gas is first passed through an acidified copper sulphate solution and then through a suspension of bleaching powder in water. As a result, lf 2S and NIL are absorbed in acidified copper sulphate solution and PH₃ and AsH₃ are absorbed in the suspension of bleaching powder in water. Acetylene gas thus obtained is moderately pure

Preparation of acetylene gas of high purity: 

1. Acetylene gas prepared in the laboratory is passed through ammoniacal cuprous chloride solution when a red precipitate of copper acetylide (CuC₂) is obtained while the other gaseous Impurities escape without any reaction

C₂H₂+ 2CuCl +2 NH₄OH Cu₂C₂  (Red)↓+2NH₄Cl+2H₂O

2. The precipitate thus obtained is filtered, washed thoroughly with water and then heated with concentrated hydrochloric acid or aqueous solution of potassium cyanide when acetylene gas is liberated.

The issuing acetylene gas, after drying over P₂O, is collected over mercury.In this way, highly pure acetylene is obtained.

Cu₂C₂+2HCl 2CuCl + C₂H₂↑

Cu₂C₂+8KCN + 2H₂O → 2K₃ [Cu(CN)₄]+ 2 KOH + C₂H₂↑

2. Other methods of preparation of acetylene:

1. By dehydrobromination of ethylene dibromide:

Acetylene may be prepared by boiling ethylene dibromide with an alcoholic solution of potassium hydroxide

The reaction occurs in two steps. In the first step, vinyl bromide (CH₂=CHBr) is produced. In the second step, the dehydrobromination of vinyl bromide results in the formation of acetylene.

However, for the second step, KOH is not a very effective reagent Sodamide (NaNH₂) dissolved in liquid ammonia is a much better and effective reagent. So acetylene can be prepared easily from ethylene dibromide by using NaNH₂ dissolved in liquid NH₃

Similarly, acetylene may be prepared from ethylidene chloride (1,1-dichloroethane)

2. By debromination of tetrabromoethane:

When vapours of 1,1,2,2-tetrabromoethane are passed over heated zinc dust, the compound undergoes debromination to yield acetylene.

3. By heating a haloform with Ag powder:

When iodoform lUMftl Physical properties (or any haloform) is heated with silver powder, acetylene gas is produced

4. By Kolbe’s electrolytic method:

Electrolysis of concentrated aqueous solution of Na or K-salt of maleic acid or fumaric acid liberates acetylene gas at the anode

5. Industrial preparation of acetylene: 

A mixture of coke and quick lime is heated to a very high temperature (2500- 3000°C) in an electric furnace when calcium carbide is produced. Acetylene is prepared by treating calcium carbide with water.

When natural gas, rich in methane, is heated at 1400-1500°C for a fraction of a second (0.1s) at ordinary atmospheric pressure, methane undergoes thermal decomposition (cracking) to yield acetylene.

Acetylene can be prepared by passing a stream of pure hydrogen gas through an electric arc struck between two carbon electrodes.

General Properties And Chemical Reactions Of Acetylene

1. Acetylene Physical properties

• Pure acetylene is a colourless sweet-smelling gas having boiling point -84°C .
• It is slightly lighter than air.
• Acetylene is sparingly soluble in water but its solubility in acetone is much higher (at 15°C and 10-12 atmospheric pressure, 1 volume of acetone dissolves 300 volumes of acetylene).
• Acetylene can be liquefied at 0°C and 26 atm pressure but on liquefaction It is converted into a highly explosive substance.
• For this reason, acetylene is transported from one place to another in cylinders by dissolving in acetone under high pressure

2. Reactions of acetylene

Combustion:

• Acetylene is a combustible gas but it does support combustion. It bums in air with a luminous flame. As the percentage of carbon content in acetylene.
• Is higher than that in the corresponding alkane, it undergoes incomplete combustion which leads to the formation of hot carbon particles for which a luminous flame is produced
•  It is for this reason acetylene Is used for illuminating purpose.
• Acetylene forms an explosive mixture with air. Acetylene burns in excess of oxygen with an explosion, forming CO and H₂O. In this luminous flame is produced which has a high temperature (3300°C). This flame is known acetylene flame

2C₂H₂ + 5O₂→4CO₂ + 2H₂O + 312kcal .mol^-1

Addition reactions:

1. Addition reactions of acetylene are:

Addition of hydrogen (hydrogenation): At 200 – 300°C in the presence of powdered nickel catalyst or at ordinary temperature and pressure, in the presence of Raney nickel or powdered platinum or palladium catalyst, the reaction of acetylene with hydrogen gas occurs in two steps. In the first step, one molecule of hydrogen adds to acetylene to give ethylene and in the second step, another molecule of hydrogen adds to ethylene to form ethane

2. Partial hydrogenation:

• In the presence of Lindlar’s catalyst, acetylene combines with only one molecule of hydrogen to form ethylene

• Partial hydrogenation of acetylene can also be affected in the presence of nickel boride (Ni₂ B), commonly known as P-2 catalyst
• Lindlar’s catalyst is finely divided palladium deposited on calcium carbonate Pd-CaCO₃, partially poisoned by lead acetate, Pb(CH₃COO)₂ and quinoline.
• 2-butyne reacts with hydrogen gas in the presence of Lindlar’s catalyst to produce mainly cis-2-butene

• Catalytic hydrogenation is an exception to the generalisation that alkenes are more reactive than alkynes towards addition reaction. The reason is as follows. Alkenes are adsorbed on the surface of the catalyst only when the plane of the n -bond approaches perpendicularly. Due to the cylindrical nature of the n bond of alkynes, any approach along the axis of the cylinder can be successful.

3. Addition of halogen (halogenation):

• Addition of chlorine:
  • Direct combination of chlorine with acetylene may occur with explosion, HC = CH + Cl₂ → 2C + 2HCl. If the reaction is carried outin the presence of kieselguhr (a silicate compound) and iron powder, the heat liberated is absorbed and no explosion takes place. In this case, the reaction takes place in two steps.

• • Westron is a poisonous substance. It is used as a solvent for oils, fats and resins. When it is made to react with steam and lime, trichloroethylene (aerosol) is obtained on the elimination of a molecule of HCl. Westrosol is also used as a solvent

• Addition of bromine:
  • The reaction of acetylene with bromine takes place at a comparatively slower rate. When acetylene gas is passed through red-coloured brominewater, 1, 2-dibromoethene or acetylene dibromide is produced and as a result, the colour of bromine-water is discharged. The reaction confirms the presence of unsaturation in acetylene

• • When acetylene is made to react with gaseous bromine or liquid bromine, it combines with two molecules of bromine to form 1,1,2,2-tetrabromoethane or acetylene tetrabromide

• Addition of Iodine:  Acetylene reacts extremely slowly with iodine. It combines with only one molecule of iodine dissolved in ethanol to form 1,2-diiodoethene.

4. Addition of halogen hydracids (HX):

Halogen hydracids react with acetylene in two steps. The reactivity of halogen hydracids in this reaction follows the order: HI > HBr > HCl.

In the first step, the addition of one molecule of HX gives vinyl halide (1-haloethene). The second step involves the addition of one more molecule of HX to vinyl halide to yield ethylidene halide (1,1-dihaloethane). This second step of the reaction occurs according to Markownikoff’s rule

Reaction with HBr:

Reaction with Hl:

Reaction with HCl:

At 65°C, in the presence of mercuric chloride (HgCl₂) catalyst, acetylene forms vinyl chloride with hydrochloric acid by addition reaction.

Hydrocarbons Chapter 13 NCERT Notes

5. Addition of hypochlorous acid (HOCl):

When acetylene is passed through a cold HOCl solution, in first, combines acetylene with one molecule of HOCl to produce 2-chloroethenoI. In the second step, another molecule of HOCl is added to the resulting enol to yield an unstable addition compound that readily eliminates a molecule of water to give dichloroacetaldehyde

6. Addition of water or hydration:

When acetylene gas is passed through a dilute H₂SO₄ solution (20%) containing 1% HgSO₄ at 60 – 80°C, it combines with one molecule of water to form the unstable addition compound vinyl alcohol which rapidly tautomerises to yield stable acetaldehyde.

In case of higher alkynes, this reaction follows Markownikoff’s rule, and ketone is obtained as the major product,

For example: Acetone is obtained from propyne

8. Addition of hydrocyanic acid (HCN):

In the presence of a barium cyanide catalyst, one molecule of hydrogen cyanide combines with acetylene to form the additional compound vinyl cyanide or acrylonitrile.

9. Addition of acetic acid (CH₃COOH): 

When vapours of acetic acid and acetylene are passed over zinc acetate placed on charcoal at 170°C, vinyl acetate is obtained. Polyvinyl acetate, an important plastic, is prepared by the polymerisation of vinyl acetate.

• When acetylene gas is passed through hot acetic acid in the presence of Hg²⁺ catalyst, in the first step one molecule and in the second step, another molecule of acetic acid is added to it to form ethylidene diacetate

• Acetaldehyde and acetic anhydride are obtained when ethylidene diacetate (liquid) is heated at 300 – 400°C.

10. Addition of arsenic trichloride (AsCl₃):

Acetylene combines with arsenic trichloride AsCl₃ in the presence of AlCl₃ or HgCl₂ to give the poisonous gas 2-chlorovinyl dichloramine or lewisite

Addition of CH₃OH:

CH₃OH adds on to acetylene in the presence of potassium methoxide (CH₃OK) to form methyl vinyl ether. This is a nucleophilic addition reaction.

Oxidation reactions

1. Oxidation by potassium permanganate:

When acetylene is treated with dilute alkaline potassium permanganate solution, it is oxidised first into glyoxal and then into oxalic acid (which exists as potassium salt) and the reddish-violet or purple colour of permanganate is discharged.

Ozonolysis:

When a stream of ozonized oxygen is passed through acetylene dissolved in an inert solvent

For example:

Chloroform, carbon tetrachloride etc.), acetylene ozonide is formed. The ozonide on decomposition (hydrolysis) is converted into glyoxal and hydrogen peroxide. H₂O₂ oxidises glyoxal partially into formic acid. Consequently, a mixture of both glyoxal and formic acid is obtained.

If the decomposition of the ozonide is carried out in the presence of zinc dust, H₂O₂ is no longer available (reduced by zinc) to convert glyoxal into formic acid and hence, glyoxal is obtained as the only product.

The acidic nature of acetylene

The H-atom attached to a triply bonded carbon atom (= C —H) is called an acetylenic hydrogen atom. Since the acetylenic carbon atom is sphybridised, its s -character is much higher (50%) and consequently electronegativity of acetylenic carbon is relatively higher.

For this reason, the Csp-H bonding electrons are strongly attracted by the carbon nucleus and the hydrogen atom can be removed as aproton (H⁺), i.e., an acetylenic hydrogen atom can exhibit mild acidic character.

The acidic character of acetylenic hydrogen may also be explained by the fact that the conjugate base (HC = C-) is considerably stable and this is because the unshared electron pair lying in an sp-hybrid orbital remains tightly held by the carbon nucleus. The acetylenic hydrogen atom can, therefore, be replaced by some metal atom to form metal acetylides.

Since the s-character decreases from sp→ sp² → sp³ – hybridised carbon atoms, the acidic character of hydrocarbons follows the order:

HC = CH (Ka = 10^-25) > , CH₂=CH₂ (Ka = 10^-35) >CH₃– CH₃ (Ka = 10^-40)

1. Reaction with sodium:

When acetylene gas is passed over molten sodium at 180°C, both mono and disodium acetylide are produced and H₂ gas is liberated. However,if excess acetylene is used, monosodium acetylide is obtained as the major product.

2. Reaction with sodamide:

Acetylene reacts with sodamide in liquid ammonia to form monosodium acetylide. However, when excess sodamide is used, disodium acetylide is obtained

Sodium acetylide undergoes rapid hydrolysis in the presence of water to give acetylene and NaOH

3. Reaction with lithium amide:

Lithium amide dissolved in liquid NH₃ reacts with acetylene to form lithium acetylide.

4. Reaction with butyllithium:

When n -n-butyllithium reacts with acetylene, lithium acetylide is produced quantitatively.

Reactions of NaNH₂, LiNH₂ and BuLi with acetylene are acid-base reactions in which acetylene acts as an acid and other compounds act as bases. These reactions occur because acetylene is more acidic than NH₃.

5. Reactions with heavy metal cations (Cu⁺, Ag⁺):

Acetylene and the terminal alkynes

For example:

CH₃C = CH, CH₃CH₂C= CH etc.) react with heavy metal cations like Cu⁺ and Ag⁺ to form insoluble salts. These salts, when absolutely dry, are explosive.

When acetylene gas is passed through ammoniacal cuprous chloride solution, a red precipitate of insoluble cuprous acetylide (Gu₂C₂) is obtained. With the help of this reaction, acetylene present even in trace amounts can be detected.

Again, when cuprous acetylide is boiled with aqueous solution of potassium cyanide, pure acetylene is regenerated.

Moreover, if cuprous acetylide is boiled with concentrated hydrochloric acid, acetylene is again obtained but not in pure form

CuC≡CCu + 2HCl →HC ≡ CH ↑ + 2CuCl

When acetylene gas is passed through ammoniacal stiver nitrate solution, an insoluble white precipitate of Ag₂C₂ is obtained.

Silver acetylide is also produced when acetylene is allowed to react with an alcoholic silver nitrate solution.

Again, when silver acetylide is boiled with aqueous solution of potassium cyanide or nitric acid, pure acetylene is regenerated

Propyne also exhibits acidic characteristics similar to that of acetylene.

For example:

Acetylene can be regenerated from the metal derivatives by taking advantage of the weak acidic property of acetylene.

The corresponding reactions are as follows:

1. With the help of the above reactions (formation of cuprous and silver acetylides):

1. Acetylene and terminal alkynes:
   • For example: CH₃C = CH ) can be detected,
2. Terminal and non-terminal:
   • For example CH₃C≡ CH₃ ) alkynes can be distinguished from each other and

3. Acetylene and any terminal alkyne can be separated and purified from a mixture containing alkane, alkene and non-terminal alkyne.

2. Purification o( impure 1-pentyne):

Aqueous solution of silver nitrate is added to a sample of impure 1-pentyne dissolved in 95% ethyl alcohol. As a result of the reaction, a white precipitate of silver 1 -pentoxide is obtained

The white precipitate is filtered, washed with ethanol, and refluxed with an aqueous solution of sodium cyanide when 1-pentyne is regenerated. Pure 1-pentyne is separated from the mixture by distillation

6. Formation of Grignard reagent:

Acetylene reacts with ethyl magnesium bromide in ether to yield the corresponding Grignard reagent. This is an acid-base reaction.

Polymerisation reactions

Acetylene molecules may polymerise to form cyclic or linear polymers.

1. Formation of cyclic trimer:

When acetylene gas is passed through Fe or Cu-tube heated at 600°C, 3 molecules of acetylene combine to form a benzene molecule. Benzene is the cyclic polymer of acetylene. From this reaction, aromatic compounds can be prepared from aliphatic compounds

Formation of cyclic trimers of propyne and 2-butyne:

When acetylene homologues are passed through red hot Fe or Cu tube, they polymerise to produce cyclic polymers.

For example:

1,3,5-trimethyl benzene and hexamethyl benzene can be prepared from propyne and 2-butyne respectively

Hydrocarbons Chapter 13 NCERT Notes

2.  Formation of cyclic tetramer:

When acetylene gas is passed over nickel cyanide catalyst at high pressure, the cyclic tetramer, cyclooctatetraene is obtained.

3. Formation of open-chain polymer:

When acetylene gas is passed through a mixed solution of cuprous chloride and ammonium chloride, the simple linear polymers, vinylacetylene and divinylacetylene, are produced.

In the additional reaction of vinylacetylene and concentrated hydrochloric acid in the presence of cuprous and ammonium chlorides, chloroprene (2-chlorobuta- 1,3-diene) is obtained. Polymerisation of chloroprene leads to the formation of the artificial rubber, neoprene.

Identification And Uses Of Acetylene

1. Proof of unsaturation in acetylene

• Acetylene discharges a reddish-brown color of bromine in carbon tetrachloride without the liberation of hydrogen CH₃ bromide (HBr)

• Acetylene decolorises reddish-violet or purple-colored cold dilute alkaline solution of KMnO₄
• Acetylene reacts with ozone to form the additional compound acetylene ozonide

2. Identification of acetylene

• A red precipitate of cuprous acetylide is obtained when acetylene gas is passed through ammoniacal cuprous chloride solution.
• A white precipitate of silver acetylide is obtained when acetylene is passed through ammoniacal silver nitrate solution.
• Acetylene can be identified by the above-mentioned tests along with the tests for unsaturation.

3. Uses of acetylene

• Acetylene is used for producing oxy-acetylene flame (temperature nearly 3300°C), and used for welding and cutting steel and other metals.
• It is used for producing bright illuminating flame in
  carbide lamp or Hawker’s lamp.
• It is used in the manufacture of acetaldehyde, acetic acid, ethyl alcohol, acetone etc
• It Is also used in the manufacture of industrial noninflammable solvents like acetylene tetrachloride (western, C₂H₂Cl₄) and trichloroethylene (aerosol, Cl₂C=CHCl ), used for dissolution of fats, oils, resins etc.
• It is an important raw material for the large-scale production of vinyl plastics, synthetic rubber such as buna-N and synthetic fibre such as orlon.

Separation of methane, ethylene and acetylene from their mixture: 

1. The mixture of methane (CH₄), ethylene (C₂H₄) and acetylene (C₂H₂) gases is at first passed through ammoniacal cuprous chloride solution. Acetylene is absorbed in ammoniacal cuprous chloride (Cu₂Cl₂) solution with the formation of red precipitate of cuprous acetylide. Methane and ethylene pass out without undergoing any reaction.

2. The red precipitate is filtered, washed with alcohol and then boiled with concentrated HCl or KCN solution when acetylene gas is evolved. It is dried by P₂O₅ and collected.

CCu ≡CCu + 2HCl →HC = CH↑ + Cu₂Cl₂

3. The gas mixture (CH₄, C₂H₄) which escapes is then passed through fuming sulphuric acid. CH₄ comes out without any reaction. It is collected after removing acid vapours by passing through KOH.

4. Ethylene reacts with fuming sulphuric acid to form ethyl hydrogen sulphate. Ethyl hydrogen sulphate thus produced is heated at 170°C when ethylene gas is liberated. It is collected after removing acid vapours bypassing through KOH

H₂C=CH₂ + H₂SO₄  →CH₃CH₂OSO₃H

Comparison among methane, ethylene and acetylene:

Alkadienes

Unsaturated hydrocarbons containing two carbon-carbon double bonds (C=C) are called alkadienes. An alkadiene molecule contains four H-atoms less than the corresponding alkane. Therefore, general molecular formula for an alkadiene isCnH2n2 (n = 3, 4–). These are isomeric with alkynes

1. Classification of alkadienes

Depending on the relative positions of the two double bonds,

Alkadienes are classified into three types:

1. Isolated dienes:

Dienes in which two double bonds are separated from each other by more than one single bond are called isolated dienes

2. Conjugated dienes:

Dienes in which the two double bonds are separated by one single bond are called conjugated dienes i.e., there are alternate single and double bonds in the compound.

3. Cumulated dienes:

Dienes in which two double bonds are adjacent to each other, i.e., two double bonds are attached to the same carbon atom are called cumulated dienes. These are also called allenes

The cumulated dienes or allenes have the general structural formula molecules and are always optically active.

2. Relative stabilities of dienes

The heat of hydrogenation of a conjugated diene is less than that of an isolated diene having the same molecular formula. Therefore, a conjugated diene is relatively thermodynamically more stable than the corresponding isolated diene

Reasons for greater stability of conjugated diene:

There are two reasons for additional stability of conjugated dienes as compared to isolated dienes. These are as follows:

1. Carbon-carbon single bond between two double bonds in a conjugated diene is derived from the overlap of two sp² – orbitals of carbon, i.e., it is sp²-sp² single bond. This is a shorter and stronger bond than one formed by sp³-sp² overlap. As a conjugated diene has one stronger single bond than an isolated diene, the conjugated diene is more stable

2. Second factor that causes a conjugated diene to be more CH₂—CH=CH-CH₂stable than an isolated diene is resonance, which means diene has delocalised electrons, n -electrons in each of the double bonds in an isolated diene are localised between 2 carbon atoms.

But in a conjugated diene, the 2p-orbitals on the 4 carbons are in parallel alignment which is necessary for overlap; the 4 electrons are delocalised over 4 carbon atoms. Due to such electron delocalisation, a conjugated diene is relatively more stable than an isolated diene

The resonance hybrid shows that in 1,3-butadiene, the single bond flanked by two double bonds is not a pure single bond but has a partial double bond character

3. Electrophilic addition reaction of1,3-butadiene 

1. Reaction with bromine (1:1 molar ratio):

In this case, two types of addition compounds are formed. These are 3,4-dibromo but-l-ene (1,2-addition product) and l,4-dibromo but-2-ene (1, 4-addition product)

Mechanism of the reaction:

Carbocation obtained in the first step is a hybrid of two resonance structures. The positive charge of the carbocation exists partially on two carbon atoms. In the second step, (Br®) (the nucleophile) attacks either of the positive carbon atoms to yield two different dibromo compounds

2.  Reaction with HBr (1:1 molar ratio):

In this reaction also, two addition compounds, 1-bromobut-2-cne and 3-bromo but-1-ene, are formed.

Mechanism to the reaction:

3. Diels-Alder reaction:

When 1,3-butadiene is heated with acetaldehyde, an addition reaction leading to the formation of a six-membered ring occurs due to the formation of two different C—C bonds.

This type of reaction is called the Diels-Alder reaction. It is an example of 1,4-addition. The conjugated diene is referred to as a diene, the compound containing the double is called a dienophile and the product is called an adduct

Preparation of Methane, Ethylene and Acetylene and their Reactions

Preparation of Methane Reactions :

Preparation of Ethylene Reactions:

Preparation of Acetylene Reactions:

Transformation

Synthesis of organic compounds is one of the main objectives of organic chemistry and for this, one compound is to be transformed into another. Soit is necessary to devise a suitable transformed into another. Soit is necessary to devise a suitable These conversions are carried out with the help of different reactions. During writing each reaction, the necessary conditions are mentioned.

In case, if any preparation involves multistep reactions, then the yield of the desired product is increased by minimising the number of steps. However, if the desired product is contaminated with other side products, the separation of which is difficult or the end product further participates in a reaction and gets partially converted into other substance(s) or the yielding any step is very low, then the route of the preparation with more number of steps is selected. The relation between different homologous series and their inter¬conversions are shown by the following flow charts.

Hydrocarbons Class 11 Chemistry Solutions

Conversion of methane and ethylene into different compounds

Conversion of acetylene into different compounds:

Various transformations obtained from the chart:

The distinction between two compounds chemical tests:

Acetylene and ethylene:

1-butyne and 1-butene:

2-butene and 1-butene:

Butane and 1-butene:

1-butyne and 2-butyne:

Problems related to alkanes, alkenes and alkynes and their solutions

1. A gaseous hydrocarbon decolourises bromine i in CCI₄. One molecule of acetone and one molecule of acetaldehyde are obtained as a result of its ozonolysis. Determine the structural formula of the compound and write its IUPAC name. Give all reactions involved
Answer:

1. The given hydrocarbon decolourises the bromine solution. Hence,it is an unsaturated hydrocarbon.
2. The hydrocarbon, on ozonolysis, produces 1 molecule of acetone and1 molecule of acetaldehyde. So, the structure of the unsaturated hydrocarbon may be obtained by writing the formula of the two carbonyl compounds side by side facing formula of the two carbonyl compounds side by side facing formula of the two carbonyl compounds side by side facing formula of the two carbonyl compounds side by side facing hydrocarbon is 2-methyl but-2-ene

The reactions are as follows:

2. One molecule of an olefinic compound, on ozonolysis, produces one molecule of acetone, one molecule of glyoxal and one molecule of formaldehyde. Identify the compound and write its IUPAC name
Answer:

The structure of the olefinic hydrocarbon may be obtained by arranging the carbonyl compounds obtained on ozonolysis properly (each of the two oxygen atoms of acetone and formaldehyde placed in front of the 2 oxygen atoms of glyoxal) followed by eliminating the four oxygen atoms and joining the carbonyl carbons by double bonds. Thus, the IUPAC name ofthe starting olefinic compound is 4-methylpenta-1,3-diene

3. An organic compound having the molecular formula C₅H₁₀O₃, on hydrolysis in the presence of Zn, gives acetone and acetaldehyde. Write the structure of the organic compound
Answer:

The molecular formula of the compound is C₅H₁₀O₃ = C_nH_2nO₃ (n = 5)> ie-the compound is an addition compound of an alkene and O₃ The compound reductive hydrolysis, produces acetone and acetaldehyde Hence, the compound must be an alkene ozonide.

The structure of the alkene is obtained by writing the carbonyl compounds side by side facing their carbonyl groups for each other followed by removing the two oxygen atoms and joining the two carbonyl carbons by a double bond. So the compound is 2- methyl but-2-ene

Hence, the starting compound is 2 methylbut – 2 – ene ozoined and its structure is:

4. A gaseous hydrocarbon (A) is converted into another (B) by consuming 2 mol H2 in the presence of Ni catalyst. B can be prepared by the reaction between methyl iodide and metallic sodium in a dry ether medium. Identify the two hydrocarbons (A andB) and mention the reactions involved.
Answer:

In the hydrogenation (H₂/Ni) reaction, one mole of the hydrocarbon (A) accepts two moles of hydrogen. So, the n hydrogenation (H₂/Ni) reaction, one mole of the hydrocarbon (A) accepts two moles of hydrogen. So, the contains two double bonds or one triple bond.

In either medium, the reaction between methyl iodide and metallic sodium yields the hydrocarbon (JB) . This is the Wurtz reaction and in this reaction, ethane (CH₃CH₃) is formed,

Therefore, the hydrocarbon (B) is ethane. Since the hydrocarbon (A) gives ethane by absorbing 2 moles of H₂, it must contain a triple bond (two double bonds cannot be formed between two carbon atoms) and obviously, it is acetylene (H—C = C—H)

5. An alkene (A) having formula C₄H₈, on ozonolysis, gives propanal & methanal. A reacts with HBr to produce a compound of molecular formula, C₄H₉Br. This compound, when heated with alcoholic KOH, produces another alkene (B) which is isomeric with A. Identify the alkenes (A) and (B).
Answer:

The products of ozonolysis of the alkene (A) are propane (CH₃CH₂CHO) and methanal (HCHO). Hence, the alkene(A) is 1-butene (CH₃CH₂CH=CH₂)

1-butene, being an unsymmetrical alkene reacts with HBr according to Markownikoff’s rule to give 2-bromobutane. Its molecular formula is C₄H₉Br.

CH₃CH₂CH=CH₂+ HBr → CH₃CH₂CHBrCH₃ (2- Bromobutane)

When 2-bromobutane is heated with alcoholic KOH solution, it undergoes dehydrobromination and according to Reaction: Saytzeff’s rule, 2-butene (an isomer of 1-butene) is obtained as the major product. Therefore, the alkene (B) is 2-butene (CH₃CH=CHCH₃).

6. A compound (A) having molecular formula C₄H₉Cl, on heating with alcoholic KOH solution, gives two isomeric alkenes (B) & (C). The mixture of (B) & (C), on ozonolysis, produces three compounds:

1. HCHO
2. CH₃CHO
3. CH₃CH₂CHO.

Ascertain the structures of(A), (B) and (C).
Answer:

The compound (A), when heated with alcoholic KOH solution, undergoes dehydrochlorination to give two isomeric alkenes (B) and (C). So, the molecular formula of these two alkenes is C₄H₈

A mixture of (B) and (C), on ozonolysis, produces three carbonyl compounds (HCHO, CH₃CHO & CH₃CH₂CHO). Now, the total number of carbon atoms of the compound of compounds obtained by ozonolysis must be equal to the number of carbon atoms present in the compound undergoing ozonolysis. So, letus suppose that as a result of ozonolysis of the alkene B, 1 molecule of CH₃CH₂CHO and 1 molecule of HCHO (totalnumber of carbon atoms of the two compounds = 4) are obtained and the ozonolysis of( C) produces 2 molecules of CH₃CHO (total number of carbon atoms of the two molecules = 4).

So, the structure of the alkene B is CH₃CH₂CH=CH₂ (1-butene) & the structure of alkene C is CH₃CH=CHCH₃ (2-butene)

CH₃CH₂CH= O + O = CH₂  ⇒ CH₃CH₂CH=  CH₂

CH₃CH= O +O = CHCH₃  ⇒ CH₃CHCH = CHCH₃

Since the compound (A), on dehydrochlorination produces 1 – butene and 2 – butene , the structure of (A) is:

CH₃CH₂CH(Cl)CH₃( 2- chlorobutane)

Reaction:

7. A hydrocarbon containing two carbon atoms decolourises bromine water and undergoes hydration by H₂SO₄ in the presence of H₂SO₄ to produce a compound which produces chloroform when heated with solution of bleaching powder. Write the name of the hydrocarbon and mention the reactions in support of your arguments.
Answer:

The hydrocarbon decolourises bromine water. Hence, it is an unsaturated hydrocarbon. Since the hydrocarbon undergoes hydration by H₂SO₄ in the presence of H₂SO₄ it is an alkyne. The compound obtained on hydration of the hydrocarbon reacts with bleaching powder solution to produce chloroform.

This reaction is a haloform reaction and the compounds containing the CH₃CO —group participate in the haloform reaction. Now, acetylene is the only hydrocarbon containing two carbon atoms which reacts with H₂SO₄/HgSO₄ to form acetaldehyde (CH₃CHO) having a CH₃CO —group.

Acetal dehyde, on reacting with bleaching powder, produces chloroform

Therefore, the hydrocarbons acetylene (HC = CH).

8. A sweet-smelling organic liquid (A) consisting of C, H and O boils at 78°C. (A) on heating with a cone. H₂SO₄ liberates a gaseous substance (B). The empirical formula of (B) is CH₂. (JB) decolourises bromine water and alkaline KMnO₄ solution. Again, each mole of(B) absorbs one mole of H₂ at high temperature in the presence of Ni catalyst. Identify A and B.
Answer:

Since the gaseous substance (B) decolourises bromine KMnO₄ water and alkaline KMnO₄ solution and consumes one mole of H₂ permolein catalytic hydrogenation, it must be alkene.

Since the empirical formula of (B) is CH₂, it may be the gaseous alkene ethylene, (CH₂), or C₂H₄. Ethylene may be obtained when the sweet-smelling organic liquid ethyl alcohol (CH₃CH₂OH) hating a boiling point 78°C is dehydrated by heating with concentrated H₂SO₄. Therefore, the organic liquid (A) is ethyl alcohol

Hydrocarbons Class 11 Chemistry Solutions

9. Ahydrocarbon having the molecular formula C₆H₁₀, on catalytic hydrogenation, absorbs one mole of H₂. The product obtained on ozonolysis of the compound is OHCCH₂CH₂CH₂CH₂CHO. Determine the structure of the compound
Answer:

As the compound absorbs one mole of hydrogen in the presence of a catalyst, the compound is an alkene. On ozonolysis, it gives a dicarbonyl compound. So, the alkene is cyclic.

The structure of the alkene is obtained by writing the 2 O-atoms of hexanedial (OHCCH₂CH₂CH₂CH₂CHO) face to face followed by removing the two oxygen atoms and joining the two carbonyl carbons by a double bond. Hence, the hydrocarbon (C₆H₁₀) is cyclohexene

10. A hydrocarbon (A) (molecular formula C₅H₁₀), on catalytic hydrogenation, produces 2- methylbutane. The compound (A) combines with HBr according to Markownikoff’s rule to form the compound (B) which reacts with silver hydroxide to produce an alcohol (C) having the molecular formula C₅H₁₂O. On oxidation, the alcohol (C) yields a ketone (D). Identify A,B,C, and D and give the reactions involved
Answer:

The given changes are as follows:

The molecular formula, C₅H₁₀ agrees with the general formula of alkenes (C_nH_2n) and on catalytic hydrogenation, (A) produces the saturated hydrocarbon, 2- methylbutane. Therefore, the compound (A) is an alkene. Now, the alcohol (C), on oxidation, produces the ketone (D). So, the alcohol must be a secondary (2°) alcohol. Again, this alcoholis obtained the reaction of (B) with AgOH.

So, (B) is a secondary bromide.

Now, (B) is produced by the reaction of (A) with HBr according to Markownikoff’s rule. So, the structure of(A) is

Reactions:

11. A hydrocarbon (X) discharges the colour of cold (reddish violet) HgS04 alkaline KMnO₄ solution and reacts with warm dilute sulphuric acid containing HgSO₄ to form another compound (Y). (Y) gives a positive iodoform test but does not react with Tollens’ reagent. (Y), when distilled with 80% H₂SO₄, gives sym-trimethylbenzene. Identify the compounds (X) and (Y) and write the reactions involved with proper reasons.
Answer:

The compound (X) discharges the colour of cold dilute 80% H₂SO₄ alkaline KMnO₄ solution. So it may be an unsaturated hydrocarbon. Since (X) reacts with dilute H₂SO₄ to yield a carbonyl compound ( Y) which gives a positive iodoform test but does not react with Tollens’ reagent, the carbonyl compound must be a ketone containing a CH₃CO— group.

And the unsaturated hydrocarbon (X) is a terminal alkyne (R- C = CH) which, on hydration, produces a methyl ketone (RCOCHg). Now, the alkyne, when distilled with 80% H₂SO₄, gives jym-trimethylbenzene. Hence, the terminal alkyne (X) is propyne. Naturally, the ketone (Y) is acetone, CH₃COCH₃

Reactions:

CBSE Class 11 Chemistry Notes For Chapter 13 Hydrocarbons Aromatic Hydrocarbons

The Greek word aroma means fragrance and so the term ‘aromatic compounds’ was originally applied to various fragrant organic compounds. Chemical analysis of most of the natural fragrant organic compounds has shown that they are made of one or more benzene rings containing six carbon atoms and the carbon content in them is higher than the corresponding aliphatic compounds. Moreover, these aromatic compounds can be converted into benzene and benzene derivatives by chemical reactions. Similarly, different aromatic compounds can be synthesised from benzene. Thus, benzene is called the parent hydrocarbon of aromatic compounds and all aromatic compounds are considered as benzene derivatives. Hence, it may be simply said that the aromatic compounds are benzene and benzene derivatives.

Aromatic compounds containing benzene ring are also called benzenoids. Later on, some polycyclic compounds like naphthalene, anthracene, etc., and some heterocyclic compounds like pyrrole, furan, pyridine etc., are also considered as aromatic compounds. Again, some 3, 4, 5 or 7-membered cyclic polyenes (cations, anions or neutral compounds) are also included in the aromatic group. These aromatic compounds are called non-benzenoids. Therefore, the presence of benzene ring in an aromatic compound is not essential. All aromatic compounds do not necessarily possess sweet smell. Many aromatic compounds are odourless while some have bad odour. There is also a large number of non-aromatic compounds having characteristic sweet smell. Hence, the concept that all sweet-smelling organic compounds are aromatic compounds and all aromatic compounds must possess sweet smell is baseless.

Arenes

Aromatic hydrocarbons containing one or more benzene rings are called arenes. If arenes contain more than one benzene ring, then the rings may remain fused or isolated.

Some examples of arenes containing only one benzene ring are as follows:

Aromatic hydrocarbons containing fused rings

Two benzene rings are said to be fused when they remain attached through a common bond. For example, in naphthalene molecule, the two benzene rings A and B are attached through the common bond C₉ — C₁₀. So, this is a fused bicyclic arene. Similarly, anthracene and phenanthrene are two examples of used tricyclic arenes. In these compounds, the benzene rings A and B are attached with each other through a common bond and the benzene rings B and C are attached with each other through a common bond

Aromatic hydrocarbons containing fused rings are also called polynuclear aromatic hydrocarbons. Their general formula is C_nH_n2n-6m, where n = the number of carbon atoms and m = the number of rings. For the bicyclic arene naphthalene, n = 10 and m = 2. For tricyclic arenes such as anthracene and phenanthrene, n = 14 and m = 3

Arenes containing isolated rings:

These are also polynuclear hydrocarbons

Structure Of Benzene Molecule

1. The molecular formula of benzene is C₆H₆ , but the molecular formula of the corresponding open-chain saturated hydrocarbon (alkane) is C₆H₁₄. It means that, benzene has eight hydrogen atoms less than the corresponding saturated hydrocarbon. Therefore, benzene is expected to be a highly unsaturated compound which will easilyform addition compounds.

2. It has been observed in practice that benzene undergoes addition reactions only under drastic conditions

These reactions suggest that benzene is an unsaturated compound containing 3 double bonds. However, from the conditions of these reactions, it is clear that benzene does not contain a high degree of unsaturation

3. In spite the presence of three double bonds, benzene does not discharge the reddish-violet colour of cold alkaline KMnO₄ solution, does not decolourise bromine in carbon tetrachloride solution and does not react with halogen acids.

Therefore, the type of unsaturation present in benzene is quite different from that of aliphatic unsaturated compounds.

4. Like saturated compounds, benzene undergoes substitution reactions

For example: Chlorination, nitration, and sulphonation) easily. In these reactions, one or more H atoms of benzene are substituted by different atoms or groups.

For example:

So, it can be said that in spite of the presence of three double bonds, benzene behaves mostly like a saturated compound.

1. Kekule structure of benzene

The first acceptable ring structure for benzene was proposed by Friedrich August Kekule (1865). He proposed that the six carbon atoms of benzene molecule are joined to each other to form a ring resembling a regular hexagon, with each carbon atom carrying one H-atom. In order to satisfy the fourth valency of each carbon atom, he proposed the presence of three double bonds at the alternate positions in the ring Presence of three double bonds and the equivalency of six supported by this structure of benzene.

Drawbacks of Kekule Structure:

There are two drawbacks of the Kekule structure of benzene and these are as follows:

1. In spite of the presence ofthree double bonds, benzene is a very stable molecule and behaves like a saturated compound.

2. Two disubstituted compounds (I & II) should result from benzene when H-atoms attached to two adjacent carbon atoms in a benzene molecule are substituted bytwo similar or different groups. The reason is that in one of the isomer, the bond between the substituted C-atoms is a single bond while in the other isomer, it is a double bond. However, in fact, only one 1,2-disubstituted compound is obtained.

To account for the non-existence of two types of ortho-disubstituted benzene derivatives, Kekule slightly modified his proposed structure for benzene. He proposed a dynamic equilibrium between two structures (III & IV)

Positions of carbon-carbon double and single bonds in benzene are not static but oscillate back and forth between adjacent positions. That means each C— C pair has a single bond for half of the time and a double bond for the other half. In other words, each molecule spends half of its time in (III) and the other half in (IV)

This new proposition of Kekule was known as the Oscillation theory. According to this theory, the two ort/io-disubstituted benzenes (1 and II) are identical, i.e., benzene will form only one type of ortho -disubstituted compound.

2. Valence bond or resonance theory regarding the structural formula of benzene

The relative stability of tire double bonds and the unusual behaviour of benzene as a whole have been explained with the help of valence bond theory.

According to this theory— 

•  Neither of the two Kekule structures (1 & 2) represents the actual structure of benzene. None of these structures of benzene can individually give the exact identity of the benzene molecule. They do not have any real existence.
• The real structure of benzene is a resonance hybrid of these two structures. Since the two structures, I and II are equivalent and equally stable, they contribute equally to the resonance hybrid, Le., the contribution of each of these resonance or canonical structures is 50%.
• Consequently, the resonance hybrid becomes highly stable. Due to resonance stability, the chemical reactivity of benzene due to unsaturation decreases and stability increases. The resonance energy of benzene is 36 kcal-mol^-1 (calculated value)

Previously benzene was considered as a resonance hybrid of two Kekule structures (1 & 2) and three Dewar structures (3, 4 & 5). However, benzene having a Dewar structure has been prepared later on in the laboratory. So structures 3, 4 and 5 are excluded from the resonance hybrid of benzene. It is to be noted that structures involved in resonance has no real existence.

Explanation of the abnormal behaviour of benzene, by resonance:

1. The three double bonds of benzene are not active in forming addition compounds like the olefmic double bonds because their participation in addition reaction causes loss of resonance stability of benzene. However, in substitution reaction there is no net effect on the hybrid structure of benzene. So, benzene prefers to participate in substitution reaction than in addition reactions.

2. Due to the hybrid structure, all the carbon-carbon bonds In benzene arc equivalent. So, there is no difference between two apparently different disubstituted ortho- isomers— 1,2 and 1,6. Two H-atoms of benzene, on being replaced by the same or different atoms or groups, can form three isomers (ortho, meta & para).

3. Since benzene exists as a resonance hybrid, there is no real existence of carbon-carbon double bonds (C=C) or carbon-carbon single bonds (C —C) in the molecule. All the carbon-carbon bonds are equivalent. It has been observed experimentally that in a benzene molecule all the carbon-carbon bonds are equal in length and its value (1.39A) is intermediate between carbon-carbon single bond length (1.54 Å) and carbon-carbon double bond length (1.34 Å).

4. Orbital structure of benzene

1. Each carbon atom in benzene molecule is sp2 – hybridised, i.e., to form  -bond, each carbon atom uses three sp2 -hybrid orbitals.

2. Out of these three hybrid orbitals, one orbital forms a C—H cr-bond by axial overlapping with ls-orbital of hydrogen atom and the other two overlap axially with two sp2 -hybrid orbitals of two adjacent carbon atoms to form two C —C tr -bonds. So, in the benzene molecule, there are six carbon-carbon (sp2 – sp2) and six carbon-hydrogen (sp² -s) cr -bonds.

3. Since all the carbon atoms in benzene forming the ring system are sp2 -hybridised, the benzene molecule has a planar regular hexagonal structure.

Calculation of resonance energy:

The resonance energy of benzene can be calculated from heat of hydrogenation data. When one mole of an unsaturated compound is hydrogenated, the amount of heat liberated is called heat of hydrogenation. The resonance energy of benzene can be calculated by the process as follows

Heat of hydrogenation involving one double bond,

ΔH = -28.6 kcal-mol^-1

The calculated heat of hydrogenation involving three double bonds is given by, All = -28.6 ×3 = -85.8 kcal-mol^-1

Experimentally observed heat of hydrogenation of benzene is given by, ΔH = -49.8 kcal-mol^-1

∴ Resonance  energy

Observed heat of hydrogenation – calculated heat of hydrogenation

= -49.8(85.8- 49.8)(-85.8)kcal-mol-1kcal-mol^-1

= 36.0 kcal-mol^-1

4. One carbon atom resides at each corner of the hexagon.

Each C —C — C and C — C — H bond angle is 120°.

5. Each carbon atom contains one unhybridised 2p_z -orbital. Each 2p_z -orbital contains one electron and the six 2p_z orbitals are perpendicular to the plane of the hexagon, i.e., they are parallel to each other

6. Each p_z -orbital can overlap laterally with either of the adjacent p_z -orbitals. So, overlap can occur in two different ways as shown below. As the extent of overlap of each p_z -orbital on both sides is equal, ;r -electrons remain no longer localised, but undergo delocalisation.

These six n -n-electrons form two continuous rings of n -electron clouds—one lying above and the other below the plane of the hexagonal ring. This ring-like electron cloud formed by six 2p_z -orbitals is called aromatic sextet The benzene molecule acquires its stability due to this delocalisation of electrons coupled with the presence of an aromatic sextet

X-ray and different spectral analysis (UV) IR, NMR and Mass spectroscopy) have offered a complete idea about the structure of benzene and now it is known that —

• Benzene is a planar cyclic compound consisting of six carbon atoms
• Each of its carbon-carbon bonds are equal in length and its value is 1.39A
• All the six H-atoms are equivalent and each C— H bond length is 1.09A and
• All H— C — C and C— C—C bond angles are equal and the value is 120°

Representation of benzene molecule

The hexagonal structure of benzene is known as benzene ring. Benzene ring is generally represented by any one of the two Kekule structures. Benzene ring is also represented by drawing a circle inside a regular hexagon. While representing benzene molecule, C & H-atoms are not generally written

Aromatic Character Or Aromaticity

Aromatic character or aromaticity is the collective representation of the characteristic properties of aromatic compounds, which are responsible for the different behaviour of such compounds from their aliphatic or alicyclic analogues.

These are as follows:

1. All aromatic compounds are relatively more stable than the system of the corresponding aliphatic compounds of similar molecular formula. They are resistant to oxidation. Low heat of combustion and hydrogenation gives an idea about their unusual stability.

2. Although aromatic compounds contain several double bonds, yet they do not easily participate in addition reactions like aliphatic unsaturated compounds. However, aromatic compounds undergo substitution reactions easily. Therefore, the nature of unsaturation associated with aromatic compounds is different from that in aliphatic compounds.

3. Aromatic rings containing — OH group exhibit acidic property

For example:  Phenol, but aliphatic compounds containing — OH group possess alcoholic properties and alcohols are neutral in nature

4. Aromatic rings containing — NH₂2 group exhibit weak basic properties

For example: Aniline), but aliphatic compounds containing the — NH₂ group are relatively stronger bases

5. In the molecules of aromatic compounds, planar or almost planar rings composed of carbon atoms (in some cases, formed by C, N, O etc., atoms) are present. Such rings are called aromatic rings. Generally, the cyclic compounds which have conjugated systems of double bonds and have very high resonance stability are called aromatic compounds. However, all conjugated monocyclic polyenes are not highly stable.

For example:

Although benzene is extremely stable, cyclobutadiene and cyclooctatetraene are not similarly stable. Therefore, it is found that the special stability of aromatic compounds is not only due to π -electron delocalisation but also due to the presence of a definite number of it -electrons. Huckel’s rule gives a clear idea about this matter

1. Huckel’s rule for aromaticity or (4n+2) rule

According to German scientist Huckel, monocyclic planar conjugated polyene systems (cation, anion or neutral species) containing (4n + 2) delocalised it -electrons [n = 0, 1, 2, 3,…) exhibit aromatic properties. Therefore, monocyclic conjugated polyene systems containing 2(n = 0), 6(n = 1), 10 (n = 2), 14 (n = 3), ••• etc., delocalised 7T -electrons possess aromatic character and they are unusually stable

From Huckel’s rule, it is evident that the following conditions must be fulfilled by a compound to be aromatic:

• The molecule must have planar ringsystem.
• Each atom involved in the formation of ring system must have an unhybridised p-orbital.
• These p -orbitals must be parallel and undergo continuous overlap to cause the delocalisation of it -electrons.
• Each p -orbital may contain 1 electron, 2 electrons or even no electron.
• The delocalised it -electron system must contain 2, 6, 10, 14… etc., electrons.

It must be remembered that due to such it -electron delocalisation, the electronic energy of an aromatic compound decreases and hence, stability increases. Aromatic compounds are more stable than their open-chain analogues.

The general formula of monocyclic conjugated polyenes is C_nH_n (where n = 4, 6, 8, 10, etc.). The general formulas of cationic & anionic polyenes are C_nH⁺ & C_nH_n^– (where ” ~ 3’5, 7’ etc….)

CBSE Class 11 Chemistry Chapter 13 Hydrocarbons Summary

Classification Of Aromatic Compounds

The compounds that exhibit aromatic properties are two Types.

These are:

1. Benzenoid aromatic compounds and
2. Non-benzenoid aromatic compounds.

1. Benzenoid aromatic compounds:

Aromatic compounds containing one ormore benzene rings are called benzenoid aromatic compounds or simply benzenoids.

2. Non-benzenoid aromatic compounds

Some compounds, where n = 3] in (Antiaromatic)Less stable c spite of having no benzene ring in their molecules, are able to display aromatic properties. These are called nonbenzenoid aromatic compounds or simply non-benzenoids

1. Antiaromatic compounds

If a compound contains 4nπ -electrons (where n – 1, 2, 3, … etc.) in its planar ring system with a continuous overlap of p-orbitals, it becomes an antiaromatic compound.

Antiaromatic compounds Definition:

Monocyclic planar conjugated polyene systems ions) containing delocalised π -electrons (n = 1, are called antiaromatic compounds.

Therefore, the compounds having 4(n = 1), 8(n = 2), 12(/i = 3)… etc., delocalised π -electrons in their planar ring m system behave as antiaromatic compounds

It should be remembered that due to the delocalisation of π -electrons, electronic energy of antiaromatic compounds increases and consequently stability decreases. Antiaromatic compounds are less stable than their open-chain analogues.

2. Non-aromatic compounds

Non-aromatic compounds Definition:

Cyclic compounds which do notpossess delocalised system of π -electrons, i.e., compounds which are neither aromatic nor antiaromatic, are called non-aromatic compound

The stabilities of non-aromatic compounds are similar to that of their open-chain analogues.

Isomerism Of Benzene Derivatives

When one or more H-atoms of benzene are substituted by any other atom or group, the product obtained is known as benzene derivative. All the six H-atoms of benzene are equivalent. So, if one H-atom of benzene is replaced by a monovalent atom or group, only 1 monosubstituted benzene derivative is obtained.

Monosubstituted benzene has no isomer, i.e., it exists in one form only.

For example: Chlorobenzene (C₆H₅Cl) or nitrobenzene (C₆H₅NO₂) exists in one form only

1. When two H-atoms in a benzene molecule are substituted by two atoms or groups (same or different), depending on the relative position of the two substituents, three positional isomers are possible.

For example: Three isomers each of dibromobenzene (C₆H₄Br₂) and nitrotoluene (CH₃C₆H₄NO₂) are known

Isomeric dibromobenzenes:

Isomeric Nitrotoluenes:

2. In the case of trisubstituted benzene derivatives, the number of isomers depends on the nature of the substituents. O If the three substituents are identical then three isomers are possible.

For example: Trinitrobenzene [C₆H₃(NO₂)₃], tribromobenzene (C₆H₃Br₃), etc., are known to exist in three isomeric forms. When two substitutents are identical and one is different, then six isomers are possible.

For example: Dibromophenol (Br₂C₆H₃OH), dichlorobenzoic acid (Cl₂C₆H₃COOH) etc., are found to exist in sixisomeric forms.

When all the three substituents are different, ten isomers are possible.

For example – Bromochlorotoluene (CH₃CgH₃ClBr) , bromochlorobenzoic acid (BrClC₆H₃COOH) etc., are found to exist in ten isomeric forms

Isomeric Tribromobenzenes:

Nomenclature Of Benzene Derivatives

1. Monosubstituted benzene derivatives

1. In the IUPAC system, some monosubstituted benzene derivatives are named by their trivial names which may have no resemblance to the name ofthe substituent

2. For many of these derivatives, the name of the substituent is simply added to the word ‘benzene’, as a prefix leaving no gap between the name of the substituent and the word, benzene.

3. In some cases, the names of the substituents are written as suffixes after the word benzene.

4. In some cases, the phenyl group, originated from the benzene molecule, is considered as the substituent to write the IUPAC names.

2. Disubstituted benzene derivatives

In disubstituted benzene, when two substituents are attached to adjacent carbon atoms, the compound is called ortho-isomer, if the substituents are on the alternate carbon atoms, then the compound is called meta-isomer and if the substituents are attached to diagonally opposite carbon atoms, the compound is called para-isomer, ortho, meta and para- are abbreviated as o-, m- and p-respectively.

Nomenclature according to nature & position of substituent:

When the two substituents are identical, then their relative positions are indicated by the prefix ortho, meta or para followed by the word to denote two substituents. Then the name of the substituent along with the word ‘benzene’ are written. In that case, 1,2- disubstituted benzene is called ortho, 1,3-disubstituted benzene is called meta-and 1,4-disubstituted benzene is called para.

Alternatively, relative positions ofthe substituents are also indicated by using numbers. In that case, any one of the carbon atoms having a substituent attached to it is marked as number ‘1’ carbon atom. The other carbon atoms of the benzene ring are numbered consecutively (clockwise or anticlockwise) in such a way as to give lowest number to the carbon atom carrying the second substituent

3. If the substituents are different, the ring containing a substituent is considered as parent, and using its name as the root name, the position of the second substituent is indicated by ortho-, meta- or para- or alternatively by 2, 3 or 4.

The name of the second substituent, in this case, comes before the root name. In the given compound, the benzene ring containing —OH group (out of —OH and — NO₂ ) i.e., phenol, is taken as the parent compound. Now, since the substituent — NO₂ exists in para-position with respect to —OH, so the compound is named as p-nitrophenol. Alternatively, the carbon atom to which the —OH group is attached is taken as number-1 carbon atom, and then the other carbon atoms of the ring are marked with numbers in succession in a clockwise or anticlockwise manner in such a way that the carbon atom containing the second substituent — NO₂ is assigned the lowest possible number. Now, the substituent — NO₂ is attached to the number-4 carbon atom. Hence, the name ofthe compound is 4-nitrophenol.

Order of priority of groups for determining root name:

—COOH > —SO₃H > —CONH₂ > — CN > —CHO > NC=O> -CH₂OH > -CH₃ > -OH > -NH₂ > — NO₂> Halogens (F, Cl, Br, I)

If the substituents are two different halogen atoms, then theyare mentioned alphabeticallyfollowing the English spelling of their names. The relative positions of the substituents are indicated bynumbers orwith the prefix ortho-, meta- or para-. Substituent whose name comes first (alphabetically) is considered to be attached to carbon number-1, and the other carbon atoms are so numbered as to give the lowest possible number to the carbon carrying the second substituent

Examples:

In naming the first compound given below, at first bromo and then chloro have been written and this is because alphabetically the initial letter ‘b’ ofbromo comes before the initial letter ‘d of chloro. The carbon to which bromine is attached has been marked with number-1 and the other carbon atoms have been numbered serially in the clockwise direction because, as a result ofsuch method of marking the carbon atom containing Cl gets the lowest possible number.

3. Tri- or polysubstituted benzene derivatives

1. When more than two (same or different) substituents are present in the benzene ring, their relative positions are indicated by numbers. Carbon atom containing principal substituent (according to relative preference order is othercarbon atoms are denoted by serial numbers 2, 3. 4, 5. 6. The name of the benzene ring containing principal substituent is takenas the root name

2. Except die principal substituent other substituents are expressed by position numbers and this is done insuch a way that the first difference between the possible position numbers assumes the lowest value. Their positions in die benzene ring are indicated by writing these numbers before the names ofthe substituents.

3. The substituents are mentioned according to the alphabetical order of the initial letters in the English spelling of their names and finally the word benzene or the root name of benzene derivative is added to the substituent concerned.

For example: In the compound shown above, principal substituent is —CH₃. So, correct name of the compound is 6-bromo-3-iodo-2-nitrotoluene.

4. 2-bromo-5-iodo-6-nitrotoluene is not correct nomenclature because in the first nomenclature, the numbers assigned to the carbon atoms containing substituents are 1, 2, 3 and 6 while in the second nomenclature, numbers assigned to the carbon atoms earning substituents are 1,2, 5 and 6. So, in the first nomenclature, the third number is ‘3’ while in the second nomenclature, it is ‘5’. Since ’3′ is less than‘5’ the first nomenclature is correct.

Common and IUPAC names of some typical compounds:

Homologous Series Of Aromatic Compounds

Like aliphatic compounds, depending on the nature of the functional group present, the aromatic compounds are also classified into different homologous series.

1. Aromatic hydrocarbons

Hydrocarbons derived by replacement of one or more H-atoms of the benzene ring by hydrocarbon substituents such as alkyl, alkenyl, alkynyl or aryl groups are called aromatic hydrocarbons or arenes. They can be divided into two categories

1. Arenes containing one benzene ring

2. Arenes containing more than one benzene ring are called polynuclear hydrocarbons

2. Aromatic halogen compounds

Compounds obtained by replacing one or more H-atoms of benzene ring by halogen atoms are called aromatic halogen compounds or aryl halides

3. Aromatic nitro compounds

Compounds obtained by replacing one or more H-atoms of benzene ring by -NO₂ group are called aromatic nitro compounds.

4. Aromatic amino compounds

Compounds obtained by replacing one or more H-atoms of benzene ring by — NH₂ group or substituted amino group ( — NHR, — NR₂) are called aromatic amino compounds

5. Aromatic hydroxy compounds

Aromatic hydroxy compounds are of two types—

1. The compounds in which the hydroxyl (—OH) group is attached directly to the benzene ring are called phenols

2. When the —OH group, instead of being directly linked to the benzene ring, is attached to the side chain, then the compounds are called aromatic alcohols

It is better not to consider aromatic alcohols as aromatic hydroxy compounds because their properties are similar to those of aliphatic alcohols. So, they are usually considered as aryl derivatives ofaliphatic alcohols

6. Aromatic carbonyl compounds

1. Aromatic aldehydes: The compounds in which a —CHO group is directly linked to the benzene ring are known as aromatic aldehydes

2. Aromatic ketones: When an aryl group and an alkyl group aryl groups are joined to a carbonyl group (>C=O), the compounds so obtained are called aromatic ketones

7. Aromatic carboxylic acids:

Compounds containing  (one carboxyl (—COOH) group directly attached to the aromatic ring are called aromatic carboxylic acids.

1. Aryl groups: The groups obtained by expulsion of one H -atom from arenes are called aryl (Ar — ) groups,

For example:

2. Aralkyl groups: The groups obtained by expulsion of one or more H-atoms from the side chain of arenes are called aralkyl groups,

For example: The groups that may derived from toluene are:

Nucleus And Side Chain Of Aromatic Compounds

An aromatic compound consists of two parts

1. Nucleus and
2. Side chain.

1. Nucleus: The benzene ring present in an aromatic compound is called the nucleus.

2. Side chain: If one or more hydrogen atoms of benzene are substituted by alkyl groups, different types of alkyl benzenes are obtained. These alkyl groups are known as side chains.

For example:

The benzene ring in the toluene molecule is its nucleus and the —CH₃ part is the side chain. The carbon chains directly attached to the benzene ring are called side chains. Both the nucleus and side chain of aromatic compounds take part in substitution reactions.

Oxidation of the side chain

When an aromatic compound is subjected to oxidation, only the side chain is oxidised but the benzene ring or the nucleus, being sufficiently stable, remains intact.

1. Oxidising agents:

Commonly used oxidising agents dilute HNO₃, alkaline KMnO₄ solution, K₂Cr₂O₇ acidified with sulphuric acid etc.

For example:  Toluene, on oxidation by alkaline KMnO₄ and subsequent acidification, yields shining white crystals of benzoic acid.

2. Special feature:

Any side chain (saturated, unsaturated or substituted) containing benzylic hydrogen (i.e., the hydrogen atom attached to the carbon atom directly linked to the benzene ring), on oxidation, becomes converted into a carboxyl ( —COOH) group.

The side chain of test-butylbenzene cannot be oxidised because it does not contain any benzylic hydrogen

Benzene rings containing two side chains give dicarboxylic acid on oxidation

Using mild oxidising agent such as chromyl chloride (CrO₂Cl₂), toluene gives benzaldehyde. This reaction is called the Etard reaction.

Oxidation of the benzene ring or nucleus:

The benzene ring or nucleus of any alkylbenzene can be oxidised to a carboxyl group by ozonolysis (oxidative workup)

CBSE Class 11 Chemistry Chapter 13 Hydrocarbons Summary

Orientation Of Substituents In The Benzene Ring

1. Ortho/para-directing groups

Ortho/para-directing groups Definition:

Substituent groups present in the benzene ring which direct the incoming group to the ortho- and para-positions of the ring are called ortho-/para- directing group

Groups:

—CH₃, —C₂H₅, — C₆H₅, —Cl, —Br, —I, —OH,

-OCH₃, -NH₂, —NHR, —NR₂, —NHCOCH₃

Examples:

In phenol molecule, an —OH group is already present in the benzene ring. So, phenol on nitration produces mainly a mixture of o- and p -p-nitrophenols.

Similarly, chlorination of toluene in which a — CH₃ group is already present in the benzene ring results in the formation of mainly mixture of o – and p -chlorotoluenes.

One characteristic feature of ortho-para- directing groups except alkyl and aryl groups ( —CH₃, —C₂H₅ , —C₆H₆ ) is that the atom, through which these groups are attached to the benzene ring, contains one or more lone pair of electrons. In fact, due to presence of these unshared pairs of electrons, these groups become ortho-/ para- orienting.

Increase in the activity of the benzene ring :

The ortho/ para directing groups (except halogen atoms), by increasing the electrondensity of the benzene ring, activate the ring more concerning unsubstituted benzene in electrophilic substitution reactions. Thus these groups are called activating groups. In fact, due to an increase in the electron density of the ring by the activating group, the electrophile (E) is more easily attracted by the ring to form a covalent bond (o’ -complex) than the unsubstituted benzene ring. As a result, the electrophilic substitution reaction of the aromatic compound containing activating group occurs at a faster rate than benzene’

2. Meta-directing groups

Definition: Substituent groups present in the benzene ring

Groups:

—NO₂, — CN, — CF₃, — CHO, — COR,

— COOH, —COOR, —SO₃ H etc.

Example: In the nitrobenzene molecule, a nitro (— NO₂) group is already present in the benzene ring. So, nitration of nitrobenzene yields mainly m-dinitrobenzene

One characteristic feature of these groups is that the atom which is directly attached to the ring is usually linked to another more electronegative atom by a double or triple bond. In fact, due to the displacement of electrons of the multiple bonds towards the more electronegative atom, these groups become meta-orienting.

Decrease in activity of the benzene ring:

The meta-directing groups, under their electron withdrawing effect, decrease the electron density in the benzene ring and make the ring less reactive towards further electrophilic substitution than unsubstituted benzene.

Thus, these groups are called deactivating groups. As the deactivating group decreases the electron density of the ring, the electrophile (E⁺) is less easily attracted by the ring than the unsubstituted benzene ring and consequently, the formation of σ -complex, rather the electrophilic substitution reaction occurs at a much slower rate compared to benzene

Ortho-/ Para And Meta Directing Groups:

3. Theory of orientation

The position occupied by the incoming group In monosubstituted benzene is determined by the electronic character, i.e., inductive effect, hyperconjugation effect and resonance effect of the substituent already present In the ring.

Orientation in case of phenol (C₆H₅ :O:H)

Due to the participation of an unshared pair of electrons of the —:O:H group In resonance (+R effect), the electron density at ortho-and para-positions of the ring increases. For this reason, the —OH group acts as an ortho-/para- directing group in the second electrophilic substitution

Since oxygen is more electronegative than carbon, —OH group in phenol withdraws electrons from the ring by -1 effect. Again, the group donates electrons to the ring by + R effect.

As + R is a more effective than-I effect, —OH group by net electron release activates ring towards further electrophilic substitution, i.e., it is an activating group.

Orientation in the case of toluene (C₆H₅CH₃):

Carbon atom in —CH₃ group of toluene contains no lone pair of electrons and therefore, — CH₃ group cannot release electrons by + R effect. However, it increases electron density at ortho and para-positions of the ring by hyperconjugation effect. For this reason, the — CH₃ group acts as an ortho-/para directing group in the second electrophilic substitution.

+1 effect of —CH₃ group has also some contribution in playing the role ofortho-/para- directing group. This can be explained in terms of the relative stabilities of different cr complexes. — CH₃ group by its +1 and hyperconjugation electron release activates the ring towards further electrophilic substitution, i.e., it is an activating group

Orientation in case of halobenzene (C₆H₅ X):

Halogens behave abnormally in electrophilic substitution reactions ofhalobenzenes. In spite of being deactivating groups, they are undoubtedly ortho-/para- directing groups. Halogen atoms withdraw electrons from the ring by -I effect and donate electrons by +R effect 1. However, in case of halobenzene, -I > + R.

Thus the net electron displacement occurs from the ring towards the halogen atom. So, the benzene ring, as a whole, becomes deactivated towards further electrophilic substitution. In fact, halogen atoms withdraw electrons from all positions of the ring by -I effect and send electrons to ortho-Zparapositions by +R effect. Thus, electron densities of ortho-/ para- positions are less reduced, i.e., these positions are comparatively electron-rich and naturally these positions are attacked more easily by electrophiles. So, halogens, in spite of being deactivating groups, are eventually o-/pdirecting. In this case, +R effect of halogen governs the orientation.

Orientation in the case of nitrobenzene (C₆H₅NO₂):

As tlie N=O moiety of — NO₂ group in nitrobenzene remains conjugated with the nucleus, the —NO₂ group decreases the electron density in the benzene ring by -R effect. Again, it also reduces the electron density of the ring by-1 effect.

In fact, — NO₂ group decreases the electron density of all positions of the ring by -I effect but reduces the electron density of only the ortho- and para- positions by -R effect. As a result, the electron density at the meta-position becomes relatively higher and naturally, this position becomes more susceptible towards electrophilic attack. Thus, in the second substitution, the —NO₂ group acts as a meto-directing group

In this case, displacement of electrons caused by -R and -I effects occurs from the ring towards the group. Hence, the —NO₂ group is a deactivating group.

Almost in each of the substitution reactions of monosubstituted benzene, a mixture of ortho-, meta- and para- isomer is obtained. For ortho-/para- orienting groups, the ortho- and para-isomers are the chief products, the metaisomer being produced in negligible amounts. However, in I the case °i meta- directing groups, the meta-isomer is the major product and the yields of ortho-and para-isomers are negligible. Again, for ortho/para-directing groups, the ortho/ para- isomers are frequently obtained in different amounts

Substitution Reactions Of Aromatic Compounds

Like aliphatic compounds, aromatic compounds undergo three types of substitution reactions:

1. Electrophilic substitution reactions: In this type of reaction, the benzene ring is attacked by electrophilic reagents or electrophiles in the first step.
2. Nucleophilic substitution reactions: In this type of reaction, the benzene ring is attacked by nucleophilic reagents or nucleophiles in the first step.
3. Free radical substitution reactions: This type of reaction involve the attack of free radicals on the benzene ring.

Cause of participation of aromatic compounds in substitution reactions:

• Aromatic compounds gain extraordinary stabilisation due to the resonance and delocalisation of n –electrons. In the formation of an additional product, the aromaticity of the parent aromatic compound is no longer retained due to loss of conjugation and consequently, the extra stability of the compound is lost.
• For this reason, the tendency of aromatic compounds to undergo addition reactions is much lower. On the other hand, in the substitution product, the’ aromatic stability or aromaticity of the starting organic compound remains intact. For this reason, benzenoid aromatic compounds prefer to undergo substitution rather than addition reaction.
• Aromatic compounds prefer electrophilic substitution rather than nucleophilic substitution: The substitution reactions of aromatic compounds are mainly ionic.
• Due to the presence of π -electron cloud above and below the plane of the system, the aromatic rings serve as a source of electron-rich centre and so welcome any attacking electrophile. On the other hand, the benzene ring being electron-rich repels any approaching nucleophile. For this reason, the tendency of aromatic compounds to undergo electrophilic substitution is much higher than nucleophilic substitution.

Mechanism type of reactions of electrophilic take place substitution in two steps reaction:

First step:

The electrophilic reagent or the electrophile (E+) being attracted by the n -electron cloud of the benzene ring forms a sigma (σ) bond with any one of the ring carbon atoms. As a result, a carbocation is formed and in forming this carbocation, the benzene ring loses its aromatic character. The carbocation, however, is stabilised by resonance. It is a resonance hybrid of three resonance structures 1, 2 and 3.

It is generally represented by a single non-Lewis structure 4. As it Is produced by the formation of a sigma bond, it is called σ -complex. It is also known as cyclohexadienyl cation, benzeniumi on or Wheland intermediate. As this step involves loss of aromatic character of the benzene ring, it is slow and hence it is the rate-determining step of the reaction

Second step:

Although the carbocation produced attains some degree of stability through resonance, yet it is less stable than benzene. For this reason, the σ -complex, by rapid expulsion of a proton from the carbon bonded to the electrophile, reverts to the more stable substituted product. Any base (B”) present in the reaction medium helps in the formation of substituted benzene by accepting a proton. In this step, benzene regains its aromatic character or aromaticity

This type of reaction pathway is called bimolecular electrophilic substitution (aromatic) or S_E2(Ar) pathway

Electrophiles involved in various electrophilic substitution reactions:

Benzene (C₆H₆)

Michael Faraday discovered benzene in 1825. In 1845,

Hof&nann was able to recover benzene from coal tar. 80-110°

1. Fractional distillation of coal tar: Isolation of benzene

When coal is subjected to destructive distillation, coal gas, coal tar (black liquid having high viscosity), ammoniacal liquor and coke are obtained. It is a mixture of about 200 compounds, the majority of which are aromatic compounds. Coal tar consists of compounds which are acidic, alkaline and neutral. Isolation of benzene from coal tar: when coal tar is subjected to fractional distillation, the fraction collected upto 170°Cis called light oil.

Its colour is yellow. Light oil contains mainly neutral hydrocarbons like benzene, toluene, xylene etc. Besides these, it contains small amounts of acidic phenol, basic aniline, pyrrole, pyridine, neutral sulphur-containing compound, thiophene and water.

Preparation of 90% benzol from light oil:

•  At first the light oil is fractionally distilled and the fractions which are distilled out at temperatures above 70°C are collected.
• The distillate is shaken with cold cone. H₂SO₄ when the basicsubstances like aniline and pyridine get converted into their sulphate salts and dissolve in acid. A portion of thiophene also dissolves in the acid after being converted into thiophene sulphonic acid. The mixture forms two separate layers. The acid layer is separated from the organic layer.
• The organic layer is then shaken with 10% NaOH solution when acidic phenol dissolves in an alkali solution forming phenatesalt. Excess H₂SO₄ being neutralised also dissolves in an alkali solution. Two separate layers are formed. The aqueous layer containing caustic soda is then separated from the organic layer.
• The upper organic layer is repeatedly washed with water to remove .excess alkali. The washed fight oil thus obtained is subjected to fractional distillation and the fractions obtained at different range of temperatures are collected

Fractions obtained at different temperatures by fractional distillation of light oil:

Preparation of pure benzene from 90% benzol:

• When 90% benzol is subjected to fractional distillation, the fraction that is collected at 80-82°C is impure benzene. This benzene is contaminated with a small amount of toluene and thiophene.
• When it is cooled in a freezing mixture, benzene is converted into a solid mass at 5.5°C. It is separated from liquid toluene by filtration. When this solid benzene is kept at room temperature, liquid benzene is obtained. Benzene, thus obtained, is not absolutely pure. It contains traces of thiophene (0.05%).
• Test for the presence of thiophene in benzene:o addition of a yellow solution of isatin (dissolved in H2S04 ) to a sample of benzene causes the development of a blue colour, then it indicates that the sample is contaminated with thiophene. This test is very sensitive even to very small amounts of thiophene.
• Alternatively, on shaking a sample of benzene with a cone. H₂SO₄ two layers are formed. If the lower layer assumes a yellow colour, then it indicates that the sample is

Removal of thiophene from benzene:

• The boiling point of benzene is 80.4°C. So, thiophene (b.p. 84°C) cannot be removed completely from benzene by fractional distillation. i] When benzene containing thiophene is shaken with cold and cone. H₂SO₄ thiophene sulphonic acid (a yellow liquid) is formed. It dissolves in sulphuric acid. contaminated with thiophene
• Benzene does not react with cold and cone. H₂SO₄  This process of washing with cone. H₂SO₄ is repeated several times till the layer of H₂SO₄ becomes no longer yellow. It suggests the complete removal of thiophene from the sample of benzene.
• Benzene thus obtained is washed with water thoroughly to make it free from acid. It is then dried with fused CaCl₂ & redistilled when pure benzene is obtained.

1. Preparation of benzene

Industrial preparation:

The chief source of benzene is coal tar. Besides this, benzene is also obtained from mineral petroleum. Manufacture of benzene from coal tar has been discussed earlier

Laboratory preparation: 

1. From acetylene:

When acetylene gas is passed through red hot copper tube (600°C) , three molecules of it combine to form benzene (polymerisation reaction)

2. By decarboxylation of sodium benzoate:

Anhydrous sodium benzoate, on being heated in the presence of soda lime, yields benzene

3. Benzene From phenol:

Phenol, when distilled in the presence of zinc dust or the vapours of phenol, when passed over zinc dust, produces benzene.

4. From diazonium salts:

When a solution of benzene diazonium salt,  C₆H₅N₂Cl  is heated with hypophosphorous acid or dry alcohol, benzene is produced.

5.  From benzenesulphonic acid :

When benzenesulphonic acid is heated to 150-200°C under pressure in the presence of dilute HCl or H₂SO₄ , benzene is obtained (This reaction is used for the removal of
—SO₃H group from benzene ring is called desulphonation)

2. Physical properties of benzene

• Benzene is a colourless liquid with a characteristic smell. Its boiling point is 80.4°C. When cooled by freezing mixture, it forms a crystalline solid.’Hie melting point of crystalline benzene is 5.5°C
• Benzene is lighter than water (specific gravity : 0.B7). It is insoluble in water but dissolves In alcohol, ether and acetone.
• Benzene is a good solvent for oils, fats, rubber, resin, iodine, sulphur, phosphorus etc.
• Benzene is a highly inflammable substance.lt burns with a sooty luminous flame. Due to the high percentage of carbon, elementary carbon is produced during the burning of benzene. Due to this black smoke is formed. The presence of hot
  carbon particles in a flame makes the flame luminous.
• Benzene is a highly toxic substance. At present, it has been identified as a carcinogenic compound.

All aromatic compounds bum with sooty flame but aliphatic compounds do not bum with such sooty flame. r So, aliphatic and aromatic compounds can be distinguished with the help of an ignition test.

Two special precautions: 

Benzene is highly

It is highly injurious to inhale benzene vapours. So, it should never be allowed to vaporise in open air and for this purpose, fume chamber must be used.

3. Chemical properties and reactions of benzene

• Despite the presence of three double bonds, the chemical properties of benzene are quite different from those of olefins.
• Although in some cases benzene takes part in addition reactions
• Its chief and characteristic reactions are substitution reactions because in substitution products, aromaticity of the ring system is preserved.

Substitution Reactions Of Benzene

1. Halogenation of benzene

1. Chlorination:

The reaction in which an H-atom of benzene ring is displaced by a Cl-atom is known as chlorination reaction. When benzene is allowed to react with chlorine at ordinary temperature in the presence of Lewis acid catalysts such as FeCl₃ , AlCl₃ , I₂ etc., (act as halogen carrier), chlorobenzene is obtained.

Iron (catalyst) is most commonly used, being converted to the Lewis acid FeCl₃ by chlorine. In the absence of halogen carrier, such a substitution reaction does not take place

Reaction mechanism:

It is an electrophilic substitution reaction. The electrophile involved in this reaction is a positive chlorine ion (Cl⁺) or the chlorine-iron (3) chloride complex which leads to the formation of that ion. This Cl⁺ ion displaces H⁺ from the ring.

Formation of electrophile:

Fe reacts with Cl₂ to form Lewis acid, FeCl₃. The Lewis acid then forms a complex with excess CI₂. The complex finally dissociates to give Cl⁺ and FeCl⁺ ions:

2Fe + 3Cl₂  → 2FeCl₃ (Lewis acid)

Substitution:

The number of  H-atoms of the ring that will be displaced by chlorine depends on the quantity of chlorine used. For   For example, 1 mole of Cl₂ (in addition to that consumed by Fe to form FeCl₃) leads to the formation of monochloro benzene (C6H5Cl) . When the reaction is continued for a long period using 2 moles of Cl₂ , a mixture of maple ortho- and para- dichlorobenzenes (C₆H₄Cl₂) is obtained. In this case, chlorobenzene is formed first. Chlorine atom of chlorobenzene is ortho-/para-directing. So, the second chlorine atom enters the ortho- or para-position with respect to the first Cl-atom

2. Bromination:

Like Cl₂,Br₂ also reacts with benzene in the presence of iron filings, AlBr₃ or iodine (halogen carrier) to form bromobenzene.

The reaction mechanism of the bromination reaction is similar to the chlorination reaction

3. Iodination:

Iodination of benzene cannot be accomplished like chlorination or bromination because iodine is the least active of the halogens. Iodination may, however, be carried out smoothly in the presence of an oxidising agent like nitric acid, mercuric oxide etc. When a mixture of benzene, iodine and cone. HNO₃ is refluxed, and one H-atom of benzene ring is substituted by one I-atom to give iodobenzene in good yield

Removal of a halogen atom from the benzene ring:

A halogen atom present in the benzene ring may be removed through the i formation of Grignard reagent i.e., halobenzene can be converted into benzene

2. Nitration of benzene

Nitration of benzene Definition:

The reaction in which a hydrogen atom of benzene is displaced by nitro (-NO) group is called a nitration reaction. 05 tot. (C₆H₄Cl₂)

Reagent:

Generally a mixture of cones. HNO₃ and H₂SO₄ is used as nitrating agent. This mixture of two acids is called mixed acid.

1. Preparation of nitrobenzene:

When benzene is heated at 50 – 60°C in the presence of a mixture of one. HNO₃ and cone. H₂SO₄ one H-atom ofbenzene ring is replaced by a nitro (— ⁺NO₂) group to produce nitrobenzene.

Reaction mechanism:

It is an electrophilic substitution reaction. The electrophile involved in this reaction is nitronium ion ( NO₂ ) & it is the actual nitrating agent in this reaction.

Formation of the electrophile:

HNO₃ accepts a proton from H₂SO₄ to form its conjugate acid (O₂N—OH₂). It then dissociates to form a nitronium ion

If only hot and cone. HNO₃ is used, and the reaction takes place at a relatively slower rate. Again aromatic compounds which are susceptible to oxidation, the explosion may take place.

Substitution:

2. Preparation of dinitrobenzene :

Whenandbenzenecone. H₂SO₄ is heated at boiling water bath), and m-dinitrobenzene is produced. At first, nitrobenzene is formed. Since the ( —NO₄) group is meta-directing, the second nitro group mainly enters the meta-position

Nitro group, being an electron-attracting group, decreases the electron density of ring. So, incoming ion cannot be easily attracted by the ring and consequently, the nitration reaction becomes somewhat difficult. For this reason, in the case of second nitration, higher temperature and fuming nitric acid are used

3. Preparation of trinitrobenzene:

When benzene is refluxed with fuming HNO₃ and fuming H₂SO₄ for 5-6 days, 1,3,5-trinitrobenzene (TNB) is produced. It is an explosive. The third nitro group takes the meta-position with respect to the two nitro groups already present in ring

• The two nitro groups present in m-dinitrobenzene cause a decrease in the electron density of the ring to such an extent that the introduction of the third nitro group becomes extremely difficult.
• For this reason, the third nitration requires much higher temperature and a mixture of fuming nitric acid and fuming sulphuric acid becomes necessary.
• Nitration of benzene can also be carried out by using stable nitronium salts like nitronium tetrafluoroborate  (NO₂BF₄), nitronium trifluoromethane sulphonate (NO₂ CF₃SO₃ etc.

3. Sulphonation of benzene

Sulphonation of benzene Definition:

The reaction in which a hydrogen atom of the benzene ring Is substituted by sulphonic acid (-SO₃H) group is known as the sulphonation reaction. Reaction and conditions: At ordinary temperature, cone. H₂SO₄ does not react with benzene. However, when a mixture of benzene and cone. H₂SO₄ is heated at 80°C about hours, benzene sulphonic acid Is produced

Sulphonation with fuming H₂SO₄ (7% SO₃ dissolved in cone. H₂SO₄) can be carried out even at ordinary temperature. Sulphonation may also be affected by chlorosulphonic acid (C1S03H).

Sulphonation of benzene Reaction mechanism :

This electrophilic substitution reaction is reversible. In this reaction, sulphur trioxide (SO₃) acts as the electrophile. In some cases, protonated sulphur trioxide (HSO₃⁺) acts as an electrophile.

Formation of electrophile:

Two molecules of sulphuric acid interact in the following way to form sulphur trioxide

Substitution:

It occurs through three steps.

First step: Formation of carbocation (σ-complex).

Second step: Expulsion of proton

The second step occurs at a slower rate than the first step. Thus, the second step is the rate-determining step

Preparation of di- & tri-sulphonic acid:

Benzene, on sulphonation with fuming H₂SO₄ at 200-245°C gives benzene-m-sulphonic acid which on further heating at 280-300°C produces1,3,5-benzenetrisulphonic acid

At first benzene sulphonic acid is formed. The sulphonic acid group(—SO₃H) is meta-directing. So, the second

—The SO₃H group occupies the meta-position and the third

—SO₃H group takes the meta-position concerning the other two sulphonic acid groups. The sulphonic acid group is a deactivating group. So, the introduction of second and third

—SO₃H group becomes more and more difficult.

Thus, to introduce the second and third

—SO₃H groups, the reaction temperature is to be gradually increased.

Removal of —SO₃ H group from benzene sulphonic acid:

• When benzene sulphonic acid is heated to 150°C with dilute HCl or H₂SO₄ or is brought in contact with superheated steam, the
• —SO₃H group is replaced by hydrogen to form benzene. The reaction is called desulphonation reaction. In this reaction, proton (H+) acts as the electrophile.

• If superheated D₂O is used instead ofsuperheated steam, the —SO₃H group is replaced by D to produce C₆H₅D. O Importance: ‘Sulphonation-de-sulphonation’ may serve as a useful tool for synthesis of various organic compounds.
• This may well be illustrated in the synthesis of o-nitroaniline from aniline. To avoid oxidation, aniline is first converted into acetanilide. The para-position of acetanilide is then blocked by the —SO₃H group.

Due to steric reason, the —SO₃H group enters mainly the para-position. In the next step, the — NO₂ group enters the position ortho to -NHCOCH₃

Finally, —SO₃H group is removed from the ring. The sulphonic acid group here acts as a blocking group.

If para-position of acetanilide is not blocked by the —SO₃ H group, — NO₂ enters mainly para- position instead of the sterically more crowded ortho-position to give p-nitroaniline.

4. Friedel-Crafts reaction:

The reaction in which a H-atom of the benzene ring is Q) + CH₃CH₂CI Anhyd. AlCl₃ substituted by alkyl (R— ) or acyl (RCO— ) group in the presence of a catalyst is called Friedel-Crafts reactions

Catalysts used: The best catalyst used for this reaction is anhydrous aluminium chloride (AlCl₂). Lewis acids such as boron trifluoride (BF₃) , ferric chloride (FeCl₃) , zinc chloride (ZnCl₂) etc., and protonic acids like HF, H₂SO₄, H₂PO₄ etc., may also be used as catalyst

Solvents used:

Suitable solvent for this reaction is nitrobenzene. Nitrobenzene with highly deactivated ring (due to -R and -I effects of -NO₂ group) does not take part in this reaction as the relatively weak electrophile R+ cannot attack the ring i.e., substitution or Fridel-Crafts reaction does not take place. Being polar, nitrobenzene dissolves anhydrous AICI₃. Again, benzene and alkylating or acylating reagents also dissolve in it. As all the reagents remain dissolved in liquid phase, the reaction takes place smoothly.

Moreover, as the boiling point of nitrobenzene is high (211°C), the reaction may be conducted atappreciably high temperature. Sometimes, carbon disulphide (CS₂) is also used as a solvent for the reaction.

Friedel-Crafts alkylation:

When benzene is allowed to react with alkyl halide (RX) in the presence of anhydrous aluminium chloride as a catalyst, a hydrogen atom of benzene ring is replaced by alkyl group to produce alkylbenzene. This reaction is called the Friedel-Crafts alkylation. The different homologues of benzene can be prepared with the help of this reaction.

Examples: 

• Methylbenzene (toluene) may be prepared by allowing benzene to react with methyl iodide in the presence of anhydrous AlCl₃.
• Methyl chloride may also be used, but as methyl iodide is a liquid at ordinary temperature, it is generally preferred

Similarly, ethylbenzene is obtained when benzene reacts with ethyl chloride in the presence of anhydrous aluminium chloride

Friedel-Crafts Reaction mechanism:

The effective electrophile in this electrophilic substitution reaction is a carbocation (R⁺). Although 2° or 3  alkyl halide reacts with the Lewis acid, AICI₃ to form R, the reaction of

alkyl halide or methyl halide with AICI₃ does not produce R⁺ ion. This is because the 1° carbocation and CH₂ are relatively very unstable. In that case, the initial complex formed in the reaction between the alkyl halide and AlCl₃ acts as the electrophile.

Formation of the electrophile:

Substitution: It occurs in two steps

First step: Formation of carbonation(-complex)

Second step: Explusion of Proton

In this reaction, besides alkyl halides, aliphatic alcohols, may also be used as alkylating agents. Apart from AlCl₃, BF₃, HP, cone. H₂SO₄ etc., are used as catalysts.

Limitations of the Friedel-Crafts alkylation reaction:

1. In this reaction, there is always a possibility of polyalkylation of the benzene ring because, once an alkyl group enters the benzene ring, the electron density of the ring increases due to electron-repelling effect of the alkyl group. Consequently, the ring becomes activated towards further electrophilic substitution. Now, all the molecules of benzene and alkyl halide do participate in the reaction simultaneously.

So, the alkyl halide molecules present in the reaction medium reacts with the resulting alkylbenzene molecules at a rate faster than benzene. As a result, more than one alkyl group enters the benzene ring, i.e., polyalkylation occurs. Thus, for the preparation of monoalkyl benzenes, the Friedel-Crafts reaction is not suitable.

Polyalkylation in Friedel-Crafts alkylation reaction may be considerably reduced by using excess of substrate (benzene).

2. If alkyl halide used in the Friedel-Crafts alkylation reaction is a primary alkyl halide containing three or more carbon atoms, then instead of the corresponding alkyl benzene, the alkyl benzene containing a secondary or tertiary alkyl group is obtained as the major or the sole product by rearrangement ofthe alkyl group

Reaction mechanism: 

Lewis acid-Lewis base complex obtained in the reaction of AlCl₃ with the 1° alkyl chloride undergoes dissociation and rearrangement simultaneously to form a stable 2° or 3° carbocation. These carbocations participate in substitution reactions and as a result, alkyl benzenes isomeric with the expected alkyl benzene are obtained as the major or the sole product.

3. The following compounds do not participate in FriedelCrafts alkylation and acylation reactions:

Groups such as nitro, carboxyl, acyl and trimethyl-ammonium (— ⁺NMe₃) etc., withdraw electrons from the benzene ring and deactivate the ring to such an extent that it cannot be attacked by tyre relatively weak electrophile (R⁺). So, the substitution reaction does not take place.

Aniline does not react because the —NH₂ group gets converted into a powerful electron-withdrawing group by coordinating with the Lewis acid.

Any vinyl halide, vinyl chloride (CH₂=CH—Cl) ] or halobenzene [e.g., chlorobenzene (C₆H₅Cl) ] cannot be used as an alkylating agent. As, due to resonance, the C—X bond acquires some double bond character, AlCl₃ cannot snatch the halogen atom to form the corresponding carbocations.

Friedel Crafts Acylation:

When benzene is treated with acyl chloride (RCOCl) in the presence of anhydrous aluminium chloride as catalyst, the H-atom of benzene ring is substituted by the acyl (RCO— ) group to form acylbenzene (aromatic ketone).

This reaction is called Friedel-Crafts acylation. Besides acyl chlorides, acid anhydrides are also used as acylating agents.

Hydrocarbons Class 11 NCERT Notes

Friedel Crafts Acylation Reaction mechanism:

The electrophile involved in this electrophilic substitution reaction is an acylium ion (R —C=O) substitution reaction is an acylium ion (R —C=O)

Formation of the electrophile:

Two special synthetic advantages of acylation reaction:

1. Although polyalkylation is a common feature of benzene, its polyacylation never occurs. An electron-attracting acyl (RCO— ) Group decreases the electron density of the ring.

As a result, despite the presence of excess RCOCl in the reaction medium, the second acylation does not take place, i.e., not more than one RCO— Group can enter the benzene ring. Therefore, pure aromatic ketone can easily be synthesised with the help of this reaction.

2. During the acylation reaction, the carbon chain of the acyl halide does not rearrange. So, there is no possibility of formation ofa ketone isomeric with the desired ketone. Due to such an advantage, the acylation reaction is used to prepare alkyl benzenes containing 3 or more carbon atoms.

For example:

The alkylation reaction for preparing propylbenzene leads to the formation of iso propyl benzene as the major product. However, if benzene is first allowed to form a ketone and the ketone is then reduced (Clemmensen reduction), the desired propylbenzene is obtained as the only product

Removal of alkyl or acyl group from benzene ring:

Toluene or acetophenone on oxidation by alkaline KMn04 solution followed by acidification produces benzoic acid which, when heated with soda lime, gives benzene

If the alkyl group is tertiary (Me₃C— ), then it cannot be removed in this way because the tertiary alkyl group cannot be oxidised. However, the tertiary alkyl group can easily be removed from the ring

By the following reaction:

Chloromethylation:

When benzene is heated with HCHO and HCI in the presence of anhydrous zinc chloride, H-atom of the benzene ring is replaced by the chloromethyl (—CH₂Cl) group to form benzyl chloride. This reaction is called chloromethylation

CH₂Cl group may be removedfrom benzene ringbythe same process applied for removal ofalkyl (—R) group.

Gattermann-Koch aldehyde synthesis:

When a mixture of carbon monoxide and hydrogen chloride is passed through benzene dissolved in nitrobenzene or etherin the presence of anhydrous AlCl₃ and small amount Cu₂Cl₂ as catalyst, benzaldehyde is obtained. This reaction is known as Gattermann-Koch aldehyde synthesis

—CHO group may be removed from benzene ring by the same procedure applied for the removal of an alkyl group.

Gattermann aldehyde synthesis:

Benzene reacts with hydrogen cyanide and hydrogen chloride in the presence of anhydrous AlCl₃ to produce an imine which on hydrolysis gives benzaldehyde. This reaction is known as Gattermannaldehydesynthesis

Addition Reactions Of Benzene

1. Reduction—addition of hydrogen

1. Reduction into cyclohexane:

When a mixture of benzene vapour and hydrogen gas is passed through powdered nickel catalyst heated at 200°C, benzene combines with three molecules of hydrogen to form hexahydrobenzene or cyclohexane.

In this reaction, cyclohexadiene and cyclohexane  are not obtained as intermediates because these compounds are more reactive than benzene.

2. Birch reduction:

Benzene is reduced by Na, K or Li in liquid ammonia in presence ofmethanol or ethanol to give 1,4-cyclohexadiene. This reaction is called Birchreduction

2. Addition of halogen

When chlorine gas is passed through boiling benzene or through benzene in the presence of

UV-rays, benzene combines with three molecules of chlorine to form benzene hexachloride or 1,2,3,4,5,6- hexachlorocyclohexane

Like chlorine, bromine also reacts with benzene to form benzene hexabromide.

lodine doesn’t undergo such additional reaction with benzene. benzen

BHC is used as an insecticide. It is a mixture of eight geometrical isomers and out of them the y-isomer (overall 18%), known as gammexane or lindane, possesses insecticidal property. So, BHC too possesses insecticidal property. However, because of its many adverse effects, its use is gradually declining.

3. Addition of ozone

When ozonised’ oxygen is passed through benzene at ordinary temperature, benzene combines with three molecules of ozone to form an unstable addition compound known as benzene triozonide. When the triozonide is hydrolysed in the presence of zinc, three molecules ofglyoxal and H₂O₂ are obtained. H₂O₂ thus produced is reduced by zinc to give water

The formation of 3 mol glyoxal from 1 mol benzene suggests that benzene is a six-membered carbocyclic compound containing 3 double bonds in alternate positions

Oxidation Of Benzene

When a mixture of benzene vapour and air is passed over a vanadium pentoxide (V₂O₅) catalyst heated at much higher temperature (450°C), benzene is oxidised to maleic anhydride

Effect of high temperature on benzene 

When benzene vapours are passed through a hot (600-800) iron tube packed with pumice stones, biphenyl or diphenyl is obtained.

Uses of benzene

The preparation of ethyl benzene is an important use of benzene because, ethyl benzene is used in the preparation of styrene (C₆H₅CH=CH₂), the starting material for the manufacture of polystyrene and artificial rubber.

It is used to prepare many important compounds,

For example: Nitrobenzene, aniline, phenol, etc., and detergents (alkyl benzenesulphonates).

It is used as a solvent for fats, oils, resins, rubber, sulphur, iodine, phosphorus etc..

Carcinogenicity And Toxicity

Benzene and polynuclear hydrocarbons containing fused rings or are highly toxic. Many of these are carcinogenic substances. These are formed by Incomplete combustion of organic materials such as coal, petroleum, tobacco etc. and extensively mix with the surrounding air, These compounds enter our body during inhalation and destroy our DNA by performing various biochemical reactions. This ultimately leads to cancer.

Hydrocarbons Class 11 NCERT Notes

Action of carcinogenic substances in the human body:

• From experimental results, it has come to our knowledge that these polynuclear hydrocarbons (PNH) after entering the human body are first converted into epoxides and then into dihydroxy epoxides.
• The resulting dihydroxy epoxides react with the purine bases such as guanine present in DNA and RNA of the human cells.
• Due to the attachment of the hydrocarbon part, the purine bases become large in size and can no longer accommodate themselves in the double helix of DNA.
• This damage in the double helix of DNA causes mutation and ultimately leads to cancer. Carcinogenic effect of polynuclear hydrocarbons (PNH)

Preparation of benzene:

Special reactions of benzene:

Flow chart for substitution reactions of benzene:

NCERT Solutions For Class 11 Chemistry Chapter 13 Hydrocarbons Very Short Questions And Answers

Question 1. Which hydrocarbon is obtained on hydrolysis of Al₄C₃?
Answer: CH₄

Question 2. Name an alkane that cannot be prepared by the Wurtzreaction.
Answer: CH₄

Question 3. Which alkane is expected to be formed when ethyl magnesium bromide is allowed to react with water?
Answer: Ethane

Question 4. How many acyclic isomers of C₅H₁₂ are possible?
Answer: Isomer

Question 5. What is the main constituent of CNG?
Answer: CH₄

Very Short Answer Questions for Class 11 Chemistry Chapter 13 Hydrocarbons

Question 6. Which type of aliphatic hydrocarbon undergoes substitution reaction?
Answer: Saturated

Question 7. What is the name of the alkene obtained when an aqueous solution of potassium succinate is electrolyzed?
Answer: Ethylene

Question 8. CH₃CH=CH₂HCl/peroxlde?
Answer: CH₃CHClCH

Question 9. Which alkene on ozonolysis yields only acetaldehyde?
Answer:  2-butene

Question 10. What is Baeyer’s reagent? What is its use?
Answer:  Alkaline KMnO₄ ,it is used to identify C=C and C=C;

Question 11. What is Lindlar’s catalyst?
Answer: Pd-CaCO₃/ (CH₃COO)₂Pb

Question 12. What is Teflon?
Answer: Polytetrafluoroethylene

Question 13. 2-butanone and ethanal are obtained when an alkene containing five carbon atoms is subjected to ozonolysis. State the position of the double bond in the alkene.
Answer: Doublebondis at C-2 of the alkene containing five carbon atoms

Question 14. What is mustard gas?
Answer:  2,2′- dichloro-diethyl sulfide;

Question 15. Name a reagent which can be used to distinguish between 2-butyne and 1-butyne.
Answer: Ammoniacal Cu₂Cl₂

Question 16. Which alkyne is used in Hawker’s Lamp?
Answer: HC=CH

Question 17. Mention the name of the compound obtained when acetylene reacts with arsenic chloride.
Answer: Lewisite

Question 18. What is the chemical name of Westron?
Answer: 1, 1,2,2- tetrachloroethane

Question 19. Mention one use ofWestrosol.
Answer: As an organic solvent

Question 20. What is obtained when acetylene is passed through a hot iron tube?
Answer: C₆H₆

Question 21. Give an example of an anti-knock compound.
Answer: Tetraethyl lead

Question 22. Which of the following cannot produce white precipitate by the action of ammoniacal AgNO₃—Acetylene, dimethyl acetylene, methyl acetylene, ethyl acetylene. one acts as the base
Answer: Dimethyl acetylene

Question 23.  Name the reagent which is used to carry out dihydroxylation of a double bond.
Answer: OsO₄ followed by hydrolysis;

Question 24. Which polymer is used to make carry bags? Name its monomer.
Answer: 5. Polyethylene or Polythene, ethylene;

Question 25. Which compound is formed as the major product when propyne reacts with 20% H₂SO₄ in the presence of 1% HgSO₄ at 80°C?
Answer: . Acetone (CH₃COCH₃)

Hydrocarbons VSAQs Chapter 13 NCERT Solutions for Class 11 Chemistry

Question 26. What is the state of hybridization of each carbon atom in an aromatic ring?
Answer: sp²

Question 27. Name the compound obtained by ozonolysis of benzene.
Answer: Glyoxal

Question 28. Give an example of a group that increases the rate of aromatic electrophilic substitution reaction.
Answer: —NH₂

Question 29. Give an example of a group that decreases the rate of aromatic electrophilic substitution reaction.
Answer: —NO₂

Question 30. Name an ortho-/para-orienting group.
Answer: Methyl (-CH₃)

Question 31. Name a meta-orienting group.
Answer: Nitro ( —NO₂)

Question 32. Give an example of a reversible electrophilic substitution reaction.
Answer: Sulphonationreaction

Question 33. Which is the smallest aromatic molecule/ion?
Answer: cyclopropenyl cation

Question 34. What is the orientation of the deactivating halogen atoms?
Answer: Ortho-/para

Question 35. Which heterocyclic compound remains as an impurity in benzene obtained from fractional distillation of coal tar?
Answer: Thiophene

Question 36. Which reagent is used in Birch reduction?
Answer: Na / liquid NH₃, ethanol

Question 37. Which type of flame is observed during the combustion of benzene?
Answer: Sooty flame

Question 38. Nitration occurs at which position of the compound
Answer: Predominantly para position

Question 39. Which electrophile is involved in the desulphonation reaction
Answer:  Proton (H⁺);

Question 40. Give an example of a carcinogen
Answer: 1, 2-benzpyrene

Question 41. Between HNO₃ and H₂SO₄ which one acts as the base during the formation of N02 ion?
Answer: HNO₃

Question 42. Which is the rate-determining step in an aromatic electrophilic substitution reaction?
Answer: First step, i.e., formation of σ -complex;

Question 43. Which step in aromatic electrophilic substitution reaction is exothermic?
Answer: Second step i.e., formation of substituted compound;

Question 44. Give an example of a neutral electrophile that participates in an electrophilic substitution reaction.
Answer: Sulphur trioxide;

Question 45. Give an example of a polynuclear hydrocarbon.
Answer: Anthracene

Question 46. Which compound other than anhydrous AlCl₃ can be used for the ethylation of benzene?
Answer: FeCl₃

Question 47. What is gamm exane? Mention its use.
Answer: BHC (Benzene hexachloride); as an insecticide

Question 48. What is the electrophile involved in the nitration reaction?
Answer: NO₂

Question 49. Name the products obtained on pyrolysis of propane.
Answer: Propene, ethene, methane, and H₂

Question 50. Which compound out of 1-butene, 1-butyne, and 2-butyne is most acidic?
Answer: 1-butyne is the most acidic.

Question 51. Write the name of the compound obtained when n-heptane is subjected to aromatization
Answer: Toluene (C₆H₅—CH₃).

NCERT Solutions for Class 11 Chemistry Hydrocarbons VSAQs

Question 52. Which alkane cannot be prepared by Kolbe’s method?
Answer: Methane (CH₄).

Question 54. Write the names of the compounds obtained on ozonolysis of o-xylene.
Answer: Glyoxal, methyl glyoxal, and dimethyl glyoxal

Question 55. What is lindane?
Answer: Benzene hexachloride (BHC), C₆H₆Cl₆.

Question 56. What is picric acid?
Answer:  2,4,6-trinitrophenol is known as picric acid

Question 57.  What is the name of the compound obtained when benzene is oxidized by air (02) in the presence of a V205 catalyst heated at 500°C?
Answer: Maleic anhydride

Question 58. Which group out of -NO, and -CgHg is an o-/p directing group and which one is an o-/p-directing group?
Answer: NO₂→m -directing; —C₆H₅→o-/p -directing

Question 59.  Which will undergo nitration at a faster rate: C₆H₆ or C₆H₅Cl?
Answer: C₆H₆ undergoes nitration at a comparatively faster rate

Question 60. Name a group that is o -/p -directing but is also a deactivating group.
Answer: Chloro(-Cl)

Question 61. A hydrocarbon on ozonolysis produces ethanal and methanal. 
Answer:  CH₃CH =CH₂

Question 62. Mention the product:
Answer: CH₃CHO

Question 63. Write the structure of an organic compound which reacts with water to yield methanal and hydrogen peroxide
Answer:

Question 64.  Benzene reacts with CH₃COCl in the presence of anhydrous AlCl₃ to form (an organic compound).
Answer: Acetophenone.

Question 65. Which reagent can be used for the following conversion?  HC≡CH→HC=CH₂
Answer: H₂,Pd-CaCO₃/Pb(OAc)₂ (Lindler’s catalyst)

NCERT Solutions For Class 11 Chemistry Chapter 13 Hydrocarbons Fill In The Blanks

Question 1. The formula of marsh gas is _______________
Answer: CH₄

Question 2.  _______________ are called paraffins.
Answer: Alkanes

Question 3. Beryllium carbide yields _______________
Answer: CH₄

Question 4. Dutch oil is _______________
Answer: 1,2-dichloroethane;

Question 5. Wurtz reaction is suitable for the preparation of _______________alkanes.
Answer: Symmetrical

Question 6. CHgCOCHg undergoes Clemmensen reduction to yield _______________
Answer: Propane

Question 7.___________ can be identified by Schryver’s colour test
Answer: CH

Question 8. Peroxide effect is applicable only for _______________
Answer: HBr

Question 9. _______________ is obtained when. a solution of sodium butanoate is electrolyzed
Answer: n-hexane

Question 10. Isobutylmagnesium bromide reacts with water to form _______________
Answer: Isobutane

Question 11. _______________ on ozonolysis produces formaldehyde and acetaldehyde.
Answer: Propene

Question 12. _______________ is obtained as the major product when 2- butanol is dehydrated.
Answer: But 2 ene

Question Benzene is a polymer of _______________
Answer: Acetylene

Class 11 Chemistry Hydrocarbons Very Short Answer NCERT Solutions

Question 14. The simplest hydrocarbon which reacts with ammoniacal silver nitrate to produce a white precipitate is _________
Answer: Acetylene

Question 15. _______________ is obtained when 2-butyne is passed through a mixture of 20% H₂SO₄ and 1% H₂SO₄
Answer: 2 – butanone

Question 16. Hexamethylbenzene is the trimer of _______________
Answer: 2 butyne

Question 17. When a mixture of _________ and Ag – powder is heated ________________ is obtained as the product
Answer: Chloroform, acetylene

Question 18. The values of boiling and melting points of alkadienes are _ than the corresponding alkanes and alkenes containing a same number of carbon atoms.
Answer: Higher

Question 19. Ozonolysis of acetylene forms _______________
Answer: Glyoxal

Question 20. Two molecules of HBr react with acetylene to form _______________
Answer: 1,1 dibromomoethane

Question 21. If an alkene forms only one type of carbonyl compound on ozonolysis, then it can be concluded that the alkene is _______________
Answer: Symmetrical

Question 22. C_xH_y _______________ xCO₂ + + y/2 H₂O Heat
Answer: \(\left(x+\frac{y}{4}\right) \mathrm{O}_2\)

Question 23. The number of isomeric tribromobenze is _______________
Answer: Three

Question 24. 1,3,5-trinitrobenzene is an_ compound. _______________
Answer: Explosive

Question 25. The resonance-stabilised carbocation formed in the first step of the electrophilic substitution reaction is called _______________
Answer: Sigma complex

Question 26. _______________ of benzene is carried out by using N⁺O₄ BF^–₄ salt.
Answer: Nitration

Question 27. When benzene is oxidized by atmospheric oxygen in the presence of V₂O₅ at high temperature, _______________ is obtained.
Answer: Maleic anhydride

Question 28. The product obtained due to Birch reduction of benzene when subjected to ozonolysis forms only _______________
Answer: Propanediol

Question 29. —COOH is a/an ______________ Group but COO is an __________ group
Answer: Deactivating, activating;

Question 30. NH₂ group _______________ electron density at ortho-/para positions of the rin
Answer: Increases;

Question 31.  —NO₂ group ______________ electron density at the meta position of the ring
Answer: Decreases

Question 32. C —C bond lengths of benzene are _________________
Answer: Equivalent

Question 33. Fill in the blank __________(organic compound) is obtained when an aqueous solution of potassium succinate is electrolyzed.
Answer: Ethylene

CBSE Class 11 Chemistry Notes For Chapter 14 Environmental Chemistry Introduction

The word ‘environment’ derived from the French word ‘environment can be defined at an assembly of physical, chemical and biological factors, which act upon an organism or an ecological community to determine its form and mode of survival.

Environment mainly consists of three major components:

1. Biotic or living: 
   • For example – All living creatures)
2. Abiotic or non-living:
   • For example –  Lithosphere, hydrosphere and atmosphere) and
3. Energy components: 
   • For example – Solar energy, thermochemical energy, nuclear energy etc.)

Environmental Chemistry

Environmental Chemistry Definition

The branch of science, which deals with the sources of the chemical components of the environment, the chemical reaction occurring among them, the products formed in the reaction and their impact on the living world is called environmental chemistry.

Environmental Chemistry Class 11 NCERT Notes

The study of environmental chemistry is important because

• It makes us aware of the adverse effects of various chemical constituents on the environment.
• It gives us an idea about the toxic effects of various chemical substances and their by-products which are extensively used to fulfill our requirements.
• It gives us an idea about the sources of various toxic chemicals, their adverse effects and the antidotes to combat their toxicity.

1. Some terms used in environmental chemistry

1. Pollutant:

A pollutant is a solid, liquid or gaseous substance (produced either by natural sources or by human activity) which is present in the environment to such an extent that it causes harmful or detrimental effects on living organisms (plants, animals and human beings) or nonliving components.

Example:

Air contains trace amounts of CO (0.1 ppm). If, for any reason, the amount of CO increases to 40 ppm or more, then it is regarded as pollutant

Pollutants are of two kinds :

1. Primary pollutant:  The pollutants which are emitted from any source, directly escape into the environment without sustaining any change are called primary pollutants.
2. Example: S, NO, NO₂, CO, CO₂, hydrocarbons etc.
3. Secondary pollutant: There are some pollutants which do not appear in the environment directly from their source. They are produced as harmful substances by Interaction with pollutants), already present in the environment. Tills type of pollutants are called secondary pollutants.
4. Example: Peroxyacyl nitrate (PAN), dimethyl mercury [(CH₃)₂Hg]

2. Biodegradable pollutants

The pollutants which are decomposed by bacteria or germs are known as biodegradable pollutants.

Examples: Household garbage, cow dung and other biomass etc.

3. Non-biodegradable pollutants:

The pollutants which are not decomposed by bacteria or germs or decomposed very slowly are known as non-biode gradable pollutants.

Example:

Mercury, DDT, Gammaxene etc. The presence of these substances even in trace amounts is injurious to human beings and other animals.

4. Contaminant:

A contaminant is a substance which does not occur in nature under normal conditions but is introduced into the environment either accidentally or through indiscriminate human use.It may or may not be harmful to the living organisms or non-living components. The contaminant is considered as a pollutant when it has some harmful effect

Examples:

• In Delhi pyrosulphuric acid (H₂S₂O₇) leakage from a defective tank killed many persons and caused skin and breathing problems to many others. As pyrosulphuric acid does not occur in the atmosphere, therefore it is a contaminant. Again, because of its dangerous effect, it is also regarded as a pollutant.
•  In Kerala, in 1953, 108 people died after consuming wheat flour contaminated with parathion (an agricultural pesticide).

5. Source:

The source ofany contaminantis a chemical substance or the place from whereitis produced.

Example:

A source of the pollutants like CO, NO etc., is the gas 2 emitted from petrol or diesel automobile engines

6. Sink:

If any medium continuously reacts with a pollutant for a long period and causes destruction to it, then it is said to be the sink of that particular pollutant.

Examples:

1.  Sea water acts as a sink of CO₂ present in the atmosphere

⇒ H₂O (sink) +CO₂ (Pollutant ) H₂CO₃

2. An automobile wall or memorial acts as a sink of sulfuric acid, present as an atmospheric pollutant.

⇒ (Sink )CaCO₃+ (polluntant)H₂SO₄→CaSO₂↓ + H₂O + C₂O

7. Receptor or target:

If any plant or animal body or any biotic component is adversely affected by a pollutant, then that particular body or component is called a receptor or the target of that pollutant.

Examples:

•  Smoke or smog causes a burning sensation in our respirator tract and eyes.In this case, man is the receptor or target of smoke or smog.
• The aquatic animals are the receptors of the oil layer, floating on the surface of seawater.

8. Pathways Of pollutants:

The mechanism by which any pollutant is liberated from its source, spreads in the environment and ultimately gets destroyed, is called the pathway of that pollutant.

Example:

Nitric oxide, a pollutant, liberated from petrol or diesel engine reacts with oxygen present in the air to form nitrogen dioxide. Nitrogen dioxide reacts with rainwater and falls on the surface of the earth as nitric acid.

2NO + O₂ → 2NO₂; 2HNO₂+O₂→2HNO₃

2NO₂ + H₂O→HNO₃ + HNO₂

9. Threshold Limit Value (TLV):

Anypollutantin in the environment is considered to be harmful to living organisms if its concentration exceeds a particular limit. This particular limit (of concentration) is called ‘threshold limit value’ (TLV) ofthat particular pollutant.  Atmosphere

Examples: Threshold limit values for CO and CO₂ are. 40 ppm and 5000 ppm respectively. Ho However, the TLV for phosgene (COCl₂) is only 0.1 ppm.

Threshold limit value for factory workers: It is the maximum limit of a pollutant present in the environment maximum limit of a pollutant present in the environment hours per day without suffering from health hazards Examples: Threshold limit values for SO₂ and CO₂ are 5 mg^-3. m^-3 and 5000 mg m^-3 respectively

Environmental Chemistry Class 11 NCERT Notes

Environmental Components Of Earth

Earth’s environment is composed of the following four components—

1. Atmosphere,
2. Hydrosphere,
3. Lithosphere and
4. Biosphere.

Among these, the first three components are abiotic while the fourth one is biotic.

1. Atmosphere: The invisible gaseous layer that surrounds and protects the Earth is called the atmosphere.
2. Hydrosphere:  It includes all sources of water such as seas, oceans, rivers, fountains, lakes, polar regions, glaciers, groundwater etc.
3.  Lithosphere:  It comprises the solid crust of the earth, made of rocks, forming the outer mineral cover.It includes soil, minerals, organic matter etc., and extends up to a depth of about 30 km from the earth’s surface.
4.  Biosphere: It is the part of the earth where living organisms exist and interact with each other and also with the non-living components. The biosphere consists of all three zones,
   • Example: Soil, water, air etc., where living beings exist.

Atmosphere

The invisible blanket of gaseous layer that surrounds the earth is called atmosphere. It extends upwards to about 1600km. It is the gravitational attraction of the earth that holds this gaseous layer closely in space around the earth’s surface. The total mass of gaseous substances in the atmosphere is nearly 5.5×1015 ton. On the basis of temperature gradients and altitude,

The atmosphere has been divided into four distinct zones.

These are

1. Troposphere,
2. Atmosphere
3. Mesosphere and
4. Thermosphere

Different zones of atmosphere

Again according to the proportion of different gases from the surface of the earth towards the vacuum of interstellar space, and

Atmosphere can be divided into two categories:

1. Homosphere
2.  Heterosphere

The homosphere extends from the surface of the Earth up to about 100 km height. In this layer, the proportions of different gases are more or less identical. Thus, this layer is called the homosphere.

The layer next to it is known as the heterosphere because the proportion of the gases in the different parts of this layer are found to be dissimilar. Gravity holds most of the air molecules close to the Earth’s surface and hence the troposphere is much more denser than the other layers.

50% of the total mass of the atmosphere exists within a height, of5.5 km from the Earth’s surface, and 99% exists within a height of 30 km from the Earth’s surface

Average gaseous composition in the homosphere

Functions of gases present in the atmosphere

Oxygen:

• The most significant gaseous constituent of the atmosphere is oxygen. Oxygen is indispensable for any kind of combustion.
• Oxygen is also used for the oxidation
  of food taken by plants and animals to produce heat and energy.
• Oxygen is a necessary component of life as all living beings (except some microorganisms) and plants take oxygen from the atmosphere for respiration.
• Plants give up oxygen to the atmosphere during the process of photosynthesis. As a result, the balance of oxygen is maintained in the atmosphere.

Nitrogen:

The major constituent of the atmosphere is nitrogen. Proteins and nucleic acids present in living bodies are nitrogenous compounds.

• But most of animals including human beings and even plants cannot utilise atmospheric nitrogen direedy for the production of proteins and amino acids.
• However, some nitrogen-fixing bacteria can take nitrogen directly from air and produce nitrate salts in the soil. These are used by plants in the synthesis of amino acids and nucleic acids.
• Herbivorous animals meet their protein demand by eating those plants. Similarly, carnivorous animals get proteins from herbivorous animals.
• After the death of plants and animals, nitrogenous compounds present in their bodies are decomposed by some bacteria releasing nitrogen gas that returns to the atmosphere.

Carbon dioxide (CO₂):

Combustion of fossil fuels and carbonaceous compounds, and respiration of plants and animals increase carbon dioxide content in the atmosphere.

• Again plants, during photosynthesis, absorb carbon dioxide from the atmosphere for the preparation of food.
• As a result, the balance of carbon dioxide is maintained in the atmosphere.
• But due to excessive … combustion of carbonaceous fuels and indiscriminate deforestation, the quantity of carbon dioxide in the atmosphere is increasing constantly leading to a constant increase in the average temperature of the earth (See greenhouse effect).

Ozone:

The quantity of ozone gas present in the atmosphere is negligible.

• Almost the entire amount of ozone (=90%) is present in the stratosphere which is about 15-35 km above the earth’s surface.
• The presence of ozone gas close to the earth’s surface hurts mankind and other animals.
• But the presence of ozone in the upper layer of tire atmosphere is beneficial since it absorbs the harmful ultraviolet rays ofthe sun

Air Pollution

When one or more undesirable chemical substances produced by natural phenomena or uncontrolled human activity get mixed with the air to bring about health hazards to human or any other living being and affect their life processes, then air pollution is said to have occurred. The substances which cause air pollution are called air pollutants

Examples:

Sulphur dioxide (SO₂), carbon monoxide (CO), carbon dioxide (CO₂), nitric oxide (NO), nitrogen dioxide (NO₂), hydrogen sulphide (H₂S), hydrocarbons (C_xH_y), ammonia (NH₃), fly ash, dust particles, smoke, fumes etc., are some air pollutants

Causes of air pollution

Natural causes of air pollution:

• Emission of SO₂, CO, H₂S gases etc., due to volcanic eruption.
• Gases are liberated due to the decomposition of living bodies.
• Dust storms, forest fires and the fall of a meteorite.
• Spreading of viruses, bacteria, fungi, pollen-grain of flowers in air etc

Causes of air pollution due to human activities:

• Sulphur dioxide gas, carbon monoxide gas and fly ash are produced by thermal power plants.
• Gases such as sulphur dioxide, sulphur trioxide, oxides of nitrogen, hydrogen chloride, chlorine etc., from acid manufacturing factories.
• Ammonia gas is liberated from fertiliser factories and cold storage.
• Sulphur dioxide, carbon monoxide and different metallic oxides were obtained during the extraction of metals.
• Fine and bulky solid particles are produced in cement and asbestos factories.
• Carbon monoxide, nitrogen dioxide, sulphur dioxide and other such gaseous substances evolved from petroleum refineries.
• Dust and sand in the region of coal mines.
• CO, SO₂ and oxides of nitrogen are released from motor vehicles.
• Extensive deforestation affects the balance of oxygen and carbon dioxide and increases the quantity of carbon dioxide.
• Highly poisonous gas evolved from destructive weapons used in warfare.0 Emission of radioactive rays due to accidents in nuclear reactors, nuclear power plants etc.
• The extensive use of fossil fuel results in the evolution of gaseous pollutants.

Major Air Pollutants

Major air pollutants are divided into two classes:

1. Inorganic and organic gaseous substances and
2. Particulates.

Inorganic and organic gaseous substances

1. Carbon monoxide (CO):

Natural sources:

• During volcanic eruptions,
• Due to decomposition of dead plants and animals in marshy land,
• During forest fires,
• During the extraction of petroleum and natural gases,
• During lightning, carbon monoxide is produced.

Man-made sources:

• Incomplete combustion of fuel in automobile engines,
• Incomplete combustion of carbonaceous matter in industrial furnaces,
• Incomplete combustion of agriculture and slash materials,
• Heating of carbon dioxide at a high temperature in the presence of coke in the blast furnace produces carbon monoxide. Sulphur dioxide (SO₂)

Harmful effects:

This colourless, odourless gas is very harmful

Natural sources:

For human beings and animals. It has a greater affinity towards haemoglobin than of oxygen. So, it readily combines with haemoglobin to give a stable compound, carboxyhaemoglobin

(HbO₂ + CO ⇌ HbCO + O₂ ).

• Consequently, oxygen carrying capacity of haemoglobin decreases which ultimately results in death.
• The presence of CO even in small amounts may result in nausea, headache, dizziness etc.
• Cigarette smoke contains a considerable amount of CO which ultimately increases CO content in the smoker’s blood.
• Thus, chain smokers often suffer from mental imbalance, headaches, heart attacks etc. Acute oxygen starvation in the body due to poisoning by CO is called anoxia or asphyxiation.

Sink of carbon monoxide:

• Some bacteria present in the soil absorb CO and convert it to CO₂.
• Inorganic radicals like HO and HOO, atomic oxygen and ozone oxidise CO to CO₂

Control of CO pollution: 

1. One of the main sources of CO pollution is the use of internal combustion engines in automobiles. These engines emit a mixture of CO, NO_x, hydrocarbons and particulates.
   • To control the CO emission suitable modifications have to be done in internal combustion engines
   • The carburettor is to be adjusted to give a proper air-fuel ratio
   • Catalytic converters can be fitted into the exhaust pipe which may bring about oxidation of CO to CO₂.
   • Internal combustion engines should be modified and improved. Some automobile makers have improved automobile engines by the use of an extra combustion chamber so that fuel undergoes complete combustion and the exhaust gas does not contain CO.
2. CNG, LNG, LPG and LHG (liquefied hydrogen gas) should be used instead of gasoline.
3. Alternative power sources such as fuel cells, solar energy etc., be used.

NCERT Solutions Class 11 Chemistry Chapter 14 Environmental Chemistry

2. Oxides of sulphur (SO_x):

Two oxides of sulphur, sulphur dioxide (SO₂) and sulphur trioxide (SO₃) are represented by the general formula SO_x. Both of them are colourless gases having suffocating odour.

Sulphur Dioxide (SO₂)

Sulphur Dioxide (SO₂)

Natural sources of  SO₂:

SO₂ gas is liberated in the atmosphere due to volcanic eruptions. 67% of the total SO₂ content of the atmosphere is contributed from this source.

Sources created by human activities:

1. Combustion of coal in thermal plants produces SO₂ gas in abundant quantities.
2. SO₂ gas is evolved during roasting of sulphide ores
3. For example:  FeS, CuFeS₂, Cu₂S, ZnS, PbS etc.) in the extraction of metals.
4. Smoke emitted from oil refineries and automobile engines contains a large quantity of SO₂

Harmful effects of SO₂ :

SO₂ is a gas having extremely pungent and suffocating odour. It adversely affects the respiratory system and causes a burning sensation to eyes.

• If inhaled, it may lead to breathing problems, asthma and bronchitis.
• The presence of excess SO₂ in the atmosphere may result in lung cancer.
• SO₂ also causes allergies.
• In the presence of metallic oxide or other catalysts,  SO₂ reacts with oxygen present in the air and water vapour to form H₂SO₄.
• This sulphuric acid is much more harmful since it corrodes building materials such as marble or limestone, and metals like iron, steel, aluminium etc.

The presence of SO₂ hampers the production of chlorophyll and the leaves turn colourless. This is known as chlorosis.

Control of SO₂ pollution:

• Removal of sulphur from fuel i.e., desulphurisation of fuel by using chemical scrubbers
• Use of fuel containing less amount of sulphur
• For example: Natural gas.
• Removal of SO₂ from gaseous fuels.
• Production of electricity in nuclear power plants instead of thermal powerplants

Sulphur Trioxide (SO₃)

1. Sources of SO₃

A significant amount of SO₂, emitted from natural sources or man-made processes is oxidised to sulphur trioxide (SO₃)

1. SO₃ molecules get activated by absorbing radiations having a wavelength of 300-400 run and react with aerial oxygen to give SO₃

2. SO₂ is also oxidised to SO₃ by ozone, hydrogen peroxide and oxides of nitrogen (NO_x) present in the atmosphere

⇒ \(\mathrm{SO}_2+\mathrm{O}_3 \longrightarrow \mathrm{SO}_3+\mathrm{O}_2 ; \mathrm{SO}_2+\mathrm{H}_2 \mathrm{O}_2 \rightarrow \mathrm{H}_2 \mathrm{SO}_4\)

 Harmful effects of SO₃:

SO₃ reacts with water vapour present in the atmosphere to yield sulphuric acid (H₂SO₄). Produced H₂SO₄ combines with water droplets present in air to form an aerosol and ultimately come down as acid rain.

Historical buildings For example:

Tajmahal, Victoria Memorial, monuments, sculptures etc., made up of limestone, marble etc., are adversely affected by

Acid rain*: CaCO₃+ H₂SO₄→ CaSO₄+H₂O + CO₂

Apart from these, metallic (aluminium and iron) structures, bridges, etc., are severely damaged by acid rain. Acid rain increases the acidity(i.e., decreases the pH level of ponds, lakes etc., and in consequence, aquatic plants and animals get dangerously affected.

Acid rain also decreases the stability and glossiness of paint and varnishes.

Sink of SO_x:

Edifices, historical memorials, monuments, and sculptures made of marble, limestone and natural sources of ammonia act as a sink of SO_x.

2. Oxides of nitrogen (NO_x)

Nitrogen forms five oxides—N₂O, NO, N₂O₃, NO₂ and N₂O₅. Out of these, NO and NO₂ act as chief air pollutants. NO and NO₂ are designated

Natural sources of NO_x:

1. During lightning discharge nitrogen and oxygen combine together to form nitric oxide (NO) and this nitric oxide on reaction with excess oxygen, produces nitrogen dioxide (NO₂ ).

N₂ + O₂ → 2NO ; 2NO + O₂ →2NO₂

2. Decomposition of ammonium salts in the soil by some bacteria leads to the formation of oxides of nitrogen (NOx), mainly NO.

Sources of NO_x created by human activities:

In thermal power plants and different industries, atmospheric N₂ and O₂ combine to produce large amounts of nitric oxide (NO). This NO reacts with aerial oxygen to give NO₂.

• The high temperature produced during the burning of fuels in petrol and diesel engines also favours the formation of NO by mutual interaction of atmospheric N₂ and O₂
• NO thus formed is subsequently converted to NO₂ by aerial O₂
• An abundant quantity of NO_x gas escapes into the atmosphere from industries producing organic acid.
• Atomic explosions also add to NO_x in the air.

Harmful effects of NO_x:

1. NO₂ gas is relatively toxic but its adverse effect depends on its concentration in the air and the extent of the reaction.

• Higher concentration of NO₂ in the air produces diseases like inflammation of the lungs, bronchitis, pneumonia etc.
• If NO₂ of higher concentration is inhaled, pulmonary oedema and haemorrhage of the lungs may occur.

2. The most harmful effect of NO_x in the atmosphere is the formation of photochemical smog.

• In the presence of sunlight, hydrogen, nitrogen dioxide and oxygen in the atmosphere, on reaction, gives a mixture of ozone, peroxyacyl nitrate (PAN) and aldehyde.
• All these substances unitedly form photochemical smog. Photochemical smog irritates the eyes and nose and induces sneezing, cough and difficulty in breathing (dyspnoea). If photochemical smog persists for a long period, it may cause death.

3. NO_x drastically reduces the rate of the photosynthesis of plants. Leaves and fruits start shedding due to the presence of NO_x in the air.

4. NO and NO₂ present in the atmosphere react with ozone to form NO₂ and NO₃ respectively. From the latter (z’.e., NO₂ and NO₃), N₂O₅ is obtained. N₂O₅ thus produced, on reaction with rainwater, forms nitric acid

NO +O₃→NO₂+ O₂ ; NO₂+ O₃→NO₃ + O₂

NO₂ + NO₃→N₂O₅; N₂O₅+ H₂O→2HNO

Sink of NOx:

In the atmosphere, the maximum stability of NO and NO₂ are of four days and three days respectively. After that, they are converted to nitric acid (HNO₃). This transformation may occur in two ways following path 1 and path 2 . Ozone plays a major role in the transformation that occurred by Path 2

Path 1: 

4NO₂+ 2H₂O(water vapour or water) + O₂→ 4HNO₃

Path 2:

NO₂ + O₃ → NO₃ + O₂

NO₂ + NO₃ → N₂O₅ ; N₂O₅+ H₂O → 2HNO₃

Nitric acid thus produced comes down during the rain. A part of it falls on waterbodies

For example: Ponds, rivers, lakes etc

The remaining part, on reaction with different basic Compounds belonging to hydrocarbon series (organic substances {e.g., ammonia, lime etc.) present in the soil, Is pollutants): The main air-polluting hydrocarbons are converted into nitrate salts. Here, water and the different basic materials in the soil work as the sink of NO_x.

Control of pollution caused by NO_x:

When the gas released from automobile engines is passed through a catalytic converter in the presence of Pt-catalyst, oxides of nitrogen (NO_x) are reduced to produce mainly N₂ and a small amount of NO₃.

Production of NO_x may be reduced by conducting the combustion at a lower temperature in the presence of excess air.

Before releasing the gas mixture containing NO_x, produced in the factories, it is passed over the metallic oxide catalyst

For example:  Cr₂O₃, ZnO, CuO etc.)

Heated to 500°C, thus nitric oxide (NO) is reduced to N₂ and O₂: 2NO ⇌ N₂ + O₂.

In the chemical absorption process, NO_x can be removed from the waste gas mixture. In this case,

1. Acidic For example H₂SO₄
2. Basic substances For example: Ca(OH)₂ or Mg(OH)₂ are used

3. Hydrogen sulphide (H₂S)

Natural sources:

• H₂S gas is liberated from volcanoes.
• Due to the decomposition of proteinaceous compounds containing sulphur, H₂S has evolved. Thus, rotten fish or eggs smell like H₂S gas.

Sources created by human activities:

• In oil refineries, during the production of paper and paints (containing sulphur) appreciable amount of H₂S is produced. Harmful effects of H₂S .’H₂S is a poisonous gas.
• It has severe harmful effects on man. It causes headache, nausea, irritation to the eyes, throat and nose, loss of appetite and diarrhoea.
• When taken in higher doses, it may lead to respiratory problems like bronchial pneumonia or even death.
• The reaction of H₂S with essential proteins is the primary cause of its toxic effects on human bodies. H₂S binds with iron in the mitochondrial cytochrome enzymes, thus preventing cellular respiration.

Sink of H₂S : H₂S undergoes slow oxidation to SO₂ in the atmosphere

2H₂ S + 3O₂→  2SO₂ + 2H₂O

H₂S + O₃ → SO₂ + H₂O

It also combines with various metallic salts to form insoluble metallic sulphides

4. Compounds belonging to the hydrocarbon series (organic pollutants)

The main air-polluting hydrocarbons are methane, benzene etc. Beiides thiete,
acetylene, ethylene, propylene, 1,3 butadiene etc., act as air pollutants. Among the gaseous air pollutants, the one which Is present In maximum quantity In air Is methane

Natural sources:

• In paddy fields and other muddy marshy lands, bacterial decomposition of plants, almost In the absence of air, produces plenty of methane.
• Putrefaction of animal excreta produces an abundant quantity of methane.
• The anaerobic decomposition of animal bodies produces methane.
• Forest fires and the evaporation of hydrocarbons of the terpenoid class from plants In forests are also important sources of hydrocarbon.

 Sources created by human activities:

• Hydrocarbons are produced due to the incomplete combustion or evaporation of the liquid fuels used in vehicles.
• Hydrocarbons also escape into the atmosphere as a result of incomplete combustion of fuels used for different purposes

For example: Cooking, lighting etc

• Liquid substances such as benzene, toluene etc., used as solvents in different chemical industrial units get easily evaporated and pollute the atmosphere
• Gaseous hydrocarbon, 1,3-butadiene used in the preparation of rubber and other polymers causes air pollution.
• During destructive distillation of coal, some air pollutants [e.g., benzopyrene) escape into the atmosphere.

Harmful effects of hydrocarbons: 

• Methane is a greenhouse gas.
• Due to photochemical reactions with oxygen and oxides of nitrogen, hydrocarbons form photochemical oxidants and photochemical smog, which are responsible for irritating eyes, nose and lungs and also cause breathing problems.
• Polycyclic aromatic hydrocarbons (PAH)
  • For example: Benzopyrene are carcinogenic and its presence in the atmosphere in large amounts may cause cancer.

Sink of hydrocarbon:

Hydrocarbons are sufficiently stable. However, they undergo slow oxidation or photochemical reactions involving several steps and ultimately form products like CO₂ or other water-soluble compounds which are then washed away by rain.

Control of pollution caused by hydrocarbons:

Since automobiles are the main sources of hydrocarbon pollution, so such pollution can be controlled by following those steps which are taken to control CO pollution.

NCERT Solutions Class 11 Chemistry Chapter 14 Environmental Chemistry

Particulates

Generally, finely divided solid and liquid particles, suspended in air are referred to as particulates. The diameter of the particulates varies from 0.0002μ to 500μ (1 micron or 1 μ= 10^-4cm).

The particles of this dimension, being dispersed in air, form aerosol. Depending on the diameter, density of the particles and the intensity of air current, these particles exist in the atmosphere for a period ranging from a few seconds to a few months.

Particulates can be of two types:

• Viable (living microorganisms such as bacteria, viruses, fungi etc.) and
• Non-viable (non-living matters such as mist, smoke, dust etc.)

1. Suspended particulate matter

Smoke, soot, dust particles, metallic oxides and chlorides, fly ash, asbestos dust, acid mist(H₂SO₄, HNO₃)etc.

Natural sources of particulates:

Small particles on the surface of the earth are scattered into the atmosphere by air currents, cyclones, volcanic eruptions etc.

Sources created human activities:

• Soot:  These are fine carbon particles formed by incomplete combustion of carbonaceous fossil fuels.
• Metal particles: These are generated in the metal extraction involving processes like grinding, calcination, smelting of ores etc.
• Particles of metal oxides: During the combustion of fuels containing metal oxides, fine particles of metal oxides are generated.
• For example: Coke containing FeS₂, on combustion, produces fine particles of Fe₃O₄.

3FeS₂ + 8 O₂ → Fe₃O₄+ 6SO₂

• PbCI₂ and PbBr₂:  Compounds like tetraethyl lead (TEL), dichloroethane, dibromoethane etc., are used in petrol as anti-knocking agents. The Pb-compound during combustion of petrol produces PbO which later turns into volatile PbCl₂ and PbBr₂. These further escape into the air and exist as small particles.
• Inorganic silicates: Fine particles of silicates are scattered into the atmosphere from cement industries.
• Asbestos particles: Asbestos is a fibre-like silicate mineral. During the formation of the asbestos sheet, fine particles of asbestos arÿÿatteredinto the atmosphere.
• Sulphuric and nitric acid mist: The SO_x and NO_x  vapours present in the atmosphere react with water vapour and form their corresponding acid mists
• Particles of metal oxides: During the combustion of fuels containing metal oxides, fine particles of metal oxides are generated.
  • For example: Coke containing FeS₂, on combustion, produces fine particles of Fe₃O₄
• Organic particulates: These are mainly the particles of alkane, alkene and aromatic hydrocarbons
  • For example: PAH.
• The formation of such particles is associated with the combustion of petrol and petroleum refining. PAH particles get attached to the soots floating in the air easily and create severe health hazards.

2. Harmful effects Of particulates

The harmful effects of particulates depend on the particle size and the nature of the substances from which these particles originate. Generally, finer particles are more harmful. This is because, the particles with a diameter greater than 5μ get trapped in the nostril but particles having a diameter less than 5μ enter the lungs through the nostril.

The magnitude of the surface area of extremely fine particles being exceedingly high, carcinogenic particulates

For example:

Polycyclic aromatic hydrocarbons, asbestos etc.) can easily find their shelter on these particles and cause cancer, asthma, tuberculosis and different lung diseases. Apart from this, finely divided suspended particles in the air, enter the body through the eyes, ears, nose etc.,

Leads to different types of diseases:

• Smoke released from automobiles contains lead particles which adversely affect the child’s brains and cause nerve diseases.
• The normal production and development of red blood corpuscles (R.B.C.) are disturbed in the presence of lead particles. If an excess amount of lead particles are inhaled regularly, haemoglobin disintegrates and is eliminated through urine
•  Accumulation of cadmium particles in the body during respiration even in trace amounts, may cause breathing trouble and heart disease.
• Beryllium compounds
  • For example: BeCl₂, BeSO, ) affect the lung and create berylliosis disease.
• Workers of coal mines and cotton mills are prone to be attacked by diseases like black lung and white lung.
•  Workers of cement factories are susceptible to five attacks of silicosis due to the intake of SiO₂ particles during respiration.
• Inhalation of asbestos particles results in asbestosis and it leads to cancer in the case of workers of asbestos factories.
• The cause of arsenicosis is due to inhalation of arsenic compounds. In this disease, the skin becomes rough and wounds appear on the skin.

The harmful effects of particulates also extend to the plants present in the atmosphere react with the Water kingdom. Accumulation of dust and other particles on the vapours and their corresponding acid mists. leaves of plants close to the stomata. As a result, transpiration as well as photosynthesis of plants get severely affected. Naturally, the growth of plants and the production of crops also get depreciated

Ozone Layer

In the lower region of the stratosphere (the region at the height of 15 km to 35 km from the ground), there exists a layer of ozone gas. This layer is known as the ozone layer or ozonosphere.

Almost 90% of total ozone gas exists in this layer. In the absence of the ozonosphere, the existence of living beings would have been at stake. It is this ozone layer in the stratosphere which absorbs the harmful ultraviolet radiations coming from the sun. Thus it prevents most of the ultraviolet rays from reaching the Earth’s surface

1. Formation of the ozone layer

Oxygen molecules (O₂) present in the stratosphere absorb ultraviolet radiations coming from the sun and decompose to form oxygen atoms (O). This atomic oxygen combines with oxygen molecules to form ozone molecule

⇒ \(\stackrel{*}{\mathrm{O}}_3+\mathrm{M} \longrightarrow \mathrm{O}_3+\stackrel{*}{\mathrm{M}}\) [*Asterisk sign indicates excited state]

M  denotes a neutral colliding species

For example:

O₂ or N₂ with which O₂  on collision, releases its excess energy] On the other hand, the ozone molecule ( O₃  ) also absorbs UVradiation and gets converted into oxygen molecule ( O₂ )

In the ozone layer of the stratosphere, these two opposite processes (i.e., the formation of ozone molecule and the decomposition of ozone molecules) occur in a cyclic order and finally attain a state of equilibrium. Due to the existence of such an equilibrium state, the quantity of ozone in the stratosphere remains fixed.

2. Role of ozone layer in the environment

The ozone sphere works as a protective layer for the fitting world. Acting as a protective umbrella, the ozone layer absorbs most of the harmful UV radiation emitted by the sun because these rays are utilised in the production and decomposition of ozone.

In the absence of the ozone layer, the earth’s surface and the adjacent air would be so heated by the UV-radiation that the existence of the plants and the animals including bacteria in land and water would have been impossible.’

Ultraviolet radiation:

• Causes cancer in human skin
• Damages the cornea of the eyes and develops premature cataracts
• Decreases immunity against diseases and fertility in living beings. Genetic diseases are also the consequence of the harmful effects of ultraviolet radiation

3. Depletion of ozone layer: Ozone hole

In 1982, the British scientist Jo Foreman first observed that the ozone layer of the stratosphere above the Antarctic (the south polar regions) was gradually getting thinner. Extensive research in the subsequent years has revealed that the ozone layer is gradually becoming thinner not only in Antarctica but almost everywhere in the stratosphere. The phenomenon of thinning of the ozone layer of the stratosphere is known as depletion of the ozone layer or the ozone hole.

Depletion of the ozone layer signifies that the equilibrium between! the two contradictory’ Processes i.e., the formation and the decomposition of ozone has been disturbed somehow. Due to the influx of several foreign substances into the atmosphere, the rate of decomposition of ozone has far exceeded than the rate of its formation. This has resulted in the thinning of the ozone layer almost every where in the stratosphere

Ozone layer before formation of hole:

Ozone layer after formation of hole:

Causes of depletion of ozone layer (formation of ozone hole):

1. Scientists believe that some chemical substances belonging to the class of chlorofluorocarbons (Freons or CFCs) are mainly responsible for the formation of ozone hole. Chlorofluorocarbons (CFCs) are the different chloro and fluoro derivatives of methane and ethane.

Environmental Chemistry Class 11 Notes PDF

Some examples of CFCs are given below:

The above-mentioned chemicals were extensively used as

• Refrigerants
• Propellants in aerosols,
• Foaming agent in plastic production,
• Ingredients of fire extinguisher,
• Solvents for various purposes etc. In the troposphere, these gases are non-corrosive, non-toxic, non-inflammable and chemically inert.

As a result, these gases after being liberated from the field of their applications, gradually reach the upper stratosphere after a long period of time.

In the stratosphere, they absorb the UV radiation coming from the sun and decompose to produce highly active chlorine atoms which subsequently with O₃ of the ozone layer to liberate oxygen and chlorine monoxide free radical (CIO) which, on further reaction with ozone, forms O₂ molecules and active chlorine atoms. The chlorine atoms again combine with O₃ molecules to form O₂ molecules. In this way, a cyclic process continues, which eventually causes depletion of the ozone layer.

⇒ \(\mathrm{CFCl}_3(\mathrm{~g})+h \nu \longrightarrow \dot{\mathrm{C} F C l} 2 \text { (free radical) }+\dot{\mathrm{Cl}} \text { (active) }\)

⇒ \(\mathrm{CF}_2 \mathrm{Cl}_2(\mathrm{~g})+h \nu \longrightarrow \dot{\mathrm{C}} \mathrm{F}_2 \mathrm{Cl} \text { (free radical) }+\dot{\mathrm{Cl}} \text { (active) }\)

⇒ \(\dot{\mathrm{Cl}}+\mathrm{O}_3 \longrightarrow \mathrm{Cl} \dot{\mathrm{O}}+\mathrm{O}_2 ; \mathrm{Cl} \dot{\mathrm{O}}+\mathrm{O}_3 \longrightarrow 2 \mathrm{O}_2+\dot{\mathrm{Cl}}(active)\)

It has been experimentally found that a single Cl -atom is capable of decomposing millions of O₃ molecules.

2. Experiments head revealed that halons widely damage the ozone layer. Halons are halocarbons. Most of them contain bromine as halogen. These are mainly used as fire extinguishers

For example: Halon—1211 (CF₂BrCl, bromochlorodifluoromethane),

Halon: 1301 (CF₃Br, bromotrifluoromethane) etc.

Halons are stable in the troposphere. But in the stratosphere, they absorb UV-radiations and decompose to produce active bromine atoms. These active bromine atoms combine with O₃ of the ozone layer to liberate oxygen and bromine monoxide free radical (BrO).

The produced BrO again reacts with ozone to evolve O₂ molecules and active bromine atoms. Bromine atoms thus obtained, in reaction with O₃, give O₂ molecule. In this way, the entire process proceeds continuously in a cyclic manner which leads to the decay of the ozone layer.

3. Extensive studies have unquestionably proved that increase in quantity of die oxides of nitrogen in the stratosphere adversely affect the ozone layer. The main source of these oxides are the supersonic aeroplanes which emit plenty of NO gas while flying through the stratosphere. Like Cl and Br-atoms, NO molecule brings about catalytic decomposition of O₃ into O₂

NO + O₃→NO₂ + O₂ ; NO₂ + O → NO + O₂

The reaction of O₃ with NO yields NO₂ yields. This NO₂ combines with the oxygen atom (which ”highways produced in the stratosphere due to the decomposition of O₃ and O₂ under the influence of UV-radiation) to regenerate NO. This explains why the O₃ molecules undergo continuous decomposition although the quantity of NO is not diminished.

4. Effect of ozone hole on the environment

Effect on climate:

If the ozone layer in the stratosphere is destroyed, the UV radiation emitted by the sun, instead of being absorbed by this region, will be incident on the earth’s surface. Consequently, the temperature of the earth’s surface will increase. Owing to this rise in temperature the earth will be continuously heated and the ice in the polar regions will melt, resulting in a rise in
the water level of the sea.

Effect on mankind:

In the absence of ozone layer in the stratosphere, the UV radiation will directly reach the earth’s surface. This radiation is extremely harmful to human beings. It causes skin cancer and premature cataract in the eyes. Exposure to the UV-radiation damages the immune system which thereby increases susceptibility to viral infections. Moreover, this radiation motivates the photochemical reactions which increases the tendency of smog formation. This in turn creates severe respiratory problems such as bronchitis, tracheal irritation, lung diseases etc.

Effect on plants, animals and other living organisms:

The incidence of UV radiation on the earth’s surface will hinder the process of photosynthesis. As a result, the production of crops will decrease. UV radiation would naturally increase the earth’s average surface temperature. Water bodies will dry up and water from the soil will evaporate. Consequently, agriculture will be greatly affected and the production of crops will fall drastically.

Also, UV radiation precludes photosynthesis. Therefore plants and aquatic phytoplanktons will die. Thus, marine life which depends on phytoplankton will also perish. Therefore, UV radiation disrupts the entire ecological system and composition of the sea

CBSE Class 11 Chemistry Environmental Chemistry Summary

Greenhouse And Greenhouse Effect

What is a greenhouse?

Greenhouse means a glass room or glass chamber. Plants of the tropical region cannot adapt themselves to the climatic conditions of the cold countries. So for sustaining plant life (particularly for plants of tropical regions in cold countries), this type of chamber made of glass is used. Sun rays enter the glass chamber through the transparent glass roof and walls and due to this the soil gets heated.

The heated soil inside the chamber radiates infrared rays of longer wavelengths which cannot pass through the glass. The glass absorbs a part of these rays and the rest are reflected to the soil inside the chamber. As a result, the temperature inside the chamber always remains higher than that of the outside temperature. Thus, proper growth of these plants becomes possible.

In fact, some gaseous substances present in the earth’s atmosphere such as CO₂ water vapour etc., together act like a glass of the greenhouse and keep the atmosphere adjacent to the earth’s surface warmer and create a favourable environment for the living world.

Greenhouse effect and its importance

Greenhouse Effect Definition:

The natural process by which CO₂  water vapour and some other gases are present in the atmosphere, prevents the return of the radiation emitted by the earth’s surface to outer space, thereby keeping the surface of the earth and the adjacent environment suitable for the effect.

‘ Some gases like CO₂, water vapour etc., present in the atmosphere allow sun rays of smaller wavelengths to be incident on the earth’s surface but prevent the rays of longer wavelength (infrared rays) emitted from the hot earth’s surface, from returning to the outer space. Those gases absorb a significant portion of the reflected radiation of longer wavelength and arc heated.

The rest of the infrared rays fall on the earth’s surface and remain in the adjacent atmosphere to keep the surface of the earth and the adjacent atmosphere warm and make it favourable for the existence of the living world. due to indiscriminate deforestation, the quantity of CO₂ absorbed by the plants is gradually decreasing.

Greenhouse gases like CO₂, water vapour etc., help to keep the atmosphere warm to a certain level of temperature (average as 5°C) which is essential for the existence of life on earth. If those gases were not present in the atmosphere then the average temperature of the earth’s surface and that of the surrounding atmosphere would have dropped to about -30°C and eventually, survival of life on earth would have been impossible

Greenhouse gases, their sources and contribution towards the greenhouse effect

The gases that absorb a significant portion of radiation of longer wavelengths (infrared rays) emitted by the hot earth’s surface and reflect the rest to the earth’s surface, to keep the adjacent environment warm, are referred to as greenhouse gases. Some greenhouse gases are—carbon dioxide (CO₂), methane (CH₄), chlorofluorocarbon (CFCs), ozone (O₃), nitrous oxide (N20), water vapour (H₂O) etc.

1. Carbon dioxide:

Out of all the greenhouse gases, CO₂ is present in the largest amount in the atmosphere. Naturally, CO₂ plays the most vital role in absorbing the radiation emitted by the earth’s surface. The contribution of CO₂ towards the greenhouse effect is approximately 50%.

During respiration, plants and animals take in O₂ and give up CO₂  while plants accept CO₂ for the preparation of their food. In this way, the CO₂  -level in the environment is maintained. But at present, due to the progressive increase in the quantity of CO₂  in air, the equilibrium of CO₂  gas in the atmosphere has been disturbed. The possible reasons for the continuous

Increase in the percentage of COz gas are as follows:

1. 1.  Due to the indiscriminate use of fossil fuels in factories, motor vehicles etc., the quantity of CO₂ released in the atmosphere is not being used up completely by different natural processes.
2. Consequently, the concentration of CO in the atmosphere keeps on increasing.
3. In industrial regions, particularly during the manufacture of cement, a large &ftU>unt of CO₂ gas is released into the atmosphere.
4. During the process of photosynthesis, plants absorb CO₂ from the air. But due to indiscriminate deforestation, the quantity of CO₂ absorbed by the plants is gradually decreasing.

2.  Methane:

The role of methane gas in preventing the outflow of the heat emitted from the earth’s surface is worth mentioning. Due to the bacterial decomposition plants in paddy fields and other marshy lands, putrefaction of dung and other excreta and anaerobic decomposition of dead animals, methane gas is produced. Besides these, different waste organic compounds, oil mines etc., are the other sources of methane gas.

The capacity of each methane molecule to prevent the outflow of heat is 25 times a much as that of a molecule of CO₂  gas. But the quantity of methane gas in the atmosphere being less than that of CO₂ gas, its contribution to the greenhouse effect is about 16-20%.

3. Chlorofluorocarbons (CFCs):

CFCs are widely used as refrigerants, propellants in aerosol sprays, fire extinguishing agents, solvents for cleaning electronic types of equipment and foaming agents. These compounds destroy the ozone layer in the stratosphere and act as greenhouse gases in the troposphere.

The capacity of the chlorofluorocarbon molecules to prevent the release of heat emitted from the earth’s surface is 15000-20000 times greater than that of CO₂ molecules. These compounds are extremely stable. So, they can exist in the atmosphere for a long time. The contribution of these compounds towards the greenhouse effectis found to be 13-18%.

4. Tropospheric ozone:

Ozone gas present in the troposphere acts as a greenhouse gas. The combustion of fossil fuels in automobile engines, thermal power plants and different chemical industries gives rise to a profuse quantity of oxides of nitrogen (NO_x). The combination of ; different hydrocarbons and oxygen present in the atmosphere with these oxides results in the formation of ozone gas. The contribution of ozone gas to the greenhouse effect is about 7-8%.

5. Nitrous oxide:

Extensive combustion of fossil fuels, motor vehicles, and bacterial decomposition of nitrogenous chemical fertilisers in agricultural lands generates nitrous oxide. Again, its heat retention capacity per molecule is 200 times greater than that of CO₂ per molecule. The contribution of this gas towards the greenhouse effect is about 4-5%.

Global warming

For tire last few centuries, the average temperature of Tire Earth has been gradually increasing.

For example: During the period from 1800-1900 AD, the average temperature of the earth has increased by nearly 0.4°G. Again, in the following century i.e., 1900-2000 AD., this increase in temperature has been almost of 1°C. So it cannot be denied that the natural environment is gradually becoming warmer.

The phenomenon of this progressive rise in temperature all over the world is called global warming. The reason for this global warming can be attributed to the increased concentration of greenhouse gases in the atmosphere, caused by various human activities

NCERT Class 11 Chemistry Chapter 14 Environmental Chemistry Notes

Harmful effects of global warming: 

1. Because of global warming 2100 AD, the polar ice caps will melt, thereby releasing an enormous amount of water. Then the situation will be almost similar to what it was 1,30,000 years ago, when the surface water level of the sea was 6 years ago, when the surface water level of the sea was 6 coastal regions like Holland, America, New Orleans, Florida, Bangladesh etc., will be inundated and will go under water forever.

2. Global warming is a great threat to human health. Respiratory problems occur frequently due to human health.

Respiratory problems occur frequently due great concern. Global warning would initiate a favourable temperature for the breeding of microorganisms resulting in the epidemic spread of dreadful diseases such as dengue, malaria, encephalitis etc. If the CO₂ content in the air becomes twice that of the present value, then many species will become extinct from the earth

Consequences of the greenhouse effect

Scientists have predicted about the effect of increased concentration of greenhouse gases such as

• The temperature of the earth’s surface and the troposphere will go to increase day by day and by the middle of this century, the temperature of the earth will be increased by at least of 2°-4°C.
• Due to the increase in earth’s temperature, the polar caps, accumulated in tyre polar regions (Greenland and Antarctica) will melt and this will cause an increase in the water level of the sea. As a consequence, vast coastal regions like India, Bangladesh, Myanmar, Maldives etc., will sink, causing colossal devastation. If the populated area be inundated in this way, the resettlement of the affected people will pose a great problem to many countries
• Owing to the increased greenhouse effect, droughts will be more frequent during summer in the countries of the mid¬ latitudes in the northern hemisphere. As a result, crop production in the fertile lands of North America and the previous Soviet Russia will be reduced.
• More devastating cyclones, supercyclones, tornados or hurricanes will occur with an increase in temperature.
• Increase in temperature may lead to the destruction of forests due to forest fires.
• Due to the inability to sustain at high temperatures, living beings will die. Consequently, the ecosystem will be severely affected. In the marshy lands, due to increased decomposition of plants, methane will be liberated.

In a word, it can be said that by the middle of this century, man will have to face severe natural calamities.

Measures to check global warming

Global warming cannot be eradicated or reduced overnight. A concerted effort is necessary to attain this goal. In overnight. A concerted effort is necessary to attain this goal.

In measures are mentioned below:

• The addition of CO₂ to the atmosphere should be minimised by reducing the use of fossil fuels such as wood, coal, petroleum etc.
• Unlawful cutting of trees should be stopped and the forests should be saved from destruction.
• Afforestation ought to be encouraged so that plants absorb more CO₂ (for the preparation of their food). Koiget !r ’Boron
• Use of unconventional forms of energy
• For example: Solar energy, wind energy, tidal energy etc.) is to be increased.
• Use of CFCs is to be prohibited

Smog Or Classical Smog

In December 1952, the city of London was covered with a dense layer of fog continuously for five days. The people, irrespective of age and sex, fell ill and 4000 people ultimately lost their lives.

Subsequently, it was found that the fog contained poisonous gases emitted from automobile engines and factories and the constituent which was present in the largest quantity, was found to be sulphur dioxide gas (SO₂). A British physician named it smog (smog = smoke + fog). As the horrible effect of such smog was first observed in London, it was called London smog

Several accidents caused by such smog (of course of less alarming proportions), occurred in different cities. Smog is frequently observed in big cities

For example:

Delhi, Mumbai Kolkata) of India, during the winter season. Mixture of particulates with gaseous oxides of Mixture of particulates with gaseous oxides of the presence of SO₂ and carbon (soot) particles, classical smog possesses a reducing character. Thus it is also called reducing smog

1. Formation of smog

During winter, particularly after evening or early in the During winter, particularly after evening or early in the earth’s surface becomes heavier If it is suddenly cooled down earth’s surface becomes heavier ifit is suddenly cooled down cannot go upwards and remains confined in that layer. Impure cannot go upwards and remains confined in that layer. Impure finely divided particles liberated from local factories motor vehicles, mix with that confined air to create finely divided particles liberated from local factories motor vehicles, mix with that confined air to create and and

Harmful effects of smog:

• Smog irritates the nose, eyes and throat, resulting in sneezing and coughing.
• It affects the respiratory system, causing bronchitis, asthma, heart disease etc.
• It lowers visibility level, posing great problems while driving cars. So, accidents are likely to happen.
• It also has adverse effects on electronic systems and plants

2. Photochemical smog or Los Angeles smog

This type of smog was first observed in the city of Los Angeles in America, in the year 1950. So, it is called Los Angeles smog. Highly poisonous substances like nitrogen dioxide(NO₂) and ozone (O₃). peroxyacyl nitrate (PAN), smog was formed due to chemical reactions in the presence of bright sunlight, it is commonly known as photochemical smog. Generally, during the mid-days of the summer when the sun shines brightly, (i.e., the intensity of solar radiation is very high) this kind of smog is observed

In big cities, where there is considerable vehicular traffic on the roads throughout the whole day and night, the atmosphere contains the oxides of nitrogen particularly, nitrogen dioxide (NO₂) in the largest proportions.

Apart from these, hydrocarbons (produced by evaporation or incomplete combustion of liquid fuels) and other gaseous substances

For example: SO₂> CO₂  are present.

In the presence of bright substances like ozone (O₃), peroxyacyl nitrate (RCO₃NO₂), aldehyde and ketone. These gaseous substances and the particulates mix together in the air to form photochemical smog.

It formation of smog is to be noted that, it is not real smog, because it contains particulates mix in air to form photochemical smog. The formation of smog is to be noted that, it is not a real smog, because it contains Hence it is also known as oxidising smog

Mechanism for the formation of photochemical smog:

In the presence of sunlight, nitrogen dioxide (NO₂) molecule decomposes into nitric oxide (NO) molecules and atomic oxygen

In the reaction of hydrocarbons with this atomic oxygen, at first highly reactive free radical RCO is produced and this radical again combines with oxygen molecule to give peroxyacetyl radical

Peroxyacyl radical is highly reactive. It combines with hydrocarbon,O₂ and XO₂ to form a mixture of aldehyde, ketone, ozone and peroxyacyl nitrate respectively. This peroxyacylnitrate is extremely harmful for eyes.

Harmful effects of photochemical smog:

• Presence of large amounts of ozone (O₃), peroxyacetyl nitrate (PAN), acetaldehyde, ketone etc., causes
  • Irritation of eyes, nose, and throat but it sill effect on the eyes is much more intense
  • Congestion of nostrils, sneezing and cough
  • Respiratory problems and chest pain.
• By brown colour, it reduces visibility and hence the car drivers and pilots face extreme difficulties.
• PAN and other oxidising materials damage plant cells and produce white spots on leaves.
• PAN hinders the process of photosynthesis.
• Rubber goods lose elastic properties and become brittle.

To control or suppress the formation of photochemical smog, the following methods can be adopted.

• Certain chemical compounds, which are capable of generating free radicals, are sprayed into the atmosphere. The free radicals readily combine with the free radicals responsible for the formation of photochemical smog (such as O^•, R^•, R^•O, ^•H etc.) and hence nullify their effects.
• Efficient catalytic converters are being developed for installation in automobiles so that emission of nitrogen oxides and hydrocarbons can be prevented or minimized

Certain plants such as Pinus, Juniperus, Pyrus, Vritis etc., can directly assimilate oxides of nitrogen for their metabolic activity. So their plantation could be helpful.

Comparison between ordinary smog and photochemical smog:

NCERT Solutions for Environmental Chemistry Class 11

Acid Rain

Ordinary water is slightly acidic (pH = 5.6) because a portion of carbon dioxide gas present in the air gets dissolved in water and forms carbonic acid

CO₂ + H₂O→H₂CO₃ .

But if rainwater contains an excess amount of dissolved acid, then it is called acid rain. Acid rain is mainly a mixture of H₂SO₄, HNO₃ and HCl. The pH of such rainwater generally lies within the range of 5.6 to 3.5. The proportion of the above acids in the rainwater of different localities depends upon the quantities of sulphur dioxide (SO₂), nitrogen oxides (NO_x) and hydrogen chloride (HCl) present in the air of that particular locality

1. Origin of Acid Rain

Huge quantities of the oxides of sulphur, nitrogen and carbon (SO_x, NO_x, CO_x etc.) are released In the air due to natural processes as well as tyre human activities. These oxides combine with oxygen, ozone and water vapour present in air to give different acids. These acids float in the air in the form of fine particles as an aerosol. Moreover, HCl gas is also liberated in sufficient quantity from the factories. These acids come down to the earth through dew, snow and rainfall

H₂SO₄ has the highest contribution (60-65%) to acid rain followed by HNO₃ having 30-35% contribution.

2. Harmful effects of acid rain

Effect on soil and plants:

Acid rain increases the acidity of the soil, changes the solubility of different metals and metallic oxides present in the soil. Thus, living creatures and bacteria living inside the soil are severely affected or die. As a result, the fertility of the soil decreases and the production of crops Is drastically reduced. Due to increased acidity of the soil, leaf pigments get spoiled, the process of photosynthesis and as a consequence the growth of plants and their immunity drastically fall, i.e., agricultural productivity is reduced.

Effect on aquatic plants and animals:

Due to acid rain, the pH of different water bodies decreases significantly. As a result, the production of spawn of fish is reduced. The biological processes of fishes are affected. An increase in acidity results in the elimination of many species of algae and zooplankton, aquatic insects, fishes etc. That is, polluted water disrupts the aquatic food chains and consequently disturbs the ecosystem. In countries like America, Sweden etc., in a large number of lakes, virtually no fish exists due to acid rain.

Effect on mankind:

Acid rain dissolves different metallic substances. These dissolved substances enter human bodies through water and result in severe health hazards. Acid rain has profound ill effects on human skin, hair and cells. H₂SO₄ and HNO₃ present in acid rain enter the human body and adversely affect the nervous, respiratory and digestive systems.

 Effect on architecture and edifice:

Because of acid rain buildings, monuments of historical importance

For example:

Tajmahal, Victoria Memorial, British Parliament House), states, sculptures etc., made of marble;, limestone, dolomite, mortar and slate suffer irreparable damages

Marble, limestone etc., react with H₂SO₄ to form an insoluble layer of CaSO₄ and lose its glossiness

CaCO₃+ H₂SO₄ → CaSO₄ ↓+ CO₂↑ + H₂O

Few years ago, scars developed on the surface of Tajmahal Few years ago, scars developed on the surface ofTajmahal Few years ago, scars developed on the surface of Tajmahal

CBSE Class 11 Environmental Chemistry Important Topics

Slone cancer:

• The scars that are developed on the surface of architectural edifices, memorials, sculptures etc., are termed stone leprosy or stone cance
• Metallic surfaces,
  • For example: Aluminium, steel or iron structures, bridges etc., exposed to acid rain, suffer steady.
• Textile materials, leather and paper products are also not spared from the ill effects of acid rain

3. Measures to check acid rain

Only the drastic reduction in the quantities of SOx and NOx in the environment can eliminate the apprehension rather than the threat of acid rain.

The following measures can be adopted to check acid rain:

Use of fossil fuels is to be decreased as far as practicable. Fuels of low sulphur content should be used so that the emission of SO₂ can be controlled.

Suitable technological devices should be developed for the removal of those gases

For example: SO_x, NO_x etc.)

Released from the thermal plants, factories, furnaces for metal extraction and various other sources

Vehicles involving engines with improved technology must be launched so that the emission of NO_x can be controlled

CBSE Class 11 Chemistry Notes For Chapter 10 S Block Elements Group 1 Elements (Alkali Metals) Introduction

All the alkali metals have one valence electron ( ns¹ ) outside the noble gas core. The loosely held s -electronin the outermost valence shell makes them the most electropositive metals.

• To get the stable electronic configurations of noble gases, they readily lose the valence electron to generate the monovalent (M⁺) ions. Hence, they are never found in a free state but in the combined state of nature.
• Since the last electron enters ns -orbital, these are called s -block elements.
• Since all these elements have similar valence shells or outer electronic configurations, all the alkali metals exhibit a striking resemblance in their physical and chemical properties and they are placed in a definite group (Gr-1).
• Lithium shows some abnormal behavior as its electronic configuration is slightly different from the rest of the members of Gr-1 and also because of its extremely small atomic and ionic radii.

Again, lithium shows some similarities with magnesium present in the group- 2 of the third period

Electronic configuration of alkali metals: 

Occurrence Of Alkali Metals

• Since the alkali metals are highly reactive, they do not exist in a free state. In nature, they mostly occur as compounds like halides, oxides, silicates, borates, and nitrates.
• According to abundance, lithium is placed at the 35th position. It mainly occurs in nature in the tire form of silicates,
• For example: Spodumene: LiAl(SiO₃)₂ and Lepidolite: Li₂Al₂(SiO₃)₃(F, OH)₂
• Sodium and potassium are respectively placed at 7th and 8th position in order of their abundance. Sea water is a major source of NaCl and KCl.
• Sodium is abundantly present in the form of rock salt (NaCl). Other important minerals are Chile salt petre: NaNO₃, borax: Na₂B₄O₇.10H₂O, mirabilite: Na₂SO₄, and trona: Na₂CO₃-NaHCO₃-2H₂O.
• Important ores of potassium are sylvite: KCl, carnallite: KCl-MgCl₂-6H₂O, and feldspar: (K₂O-Al₂O₃-6SiO₂).
• Rubidium and cesium are much less abundant than lithium. Radioactive francium does not occur appreciably in nature. It is obtained from the radioactive decay of actinium
• ²²⁷Ac ₈₉ → ²²³Fr₈₇  +⁴He₂  Its longest-lived isotope ²²³Fr₈₇  has a half¬life period of only 21 minutes.
• Since most of the compounds of alkali metals are water soluble, they are found in adequate amounts in seawater.

CBSE Class 11 Chemistry Notes Chapter 10 S Block Elements

General Trends In Atomic And Physical Flv Properties Of Alkali Metals

The alkali metals show regular trends in their physical and chemical properties with an increase in atomic number. Some important atomic and physical properties of alkali metals are given in the following table:

Atomic and physical properties of alkali metals:

General trends in different atomic and physical properties of alkali metals and their explanations:

1. Atomic and ionic radii 

The atomic and ionic radii of alkali metals are the largest in their respective periods and these values further increase on moving down the group from Li to Cs.

Atomic and ionic radii  Explanation:

On moving from left to right in a period, the number of electronic shells remains the same but the nuclear charge increases with each succeeding element Thus, the valence shell electrons experience a greater pull towards the nucleus and this results in successive decreases in atomic and ionic radii with an increase in atomic number

• Thus, the atomic and ionic radii of alkali metals are the largest in their respective periods.
• On moving down the group, a new electronic shell is ) added to each element and the nuclear charge increases with an increase in atomic number.
• The addition of an electronic shell tends to increase the size of the atom but the increase in nuclear charge tends to decrease the atomic radii by attracting the electron cloud inward. Thus, the two factors oppose each other.
• However, the increase in the number of shells increases the screening effect of the inner electrons on the outermost s -electron and as the screening effect is, quite large, it overcomes the contractive effect of the increased nuclear charge.
• The net result is an increase in atomic and ionic radii down the group from Li to Cs.

2. Ionization enthalpy

The first ionization enthalpies of alkali metals are the lowest in their respective periods. Explanation: Since the alkali metal atoms are largest in their respective periods, their outermost electrons being far away from the nucleus experience less force of attraction and hence, can be removed easily.

1. Ionization enthalpy of group-1 alkali metals decreases down the group.

Explanation:

Since the alkali metal atoms are largest in their respective periods, their outermost electrons being far away from the nucleus experience less force of attraction and hence, can be removed easily.

2. Ionisation enthalpy7 of group-1 alkali metals decreases down the group

Explanation:

On moving down the group from Li to Cs, the distance of the valence s-electron from the nucleus progressively increases due to the addition of a new shell with each succeeding element With an increase in the number of inner shells, the screening effects progressively increase and as a result, the effective nuclear charge experienced by the valence electron progressively decreases and hence, the ionization enthalpies decrease down the group.

3. The second ionization enthalpies of alkali metals are very high.

Explanation:

The monovalent cation formed by the removal of an electron from the alkali metal atom has a very stable noble gas configuration,

For example – 

1. Li⁺: 1s² or [He], Na⁺: 1s²2s²2p⁶ 
2. [Ne], k⁺: 1s²2s²2p⁶3s²3p⁶ or [Ar] etc.

Removal of another electron from the monovalent ion having a stable noble gas configuration is very difficult and requires a huge amount of energy. For this reason, the second ionization enthalpies of alkali metals are very high.

3. Hydration of ions, hydrated radii, and hydration enthalpy

The salts of alkali metals are generally ionic and are soluble in water because the cations get hydrated in water to form hydrated cations: M⁺ + aq —> [M(aq)]⁺.

1. The degree of hydration of ions and the hydrated radii decrease as we move down the group

Explanation:

The smaller the cation, the greater its degree of hydration. Since ionic radii increase down the group, the degree of hydration decreases, and consequently, the radii of die-hydrated ions decrease from Li⁺ to Cs⁺.

2. The order of mobilities of die alkali metal ions in aqueous solution is: Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺

Explanation:

Smaller ions are more easily hydrated. As Li+ is the smallest ion among the given ions, it is most easily hydrated and has the least ionic mobility in an aqueous solution whereas Cs+ is the largest and is least hydrated. So its mobility is die highest.

3. Ionic conductance of the hydrated ions increases from [Li(m7)]+ to [Cs(aq)]+.

Explanation:

The ionic conductance of these hydrated ions increases from [Li(aq)]⁺ to [Cs(ag)]⁺ because die size decreases and mobility increases in this order. Hydration of ions is an exothermic process. The energy released when 1 gram-mol of an ion undergoes hydration is called hydration energy or hydration enthalpy,

4. Hydration enthalpy of alkali metal ions decreases from Li⁺ to Cs⁺.

Explanation:

The hydration enthalpy of an ion depends upon the ratio of charge to radius (q: r). Since the radii of alkali metal ions increase down the group, the hydration enthalpies decrease from Li⁺ to Cs⁺. Li⁺ ion has the maximum degree of hydration and for this reason, most of the lithium salts are found to be hydrated

For example: LiCl-2H₂O, LiClO₄-3H₂O etc.

4. Oxidation State

Alkali metals exhibit a +1 oxidation state in their compounds and it remains restricted in a +1 state only.

Oxidation State Explanation: 

Alkali metals have low ionization enthalpies and by losing their valence s -electrons they acquire the stable electronic configurations of the nearest noble gases. Thus, they have a strong tendency to form M⁺ ions and exhibit a +1 oxidation state in their compounds.

The second ionization enthalpies required to pull out another electron from M⁺ ions having very unstable noble gas electronic configuration are very high indeed and are not available under the conditions of chemical bond formation. Hence,v the alkali metals do not form M²⁺ ions, i.e. their oxidation state remains restricted to +1 state.

5. Metallic character

The elements of this group are typical metals that are soft (can be easily cut with a knife) and light. When freshly cut, they are silvery white but on exposure to air, they turn tarnished. The metallic character, which refers to the level of reactivity of a metal, increases on moving down the group.

Metallic character Explanation:

As the ionization enthalpy decreases down the group,  the tendency to lose the valence electron increases, and consequently, the metallic character increases.

6. Photoelectric effect

Alkali metals (except Li) exhibit a photoelectric effect. The emission of electrons from the surface of a metal exposed to electromagnetic radiations of suitable wavelength is called the photoelectric effect.

S Block Elements Class 11 NCERT Notes

Photoelectric effect Explanation:

Due to low ionization enthalpies, the alkali metals exhibit a photoelectric effect. It is to be noted that lithium having the highest ionisation enthalpy does not exhibit a photoelectric effect. Cesium having the lowest ionisation enthalpy possesses the highest tendency to exhibit a photoelectric effect.

Potassium anti-cesium, rather than lithium is used in photoelectric cells:

The ionization enthalpies of potassium and cesium are much lower than that of lithium. For this reason, these two metals on exposure to light easily emit electrons from their surface but lithium does not. Hence, potassium and cesium rather than lithium are used in photoelectric cells.

7. Electronegativity

The alkali metals have low electronegativity which further decreases down the group.

Electronegativity Explanation:

The alkali metals having ns¹ electronic configuration preferably show electron releasing tendency rather than electron accepting. Thus, they have low electronegativities. Since the atomic sizes increase down the group, the tendency of atoms to hold their valence electrons decreases down the group, and consequently, electronegativity decreases down the group.

8. Conductivity

Alkali metals are good conductors of heat and electricity.

Conductivity Explanation:

Due to the presence of loosely bound valence electrons (ns¹) which are free to move throughout the metal structure, the alkali metals are good conductors of heat and electricity.

9. Melting and boiling points 

Melting and boiling points: Melting and boiling points of alkali metals are low and decrease down tire group.

Melting and boiling point Explanation:

The cohesive energy that binds the atoms in the crystal lattices of these metals is relatively low (weak metallic bonding) due to the presence of only one valence electron (ns¹) which can take part in bonding. Hence, their melting and boiling points are low. These further decrease down the group as the strength of the metallic bonds and cohesive energy decrease with increasing atomic size.

10. Nature of bonds formed

Alkali metals form ionic compounds and the ionic character of compounds increases down the group from Li to Cs.

Nature of bonds formed Explanation:

For low ionization enthalpies, alkali metals readily form monovalent cations by losing their valence electrons. As ionization enthalpies decrease down the group, the ionic character of the compounds increases down the group.

11. Density

The densities of alkali metals are quite low and increase down the group from Li to Cs.

Density Explanation:

Due to their large atomic size and weak metallic bond, alkali metals have low density. Both the atomic volume and the atomic mass increase down the group but the corresponding increase in atomic mass is not balanced by the increase in atomic volume. As a result, the densities of alkali metals increase down the group. However, the density of K is less than that of Na because the atomic size and atomic volume of potassium are quite higher than that of sodium. As a result, the ratio of mass/ volume decreases.

Li is the lightest metal having a density of 0.53 g. cm^-3. It cannot be preserved in kerosene because it floats over it. Generally kept wrapped in paraffin wax.

12. Flame coloration

Alkali metals or their salts on heating in the flame of the bunsen burner, impart characteristic colors and they can be easily identified from the color of the flame

Flame coloration Explanation:

Ionization enthalpies of alkali metals are not much higher. Thus, when an alkali metal or its salt (especially chloride due to its more volatile nature) is heated in a Bunsen burner flame, the electrons in the valence shell get excited and jump to higher energy levels by absorbing energy. When the excited electron drops back to its ground state, the emitted radiation falls in the visible region and as a result, alkali metals or their salts impart color to the flame.

Alkali metals can be detected by flame tests and can be estimated by flame photometry or atomic absorption spectroscopy.

13. Softness

Alkali metals are soft (can be cut easily with die help of a knife) and their softness increases down the group.

Softness Explanation:

The softness of alkali metals is due to their low cohesive energy and weak metallic bonding. Further, on moving down the group, the strength of metallic bonding decreases due to an increase in atomic size and as a result, the softness of the metals increases down the group.

Chemical Properties Of Alkali Metals

Alkali metals are highly reactive. Such reactivity may be attributed to their large atomic size, low ionization enthalpies, and low heats of atomization. –

Action of air and moisture

Alkali metals, being highly reactive, react readily with atmospheric oxygen to form oxides. These oxides further react with moisture to form hydroxides which in turn produce carbonates by reacting with atmospheric CO₂

Metal oxides also react with CO₂ to form carbonates.

These metals lose their glossiness and become tarnished due to the formation of carbonate layers on their surface. To protect from atmospheric oxygen and moisture, these metals are always stored in inert hydrocarbon solvents such as kerosene, petroleum ether, etc.

Reaction with oxygen

When the alkali metals are heated with oxygen or excess air, they form different types of oxides depending upon the nature of the metal involved. Lithium mainly forms monoxide (Li₂O), sodium forms peroxide (Na₂O₂), and the other alkali metals (K, Rb, and Cs) mainly form superoxides having the general formula MO₂. The temperature required for the reaction decreases down the group from Li to Cs.

4Li + O₂ → 2Li₂ O₂ (Lithium monoxide)

2Na + O₂ → Na₂O₂ (Sodium peroxide)

M +O₂ → MO₂ (Superoxide) [here, M = K, Rb, Cs]

 Oxygen Explanation

A smaller cation can stabilize a smaller anion while a larger cation can stabilize a larger anion. If both the ions are similar in size, the coordination number will be high and this results in higher lattice energy.

A cation having a weak positive electric field can stabilize an anion having a weak negative electric field. Li⁺  ion and oxide ion (O^2-) have small ionic radii and high charge densities. Hence, these small ions combine to form a very’ stable lattice of Li₂O.

Sodium forms peroxide (Na₂O₂) but potassium forms superoxide (KO₂), even though the peroxide ion,   is larger in size than the superoxide ion, This can be explained in terms of charge densities. Due to the bigger size

Na+ ion has a weaker positive field around It and therefore it can stabilise peroxide Ion which also has a weaker negative field around it. Thus, Na⁺ forms peroxide. K⁺ ion is still bigger in size and the magnitude of the positive field around it is much weaker so it can’t stabilize superoxide ion which also has a much weaker negative field around it. Hence, K forms superoxide.

According to valence bond theory, two O-atoms in superoxide ion (O₂^–) are attached by a 2- 2-electron bond (common covalent bond) and a 2-electron bond. As the unpaired electron is present in the 2-electron bond, the superoxide ion is paramagnetic and all tire superoxides are colored (LiO₂ and NaO₂ are yellow, KO₂ & CsO₂ are orange, RbO₂ is brown).

According to MO theory, there is an unpaired electron in one of the n antibonding orbitals and because of this, the superoxide ion is paramagnetic. The electronic configuration of O₂^–

O₂^–  ion present in common oxides [For example,  Li₂O, NaO₂etc.) and the 02~ ion in peroxides (For example,  Na₂O₂), contain no unpaired electron and for this reason, these are diamagnetic and colorless.

Reaction with water

The alkali metals having high negative reduction potentials (E°) can act as a better-reducing agent than hydrogen. Hence, they react with water to form water-soluble hydroxides and liberate hydrogen gas.

2M + 2H₂O → 2MOH + H₂T [M = alkali metal]

Reactivity with water increases down the group as the electropositive character of the metals increases clown the group. Lithium decomposes water slowly. Sodium reacts with water vigorously. K, Rb, and Cs react with water explosively and the evolved hydrogen gas catches fire.

Alkali metals also react with compounds containing acidic H-atoms

For example:  halogen halides (HX), alcohols (ROH), acetylene HC=CH, etc., to form their corresponding salts and H₂

2Na + 2HX → 2NaX  (Sodium halide) + H₂ ↑

Li + 2C₂H₅OH → 2 C₂ H₅ OLi (Lithium ethoxide) + H₂ ↑

2Na + 2HC=CH →  2NaC = CH (Sodium acetylide)+ H₂ ↑

The standard electrode potential of Li is most negative while that of sodium is least negative i.e., in the reaction with water, Li releases a greater amount of heat than sodium. Despite that, Li reacts less vigorously with water than Na. j This can be explained concerning chemical kinetics. Na has a low melting point and the heat of the reaction is sufficient j to melt it. Molten metal spreads out and exposes a relatively large surface to water and as a result, it reacts with water 1 readily and violently. On the other hand, the melting point of Li is much higher and the heat of the reaction is not sufficient to melt it. Hence, its surface area does not increase and it reacts slowly with water.

NCERT Solutions Class 11 Chemistry Chapter 10 S Block Elements

Reaction with dihydrogen

All alkali metals react with dihydrogen at about 673 K (Li at 1073 K) to form colorless, crystalline hydrides (MH). These hydrides have a high melting point.

1. The reactivity of the alkali metals towards dihydrogen decreases down the group.

Explanation:

As the size of the metal cation increases down the group, the lattice energy of the hydrides decreases down the group. Consequently, the reactivity of the alkali metals towards hydrogen decreases down the group,

2. The ionic character of the alkali metal hydrides increases from Li to Cs.

Explanation:

Since the ionization enthalpy of alkali metals decreases down the group, the tendency to form cations as well as the ionic character of the hydrides increases.

Reaction with halogens

All alkali metals react vigorously with halogens to form crystalline halide compounds having the general formula MX. Lithium halides are covalent due to the very high polarising power of the Li⁺ ion. Halides of other alkali metals are ionic in nature’

⇒ \(2 \mathrm{M}+\mathrm{X}_2 \rightarrow 2 \mathrm{M}^{+} \mathrm{X}^{-}\) ( X = F, Cl, Br or I)

The reactivity of the alkali metals towards a particular halogen increases down the group.  Due to a decrease in ionization enthalpies or an increase in the electropositive character of the metals down the group, the reactivity increases down the group.

The reactivity of halogens towards a particular alkali metal decreases in the order:

F₂> Cl₂ > Br₂ > I₂

Reducing nature

The alkali metals act as strong reducing agents because of their low ionization enthalpies. Since the ionization enthalpies decrease on moving down the group, therefore, in the free state the reducing power also increases in the same order, i.e., Li  < Na < K < Rb < Cs.

The tendency of a metal to lose an electron in solution is measured by its standard electrode potential (E°). The alkali metals have low values (higher negative values) of E° and so they have a strong tendency to lose electrons and can act as strong reducing agents. Lithium, although, has the highest ionization enthalpy, is the strongest reducing agent in solution (E° = -3.04 V). On the other hand, Na is the weakest reducing agent and the reducing character increases from Na to Cs, i.e., Na < K < Rb < Cs.

Explanation of anomalous behavior of lithium

The anomalous behavior of lithium can be explained because the ionization enthalpy is the property of an isolated atom in the gaseous state while the standard electrode potential is concerned when the metal atom goes into solution.

The ionization enthalpy involves the change:

M(g) → M⁺+(g) + e, while the standard electrode potential involves the change: M(s) → M⁺(aq) + e.

The latter change occurs in three steps as follows:

1. M(s)→ M(g) – sublimation enthalpy
2. M(g)→ M+(g) + e – ionisation enthalpy
3. M⁺ (g) + H₉O → M⁺(aq) + hydration enthalpy

The overall tendency for the change depends on the net effect of these three steps. Among the alkali metal cations, Li⁺ ion has the maximum tendency to get hydrated due to its very small size. The high hydration enthalpy compensates the energy required in the first two steps to a large extent and the overall energy required to convert M(s) to M⁺ (aq) is minimum for lithium.

Thus, small size and high hydration enthalpy are responsible for the strong reducing character of lithium.

The solution in liquid ammonia

Alkali metals dissolve in liquid ammonia to give highly conducting deep blue solutions which are highly reducing and paramagnetic. As the concentration increases (> 3M), the color of the solution changes to copper-bronze. These concentrated solutions are diamagnetic.

Solution in liquid ammonia Explanation:

1. When an alkali metal is dissolved in liquid v ammonia, ammoniated cations, and ammoniated electrons are formed as shown below:

M + (x+ y)NH₃ →  [M(NH₃)_x]⁺ (Ammoniated cation) + [e(NH₃)_y]^–(Ammoniated electron)

2. The blue color of these solutions is due to the excitation of the free ammoniated electrons to higher energy levels by absorbing energy corresponding to the red region of visible light.  The transmitted light is blue which imparts a blue color to the solutions.

3. With the increase in the concentration of the alkali metal, the formation of clusters of metal ions starts and because of this, at a much higher concentration (> 3M) the solutions possess metallic luster and attain the color of copper-bronze.

4. These blue solutions are highly conducting because of the presence of ammoniated electrons and ammoniated cations but the conductivity decreases with increasing concentration as the ammoniated cations get attached to the free unpaired electrons.

5. These blue solutions are paramagnetic due to the presence of unpaired electrons. However, tire paramagnetism decreases with increasing concentration due to the association of ammoniated electrons to yield diamagnetic species.

6. The free ammoniated electrons make these solutions very powerful reducing agents.

7. These solutions when kept, form metal amides and release H₂. However, these solutions can be stored in anhydrous conditions in the absence of impurities like Fe, Pt, Zn, etc.

M⁺(am) + e(am) + NH₃(l)→ MNH₂(am) + ½H₂(g)

Where ‘am’ stands for ‘solution in ammonia

2M + 2NH₃→ 2MNH₂ (metal amide)+ H₂

Extraction of alkali metals

Alkali metals cannot be extracted by applying common processes used for the extraction of other metals.

Alkali metals Explanation:

• The alkali metals are strong reducing agents. Hence, they cannot be extracted by reduction of their oxides or other compounds.
• Since they are highly electropositive, the method of displacing them from their salts by any other element is not possible.
• The aqueous solution of their salts cannot be used for extraction by electrolytic method because hydrogen, instead of the alkali metal is discharged at the cathode (discharge potentials of alkali metals are much higher).
• However, by using Hg as a cathode, the alkali metals can be deposited but in that case, the alkali metals readily combine with mercury to form amalgams from which the recovery of metals becomes quite difficult.
• The electrolysis of their fused salts (usually chlorides) is the only successful method for their extraction, Another metal . ‘ chloride is generally added to lower its fusion temperature

General Characteristics Of The Compounds Of Alkali Metals

The compounds of alkali metals are predominantly ionic. Some of the general characteristics of these compounds are described below.

1. Oxides and hydroxides

1. Typical oxides or monoxides of alkali metals

For example: Li₂O and Na₂O are white ionic solids and basic. These oxides react with water to form strong alkalis (MOH).

Example:  Na₂O + HO → 2NaOH

2. All peroxides are strong oxidizing agents. They react with water or acid to give hydrogen peroxide (H₂O₂) and the corresponding metal hydroxide. Na₂O₂ is widely used as an oxidizing agent in inorganic chemistry.

M₂O₂ + 2H₂O →  2MOH + H₂O₂

Example: Na₂O₂ + 2H₂O₂→ 2NaOH + H₂O₂

3. Superoxides are stronger oxidizing agents than peroxides and react with water or acid to give both H₂O₂ and O₂ along with metal hydroxide.

2MO₂ + 2H₂O→  2MOIH + H₂O₂+ O₂

Example: 2KO₂ + 2H₂O₂ →2KOH + H₂O₂ + O₂

The alkali metal hydroxides (MOH) are all white crystalline solids and corrosive. They are the strongest of all bases and readily dissolve in water. Due to excess hydration, a large amount of heat is released. These hydroxides are thermally stable except Li OH. The basic strength of alkali metal hydroxides increases on moving down the group from Li to Cs.

Explanation:

The ionization enthalpies of alkali metals decrease on moving down the group and this causes a weakening of the bond between the alkali metal and the hydroxyl group (M —OH). This results in an increase in the concentration of hydroxyl ions in the solution, i.e.,  the basic character of the solution increases on moving down the group.

Thus, the basic strength of the hydroxides follows the order:

CsOH > RbOH > KOH > NaOH > LiOH

2. Halides

The alkali metal halides can be prepared by combining metals directly with halogens or by reacting appropriate oxides, hydroxides or carbonates with aqueous halogen acids (HX).

2M + X₂  →  2MX ; M₂ O + 2HX↓ 2MX + H₂ O

MOH + HX→ MX + H₂ O

M₂ CO₃+ 2HX→ 2MX + HO₂ + CO₂

The enthalpy of formation (ΔH°_f) of alkali metal halides is highly negative. For a given metal, AHj values decrease from fluoride to iodide. These halides are colorless crystalline solids having high melting and boiling points.

1. The melting point of halides of a particular alkali metal decreases as:

Fluoride > Chloride > Bromide > Iodide.

Explanation:

For a particular alkali metal ion, the lattice enthalpies decrease as the size of the halide ion increases.

Lattice enthalpies of NaP, NaCl, Nalir, and Nal are 893, 770,730, and 685 kJ. mol^-1 respectively, A.s the lattice enthalpy decreases, the energy required to break the crystal lattice decreases, and consequently, the melting points decrease. Thus, the melting points of NaF, NaCl, NaBr, and Nal are found to be 1201K, 10IMK, 1028 K, and 944 K respectively.

2. For a particular halide ion, the melting point of IJX is less than that of

NaX and thereafter the melting points decrease on moving down the group from Na to Cs.

Explanation:

The melting point of LiCl (887K) is less than that of NaCl (I084K), because LiCl is covalent (for smaller atomic size of Li compared to that of Na), but NaCl is ionic. Thereafter, the order of melting point is:

NaCl(1084K)>KCl(1039K)>RbCI(988K)>CsCl(925K) This is observed because the lattice enthalpies decrease as the size of the alkali metal atom increases.

3. Solubilities of the alkali metal halides (except fluorides) decrease on moving down the group since the decrease in hydration enthalpy is more than the corresponding decrease in the lattice enthalpy.

For example, the difference in lattice enthalpy between NaCl and KCl is 67kJ. mol^-1 whereas the difference in hydration enthalpy between Na⁺ and K⁺ ion is 76 kj -mol^-1 Thus, KCl is relatively less soluble in water compared to NaCl.

Explanation:

The solubility of a salt in water depends on its lattice enthalpy as well as its hydration enthalpy. In general, if hydration enthalpy > lattice enthalpy, the salt dissolves in water but if the hydration enthalpy < lattice enthalpy, the salt does not dissolve.

Further, the extent of hydration depends on the ionic size. The smaller the size of the ion, the more it will get hydrated and the greater will be its hydration enthalpy. LiF, for example, is almost insoluble in water because of its higher lattice enthalpy (-1005 kJ . mol^-1 ).

On the other hand, the low solubility of Csl in water is due to smaller hydration enthalpies of the two large ions [-276(Cs⁺)-305(I^–) = -581 kJ.-mol^-1]. o Due to the smaller size and relatively higher electronegativity of Li, lithium halides except LiF are predo¬minantly covalent and hence, are soluble in organic solvents such as acetone, alcohol, ethyl acetate etc.

In contrast, sodium chloride, being ionic, is insoluble in organic solvents.

3. Soils of oxoacids

Alkali metals react with c to acids such as carbonic acid (H₂CO₃), nitric acid (HNO₃), sulphuric acid (H₈SO₄), etc., to form corresponding salts and release H₂. Due to the high polarising power and lattice energy of small Li ions, lithium salts behave abnormally.

4. Nature of carbonates and bicarbonates

All alkali metals form carbonates of the type M₂CO₃. Since the alkali metals are highly electropositive, their carbonates are remarkably stable up to l000°C above which they first melt and then decompose to form oxides. These salts are readily soluble in water. As electropositive character increases down the group, the stability of carbonates increases in the same order:

Cs₂CO₃ > Rb₂CO₃ > K₂CO₃ > Na₂CO₃ > Li₂CO₃

Li₂CO₃ is insoluble in water and unstable towards heat. It decomposes readily to give Li₂O and CO₂.

Explanation:

1. The very small L ion exerts a strong polarising power on the large carbonate (CO₃^2-) ion and distorts the electron cloud of its nearby oxygen atom.

This results in the weakening of the C—O bond and the strengthening of the Li — O bond. This eventually facilitates the decomposition of Li₂CO₃ leading to the formation of Li₂O and CO₂. %

2. The crystal lattice formed by a smaller Li⁺ ion with a smaller O₂ ion is more stable than that 2  formed by a larger CO₃ ion and a smaller Li+ ion. This also favors the decomposition of Li₂CO₃

3. The aqueous solution of carbonates is alkaline. This is because carbonates being the salts of strong bases and weak acids (H₂CO₃) undergo hydrolysis.

M₂CO₃ + 2H₂O  ⇌  2MOH (strong base) + H₂CO₃ (weak acid)

4. Bicarbonates or hydrogen carbonates (MHCO₃) of the alkali metals except LiHCO₃ are obtained in the solid state. These bicarbonates are soluble in water and stable towards heat. On strong heating, all the bicarbonates undergo decomposition to yield carbonates with the evolution of carbon dioxide.

2MHCO₃ (heat)→ M₂CO₃ + CO₂ + H₂O

As the electropositive character of the metals increases down the group from Li to Cs, the stability of the bicarbonates increases in the same order.

5. Nature of nitrates

The alkali metal nitrates (MNO₃) are prepared by the action of HNO₃ on the corresponding carbonates or hydroxides. They are ionic crystalline solids having low melting points and are highly soluble in water. On strong heating, they (except LiNO₃ ) decompose into nitrites and at higher temperatures oxides.

S Block Elements Chapter 10 NCERT Notes

For example:

LiNO₃ decomposes readily on heating to give

6. Nature of sulphates

The alkali metals form sulfates of the type M₂SO₄. All the sulfates except Li₂SO₄ are soluble in water. The sulfates when fused with charcoal, form sulphides.