NCERT Textbook: Hydrocarbons Notes | EduRev

 Page 1


373 HYDROCARBONS
UNIT 13
After studying this unit, you will be
able to
•• • • • name hydrocarbons according to
IUPAC system of nomenclature;
• • •• • recognise and write structures
of isomers of alkanes, alkenes,
alkynes and aromatic
hydrocarbons;
•• •• • learn about various methods of
preparation of hydrocarbons;
•• •• • distinguish between alkanes,
alkenes, alkynes and aromatic
hydrocarbons on the basis of
physical and chemical properties;
•• •• • draw and differentiate between
various conformations of ethane;
•• •• • appreciate the role of
hydrocarbons as sources of
energy and for other industrial
applications;
•• •• • predict the formation of the
addition products of
unsymmetrical alkenes and
alkynes on the basis of electronic
mechanism;
•• •• • comprehend the structure of
benzene, explain aromaticity
and understand mechanism
of electrophilic substitution
reactions of benzene;
•• •• • predict the directive influence of
substituents in monosubstituted
benzene ring;
•• •• • learn about carcinogenicity and
toxicity.
HYDROCARBONS
The term ‘hydrocarbon’ is self-explanatory which means
compounds of carbon and hydrogen only. Hydrocarbons
play a key role in our daily life. You must be familiar with
the terms ‘LPG’ and ‘CNG’ used as fuels. LPG is the
abbreviated form of liquified petroleum gas whereas CNG
stands for compressed natural gas. Another term ‘LNG’
(liquified natural gas) is also in news these days. This is
also a fuel and is obtained by liquifaction of natural gas.
Petrol, diesel and kerosene oil are obtained by the fractional
distillation of petroleum found under the earth’s crust.
Coal gas is obtained by the destructive distillation of coal.
Natural gas is found in upper strata during drilling of oil
wells. The gas after compression is known as compressed
natural gas. LPG is used as a domestic fuel with the least
pollution. Kerosene oil is also used as a domestic fuel but
it causes some pollution. Automobiles need fuels like petrol,
diesel and CNG. Petrol and CNG operated automobiles
cause less pollution. All these fuels contain mixture of
hydrocarbons, which are sources of energy. Hydrocarbons
are also used for the manufacture of polymers like
polythene, polypropene, polystyrene etc. Higher
hydrocarbons are used as solvents for paints. They are also
used as the starting materials for manufacture of many
dyes and drugs. Thus, you can well understand the
importance of hydrocarbons in your daily life. In this unit,
you will learn more about hydrocarbons.
13.1 CLASSIFICATION
Hydrocarbons are of different types. Depending upon the
types of carbon-carbon bonds present, they can be
classified into three main categories – (i) saturated
Hydrocarbons are the important sources of energy.
2019-20
Page 2


373 HYDROCARBONS
UNIT 13
After studying this unit, you will be
able to
•• • • • name hydrocarbons according to
IUPAC system of nomenclature;
• • •• • recognise and write structures
of isomers of alkanes, alkenes,
alkynes and aromatic
hydrocarbons;
•• •• • learn about various methods of
preparation of hydrocarbons;
•• •• • distinguish between alkanes,
alkenes, alkynes and aromatic
hydrocarbons on the basis of
physical and chemical properties;
•• •• • draw and differentiate between
various conformations of ethane;
•• •• • appreciate the role of
hydrocarbons as sources of
energy and for other industrial
applications;
•• •• • predict the formation of the
addition products of
unsymmetrical alkenes and
alkynes on the basis of electronic
mechanism;
•• •• • comprehend the structure of
benzene, explain aromaticity
and understand mechanism
of electrophilic substitution
reactions of benzene;
•• •• • predict the directive influence of
substituents in monosubstituted
benzene ring;
•• •• • learn about carcinogenicity and
toxicity.
HYDROCARBONS
The term ‘hydrocarbon’ is self-explanatory which means
compounds of carbon and hydrogen only. Hydrocarbons
play a key role in our daily life. You must be familiar with
the terms ‘LPG’ and ‘CNG’ used as fuels. LPG is the
abbreviated form of liquified petroleum gas whereas CNG
stands for compressed natural gas. Another term ‘LNG’
(liquified natural gas) is also in news these days. This is
also a fuel and is obtained by liquifaction of natural gas.
Petrol, diesel and kerosene oil are obtained by the fractional
distillation of petroleum found under the earth’s crust.
Coal gas is obtained by the destructive distillation of coal.
Natural gas is found in upper strata during drilling of oil
wells. The gas after compression is known as compressed
natural gas. LPG is used as a domestic fuel with the least
pollution. Kerosene oil is also used as a domestic fuel but
it causes some pollution. Automobiles need fuels like petrol,
diesel and CNG. Petrol and CNG operated automobiles
cause less pollution. All these fuels contain mixture of
hydrocarbons, which are sources of energy. Hydrocarbons
are also used for the manufacture of polymers like
polythene, polypropene, polystyrene etc. Higher
hydrocarbons are used as solvents for paints. They are also
used as the starting materials for manufacture of many
dyes and drugs. Thus, you can well understand the
importance of hydrocarbons in your daily life. In this unit,
you will learn more about hydrocarbons.
13.1 CLASSIFICATION
Hydrocarbons are of different types. Depending upon the
types of carbon-carbon bonds present, they can be
classified into three main categories – (i) saturated
Hydrocarbons are the important sources of energy.
2019-20
374 CHEMISTRY
(ii) unsaturated and (iii) aromatic
hydrocarbons. Saturated hydrocarbons
contain carbon-carbon and carbon-hydrogen
single bonds. If different carbon atoms are
joined together to form open chain of carbon
atoms with single bonds, they are termed as
alkanes as you have already studied in
Unit 12. On the other hand, if carbon atoms
form a closed chain or a ring, they are termed
as cycloalkanes. Unsaturated hydrocarbons
contain carbon-carbon multiple bonds –
double bonds, triple bonds or both. Aromatic
hydrocarbons are a special type of cyclic
compounds. You can construct a large number
of models of such molecules of both types
(open chain and close chain) keeping in mind
that carbon is tetravalent and hydrogen is
monovalent. For making models of alkanes,
you can use toothpicks for bonds and
plasticine balls for atoms. For alkenes, alkynes
and aromatic hydrocarbons, spring models can
be constructed.
13.2  ALKANES
As already mentioned, alkanes are saturated
open chain hydrocarbons containing
carbon - carbon single bonds. Methane (CH
4
)
is the first member of this family. Methane is a
gas found in coal mines and marshy places. If
you replace one hydrogen atom of methane by
carbon and join the required number of
hydrogens to satisfy the tetravalence of the
other carbon atom, what do you get? You get
C
2
H
6
. This hydrocarbon with molecular
formula C
2
H
6
 is known as ethane. Thus you
can consider C
2
H
6
 as derived from CH
4
 by
replacing one hydrogen atom by -CH
3
 group.
Go on constructing alkanes by doing this
theoretical exercise i.e., replacing hydrogen
atom by –CH
3
 group. The next molecules will
be C
3
H
8
, C
4
H
10 
…
These hydrocarbons are inert under
normal conditions as they do not react with
acids, bases and other reagents. Hence, they
were earlier known as paraffins (latin : parum,
little; affinis, affinity). Can you think of the
general formula for alkane family or
homologous series? If we examine the
formula of different alkanes we find that the
general formula for alkanes is C
n
H
2n+2
. It
represents any particular homologue when n
is given appropriate value. Can you recall the
structure of methane? According to VSEPR
theory (Unit 4), methane has a tetrahedral
structure (Fig. 13.1), in which carbon atom lies
at the centre and the four hydrogen atoms lie
at the four corners of a regular tetrahedron.
All H-C-H  bond angles are of 109.5°.
In alkanes, tetrahedra are joined together
in which C-C and C-H bond lengths are
154 pm and 112 pm respectively (Unit 12). Y ou
have already read that C–C and C–H s bonds
are formed by head-on overlapping of sp
3
hybrid orbitals of carbon and 1s orbitals of
hydrogen atoms.
13.2.1 Nomenclature and Isomerism
You have already read about nomenclature
of different classes of organic compounds in
Unit 12. Nomenclature and isomerism in
alkanes can further be understood with the
help of a few more examples. Common names
are given in parenthesis. First three alkanes
– methane, ethane and propane have only
one structure but higher alkanes can have
more than one structure. Let us write
structures for C
4
H
10
. Four carbon atoms of
C
4
H
10 
can be joined either in a continuous
chain or with a branched chain in the
following two ways :
Fig. 13.1  Structure of methane
Butane (n- butane), (b.p. 273 K)
I
2019-20
Page 3


373 HYDROCARBONS
UNIT 13
After studying this unit, you will be
able to
•• • • • name hydrocarbons according to
IUPAC system of nomenclature;
• • •• • recognise and write structures
of isomers of alkanes, alkenes,
alkynes and aromatic
hydrocarbons;
•• •• • learn about various methods of
preparation of hydrocarbons;
•• •• • distinguish between alkanes,
alkenes, alkynes and aromatic
hydrocarbons on the basis of
physical and chemical properties;
•• •• • draw and differentiate between
various conformations of ethane;
•• •• • appreciate the role of
hydrocarbons as sources of
energy and for other industrial
applications;
•• •• • predict the formation of the
addition products of
unsymmetrical alkenes and
alkynes on the basis of electronic
mechanism;
•• •• • comprehend the structure of
benzene, explain aromaticity
and understand mechanism
of electrophilic substitution
reactions of benzene;
•• •• • predict the directive influence of
substituents in monosubstituted
benzene ring;
•• •• • learn about carcinogenicity and
toxicity.
HYDROCARBONS
The term ‘hydrocarbon’ is self-explanatory which means
compounds of carbon and hydrogen only. Hydrocarbons
play a key role in our daily life. You must be familiar with
the terms ‘LPG’ and ‘CNG’ used as fuels. LPG is the
abbreviated form of liquified petroleum gas whereas CNG
stands for compressed natural gas. Another term ‘LNG’
(liquified natural gas) is also in news these days. This is
also a fuel and is obtained by liquifaction of natural gas.
Petrol, diesel and kerosene oil are obtained by the fractional
distillation of petroleum found under the earth’s crust.
Coal gas is obtained by the destructive distillation of coal.
Natural gas is found in upper strata during drilling of oil
wells. The gas after compression is known as compressed
natural gas. LPG is used as a domestic fuel with the least
pollution. Kerosene oil is also used as a domestic fuel but
it causes some pollution. Automobiles need fuels like petrol,
diesel and CNG. Petrol and CNG operated automobiles
cause less pollution. All these fuels contain mixture of
hydrocarbons, which are sources of energy. Hydrocarbons
are also used for the manufacture of polymers like
polythene, polypropene, polystyrene etc. Higher
hydrocarbons are used as solvents for paints. They are also
used as the starting materials for manufacture of many
dyes and drugs. Thus, you can well understand the
importance of hydrocarbons in your daily life. In this unit,
you will learn more about hydrocarbons.
13.1 CLASSIFICATION
Hydrocarbons are of different types. Depending upon the
types of carbon-carbon bonds present, they can be
classified into three main categories – (i) saturated
Hydrocarbons are the important sources of energy.
2019-20
374 CHEMISTRY
(ii) unsaturated and (iii) aromatic
hydrocarbons. Saturated hydrocarbons
contain carbon-carbon and carbon-hydrogen
single bonds. If different carbon atoms are
joined together to form open chain of carbon
atoms with single bonds, they are termed as
alkanes as you have already studied in
Unit 12. On the other hand, if carbon atoms
form a closed chain or a ring, they are termed
as cycloalkanes. Unsaturated hydrocarbons
contain carbon-carbon multiple bonds –
double bonds, triple bonds or both. Aromatic
hydrocarbons are a special type of cyclic
compounds. You can construct a large number
of models of such molecules of both types
(open chain and close chain) keeping in mind
that carbon is tetravalent and hydrogen is
monovalent. For making models of alkanes,
you can use toothpicks for bonds and
plasticine balls for atoms. For alkenes, alkynes
and aromatic hydrocarbons, spring models can
be constructed.
13.2  ALKANES
As already mentioned, alkanes are saturated
open chain hydrocarbons containing
carbon - carbon single bonds. Methane (CH
4
)
is the first member of this family. Methane is a
gas found in coal mines and marshy places. If
you replace one hydrogen atom of methane by
carbon and join the required number of
hydrogens to satisfy the tetravalence of the
other carbon atom, what do you get? You get
C
2
H
6
. This hydrocarbon with molecular
formula C
2
H
6
 is known as ethane. Thus you
can consider C
2
H
6
 as derived from CH
4
 by
replacing one hydrogen atom by -CH
3
 group.
Go on constructing alkanes by doing this
theoretical exercise i.e., replacing hydrogen
atom by –CH
3
 group. The next molecules will
be C
3
H
8
, C
4
H
10 
…
These hydrocarbons are inert under
normal conditions as they do not react with
acids, bases and other reagents. Hence, they
were earlier known as paraffins (latin : parum,
little; affinis, affinity). Can you think of the
general formula for alkane family or
homologous series? If we examine the
formula of different alkanes we find that the
general formula for alkanes is C
n
H
2n+2
. It
represents any particular homologue when n
is given appropriate value. Can you recall the
structure of methane? According to VSEPR
theory (Unit 4), methane has a tetrahedral
structure (Fig. 13.1), in which carbon atom lies
at the centre and the four hydrogen atoms lie
at the four corners of a regular tetrahedron.
All H-C-H  bond angles are of 109.5°.
In alkanes, tetrahedra are joined together
in which C-C and C-H bond lengths are
154 pm and 112 pm respectively (Unit 12). Y ou
have already read that C–C and C–H s bonds
are formed by head-on overlapping of sp
3
hybrid orbitals of carbon and 1s orbitals of
hydrogen atoms.
13.2.1 Nomenclature and Isomerism
You have already read about nomenclature
of different classes of organic compounds in
Unit 12. Nomenclature and isomerism in
alkanes can further be understood with the
help of a few more examples. Common names
are given in parenthesis. First three alkanes
– methane, ethane and propane have only
one structure but higher alkanes can have
more than one structure. Let us write
structures for C
4
H
10
. Four carbon atoms of
C
4
H
10 
can be joined either in a continuous
chain or with a branched chain in the
following two ways :
Fig. 13.1  Structure of methane
Butane (n- butane), (b.p. 273 K)
I
2019-20
375 HYDROCARBONS
In how many ways, you can join five
carbon atoms and twelve hydrogen atoms of
C
5
H
12
? They can be arranged in three ways as
shown in structures III–V
structures, they are known as structural
isomers. It is also clear that structures I and
III have continuous chain of carbon atoms but
structures II, IV and V have a branched chain.
Such structural isomers which differ in chain
of carbon atoms are known as chain isomers.
Thus, you have seen that C
4
H
10
 and C
5
H
12
have two and three chain isomers respectively.
Problem 13.1
Write structures of different chain isomers
of alkanes corresponding to the molecular
formula C
6
H
14
. Also write their IUPAC
names.
Solution
(i) CH
3
 – CH
2
 – CH
2
 – CH
2
– CH
2
– CH
3
n-Hexane
2-Methylpentane
3-Methylpentane
2,3-Dimethylbutane
2,2 - Dimethylbutane
Based upon the number of carbon atoms
attached to a carbon atom, the carbon atom is
termed as primary (1°), secondary (2°), tertiary
(3°) or quaternary (4°). Carbon atom attached
to no other carbon atom as in methane or to
only one carbon atom as in ethane is called
primary carbon atom. Terminal carbon atoms
are always primary. Carbon atom attached to
two carbon atoms is known as secondary.
Tertiary carbon is attached to three carbon
atoms and neo or quaternary carbon is
attached to four carbon atoms. Can you identify
1°, 2°, 3° and 4° carbon atoms in structures I
II
2-Methylpropane (isobutane)
(b.p.261 K)
Structures I and II possess same
molecular formula but differ in their boiling
points and other properties. Similarly
structures III, IV and V possess the same
molecular formula but have different
properties. Structures I and II are isomers of
butane, whereas structures III, IV and V are
isomers of pentane. Since difference in
properties is due to difference in their
III
Pentane (n-pentane)
(b.p. 309 K)
2-Methylbutane (isopentane)
(b.p. 301 K)
IV
2,2-Dimethylpropane (neopentane)
(b.p. 282.5 K)
V
2019-20
Page 4


373 HYDROCARBONS
UNIT 13
After studying this unit, you will be
able to
•• • • • name hydrocarbons according to
IUPAC system of nomenclature;
• • •• • recognise and write structures
of isomers of alkanes, alkenes,
alkynes and aromatic
hydrocarbons;
•• •• • learn about various methods of
preparation of hydrocarbons;
•• •• • distinguish between alkanes,
alkenes, alkynes and aromatic
hydrocarbons on the basis of
physical and chemical properties;
•• •• • draw and differentiate between
various conformations of ethane;
•• •• • appreciate the role of
hydrocarbons as sources of
energy and for other industrial
applications;
•• •• • predict the formation of the
addition products of
unsymmetrical alkenes and
alkynes on the basis of electronic
mechanism;
•• •• • comprehend the structure of
benzene, explain aromaticity
and understand mechanism
of electrophilic substitution
reactions of benzene;
•• •• • predict the directive influence of
substituents in monosubstituted
benzene ring;
•• •• • learn about carcinogenicity and
toxicity.
HYDROCARBONS
The term ‘hydrocarbon’ is self-explanatory which means
compounds of carbon and hydrogen only. Hydrocarbons
play a key role in our daily life. You must be familiar with
the terms ‘LPG’ and ‘CNG’ used as fuels. LPG is the
abbreviated form of liquified petroleum gas whereas CNG
stands for compressed natural gas. Another term ‘LNG’
(liquified natural gas) is also in news these days. This is
also a fuel and is obtained by liquifaction of natural gas.
Petrol, diesel and kerosene oil are obtained by the fractional
distillation of petroleum found under the earth’s crust.
Coal gas is obtained by the destructive distillation of coal.
Natural gas is found in upper strata during drilling of oil
wells. The gas after compression is known as compressed
natural gas. LPG is used as a domestic fuel with the least
pollution. Kerosene oil is also used as a domestic fuel but
it causes some pollution. Automobiles need fuels like petrol,
diesel and CNG. Petrol and CNG operated automobiles
cause less pollution. All these fuels contain mixture of
hydrocarbons, which are sources of energy. Hydrocarbons
are also used for the manufacture of polymers like
polythene, polypropene, polystyrene etc. Higher
hydrocarbons are used as solvents for paints. They are also
used as the starting materials for manufacture of many
dyes and drugs. Thus, you can well understand the
importance of hydrocarbons in your daily life. In this unit,
you will learn more about hydrocarbons.
13.1 CLASSIFICATION
Hydrocarbons are of different types. Depending upon the
types of carbon-carbon bonds present, they can be
classified into three main categories – (i) saturated
Hydrocarbons are the important sources of energy.
2019-20
374 CHEMISTRY
(ii) unsaturated and (iii) aromatic
hydrocarbons. Saturated hydrocarbons
contain carbon-carbon and carbon-hydrogen
single bonds. If different carbon atoms are
joined together to form open chain of carbon
atoms with single bonds, they are termed as
alkanes as you have already studied in
Unit 12. On the other hand, if carbon atoms
form a closed chain or a ring, they are termed
as cycloalkanes. Unsaturated hydrocarbons
contain carbon-carbon multiple bonds –
double bonds, triple bonds or both. Aromatic
hydrocarbons are a special type of cyclic
compounds. You can construct a large number
of models of such molecules of both types
(open chain and close chain) keeping in mind
that carbon is tetravalent and hydrogen is
monovalent. For making models of alkanes,
you can use toothpicks for bonds and
plasticine balls for atoms. For alkenes, alkynes
and aromatic hydrocarbons, spring models can
be constructed.
13.2  ALKANES
As already mentioned, alkanes are saturated
open chain hydrocarbons containing
carbon - carbon single bonds. Methane (CH
4
)
is the first member of this family. Methane is a
gas found in coal mines and marshy places. If
you replace one hydrogen atom of methane by
carbon and join the required number of
hydrogens to satisfy the tetravalence of the
other carbon atom, what do you get? You get
C
2
H
6
. This hydrocarbon with molecular
formula C
2
H
6
 is known as ethane. Thus you
can consider C
2
H
6
 as derived from CH
4
 by
replacing one hydrogen atom by -CH
3
 group.
Go on constructing alkanes by doing this
theoretical exercise i.e., replacing hydrogen
atom by –CH
3
 group. The next molecules will
be C
3
H
8
, C
4
H
10 
…
These hydrocarbons are inert under
normal conditions as they do not react with
acids, bases and other reagents. Hence, they
were earlier known as paraffins (latin : parum,
little; affinis, affinity). Can you think of the
general formula for alkane family or
homologous series? If we examine the
formula of different alkanes we find that the
general formula for alkanes is C
n
H
2n+2
. It
represents any particular homologue when n
is given appropriate value. Can you recall the
structure of methane? According to VSEPR
theory (Unit 4), methane has a tetrahedral
structure (Fig. 13.1), in which carbon atom lies
at the centre and the four hydrogen atoms lie
at the four corners of a regular tetrahedron.
All H-C-H  bond angles are of 109.5°.
In alkanes, tetrahedra are joined together
in which C-C and C-H bond lengths are
154 pm and 112 pm respectively (Unit 12). Y ou
have already read that C–C and C–H s bonds
are formed by head-on overlapping of sp
3
hybrid orbitals of carbon and 1s orbitals of
hydrogen atoms.
13.2.1 Nomenclature and Isomerism
You have already read about nomenclature
of different classes of organic compounds in
Unit 12. Nomenclature and isomerism in
alkanes can further be understood with the
help of a few more examples. Common names
are given in parenthesis. First three alkanes
– methane, ethane and propane have only
one structure but higher alkanes can have
more than one structure. Let us write
structures for C
4
H
10
. Four carbon atoms of
C
4
H
10 
can be joined either in a continuous
chain or with a branched chain in the
following two ways :
Fig. 13.1  Structure of methane
Butane (n- butane), (b.p. 273 K)
I
2019-20
375 HYDROCARBONS
In how many ways, you can join five
carbon atoms and twelve hydrogen atoms of
C
5
H
12
? They can be arranged in three ways as
shown in structures III–V
structures, they are known as structural
isomers. It is also clear that structures I and
III have continuous chain of carbon atoms but
structures II, IV and V have a branched chain.
Such structural isomers which differ in chain
of carbon atoms are known as chain isomers.
Thus, you have seen that C
4
H
10
 and C
5
H
12
have two and three chain isomers respectively.
Problem 13.1
Write structures of different chain isomers
of alkanes corresponding to the molecular
formula C
6
H
14
. Also write their IUPAC
names.
Solution
(i) CH
3
 – CH
2
 – CH
2
 – CH
2
– CH
2
– CH
3
n-Hexane
2-Methylpentane
3-Methylpentane
2,3-Dimethylbutane
2,2 - Dimethylbutane
Based upon the number of carbon atoms
attached to a carbon atom, the carbon atom is
termed as primary (1°), secondary (2°), tertiary
(3°) or quaternary (4°). Carbon atom attached
to no other carbon atom as in methane or to
only one carbon atom as in ethane is called
primary carbon atom. Terminal carbon atoms
are always primary. Carbon atom attached to
two carbon atoms is known as secondary.
Tertiary carbon is attached to three carbon
atoms and neo or quaternary carbon is
attached to four carbon atoms. Can you identify
1°, 2°, 3° and 4° carbon atoms in structures I
II
2-Methylpropane (isobutane)
(b.p.261 K)
Structures I and II possess same
molecular formula but differ in their boiling
points and other properties. Similarly
structures III, IV and V possess the same
molecular formula but have different
properties. Structures I and II are isomers of
butane, whereas structures III, IV and V are
isomers of pentane. Since difference in
properties is due to difference in their
III
Pentane (n-pentane)
(b.p. 309 K)
2-Methylbutane (isopentane)
(b.p. 301 K)
IV
2,2-Dimethylpropane (neopentane)
(b.p. 282.5 K)
V
2019-20
376 CHEMISTRY
to V ? If you go on constructing structures for
higher alkanes, you will be getting still larger
number of isomers. C
6
H
14
 has got five isomers
and C
7
H
16
 has nine. As many as 75 isomers
are possible for C
10
H
22
.
In structures II, IV and V, you observed
that –CH
3
 group is attached to carbon atom
numbered as 2. You will come across groups
like –CH
3
, –C
2
H
5
, –C
3
H
7
 etc. attached to carbon
atoms in alkanes or other classes of
compounds.  These groups or substituents are
known as alkyl groups as they are derived from
alkanes by removal of one hydrogen atom.
General formula for alkyl groups is C
n
H
2n+1
(Unit 12).
Let us recall the general rules for
nomenclature already discussed in Unit 12.
Nomenclature of substituted alkanes can
further be understood by considering the
following problem:
Problem 13.2
Write structures of different isomeric alkyl groups corresponding to the molecular formula
C
5
H
11
. Write IUPAC names of alcohols obtained by attachment of  –OH groups at different
carbons of the chain.
Solution
Structures of – C
5
H
11
 group Corresponding alcohols Name of alcohol
 (i) CH
3
 – CH
2
 – CH
2
 – CH
2
– CH
2 
– CH
3
 – CH
2
 – CH
2
 – CH
2
– CH
2 
– OH Pentan-1-ol
(ii) CH
3
 – CH – CH
2
 – CH
2
 – CH
3
CH
3
 – CH – CH
2
 – CH
2
– CH
3
Pentan-2-ol
| |
OH
(iii) CH
3
 – CH
2
 – CH – CH
2 
– CH
3
CH
3
 – CH
2
 – CH – CH
2
– CH
3
Pentan-3-ol
           | |
OH
CH
3
CH
3
3-Methyl-
| | butan-1-ol
(iv) CH
3
 – CH – CH
2
 – CH
2 
– CH
3
 – CH – CH
2
 – CH
2
– OH
 CH
3
CH
3              
2-Methyl-
 | | butan-1-ol
(v) CH
3
 – CH
2
 – CH – CH
2 
– CH
3
 – CH
2
 – CH – CH
2
– OH
CH
3
CH
3
2-Methyl-
| | butan-2-ol
(vi) CH
3
 – C – CH
2
 – CH
3
CH
3
 – C – CH
2
 – CH
3
| |
OH
CH
3
CH
3
2,2- Dimethyl-
| | propan-1-ol
(vii) CH
3
 – C – CH
2
 – CH
3
 – C – CH
2
OH
                 | |
  CH
3
CH
3
CH
3
CH
3
OH 3-Methyl-
|       | | |       butan-2-ol
(viii) CH
3
 – CH – CH –CH
3
CH
3
 – CH – CH –CH
3
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373 HYDROCARBONS
UNIT 13
After studying this unit, you will be
able to
•• • • • name hydrocarbons according to
IUPAC system of nomenclature;
• • •• • recognise and write structures
of isomers of alkanes, alkenes,
alkynes and aromatic
hydrocarbons;
•• •• • learn about various methods of
preparation of hydrocarbons;
•• •• • distinguish between alkanes,
alkenes, alkynes and aromatic
hydrocarbons on the basis of
physical and chemical properties;
•• •• • draw and differentiate between
various conformations of ethane;
•• •• • appreciate the role of
hydrocarbons as sources of
energy and for other industrial
applications;
•• •• • predict the formation of the
addition products of
unsymmetrical alkenes and
alkynes on the basis of electronic
mechanism;
•• •• • comprehend the structure of
benzene, explain aromaticity
and understand mechanism
of electrophilic substitution
reactions of benzene;
•• •• • predict the directive influence of
substituents in monosubstituted
benzene ring;
•• •• • learn about carcinogenicity and
toxicity.
HYDROCARBONS
The term ‘hydrocarbon’ is self-explanatory which means
compounds of carbon and hydrogen only. Hydrocarbons
play a key role in our daily life. You must be familiar with
the terms ‘LPG’ and ‘CNG’ used as fuels. LPG is the
abbreviated form of liquified petroleum gas whereas CNG
stands for compressed natural gas. Another term ‘LNG’
(liquified natural gas) is also in news these days. This is
also a fuel and is obtained by liquifaction of natural gas.
Petrol, diesel and kerosene oil are obtained by the fractional
distillation of petroleum found under the earth’s crust.
Coal gas is obtained by the destructive distillation of coal.
Natural gas is found in upper strata during drilling of oil
wells. The gas after compression is known as compressed
natural gas. LPG is used as a domestic fuel with the least
pollution. Kerosene oil is also used as a domestic fuel but
it causes some pollution. Automobiles need fuels like petrol,
diesel and CNG. Petrol and CNG operated automobiles
cause less pollution. All these fuels contain mixture of
hydrocarbons, which are sources of energy. Hydrocarbons
are also used for the manufacture of polymers like
polythene, polypropene, polystyrene etc. Higher
hydrocarbons are used as solvents for paints. They are also
used as the starting materials for manufacture of many
dyes and drugs. Thus, you can well understand the
importance of hydrocarbons in your daily life. In this unit,
you will learn more about hydrocarbons.
13.1 CLASSIFICATION
Hydrocarbons are of different types. Depending upon the
types of carbon-carbon bonds present, they can be
classified into three main categories – (i) saturated
Hydrocarbons are the important sources of energy.
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374 CHEMISTRY
(ii) unsaturated and (iii) aromatic
hydrocarbons. Saturated hydrocarbons
contain carbon-carbon and carbon-hydrogen
single bonds. If different carbon atoms are
joined together to form open chain of carbon
atoms with single bonds, they are termed as
alkanes as you have already studied in
Unit 12. On the other hand, if carbon atoms
form a closed chain or a ring, they are termed
as cycloalkanes. Unsaturated hydrocarbons
contain carbon-carbon multiple bonds –
double bonds, triple bonds or both. Aromatic
hydrocarbons are a special type of cyclic
compounds. You can construct a large number
of models of such molecules of both types
(open chain and close chain) keeping in mind
that carbon is tetravalent and hydrogen is
monovalent. For making models of alkanes,
you can use toothpicks for bonds and
plasticine balls for atoms. For alkenes, alkynes
and aromatic hydrocarbons, spring models can
be constructed.
13.2  ALKANES
As already mentioned, alkanes are saturated
open chain hydrocarbons containing
carbon - carbon single bonds. Methane (CH
4
)
is the first member of this family. Methane is a
gas found in coal mines and marshy places. If
you replace one hydrogen atom of methane by
carbon and join the required number of
hydrogens to satisfy the tetravalence of the
other carbon atom, what do you get? You get
C
2
H
6
. This hydrocarbon with molecular
formula C
2
H
6
 is known as ethane. Thus you
can consider C
2
H
6
 as derived from CH
4
 by
replacing one hydrogen atom by -CH
3
 group.
Go on constructing alkanes by doing this
theoretical exercise i.e., replacing hydrogen
atom by –CH
3
 group. The next molecules will
be C
3
H
8
, C
4
H
10 
…
These hydrocarbons are inert under
normal conditions as they do not react with
acids, bases and other reagents. Hence, they
were earlier known as paraffins (latin : parum,
little; affinis, affinity). Can you think of the
general formula for alkane family or
homologous series? If we examine the
formula of different alkanes we find that the
general formula for alkanes is C
n
H
2n+2
. It
represents any particular homologue when n
is given appropriate value. Can you recall the
structure of methane? According to VSEPR
theory (Unit 4), methane has a tetrahedral
structure (Fig. 13.1), in which carbon atom lies
at the centre and the four hydrogen atoms lie
at the four corners of a regular tetrahedron.
All H-C-H  bond angles are of 109.5°.
In alkanes, tetrahedra are joined together
in which C-C and C-H bond lengths are
154 pm and 112 pm respectively (Unit 12). Y ou
have already read that C–C and C–H s bonds
are formed by head-on overlapping of sp
3
hybrid orbitals of carbon and 1s orbitals of
hydrogen atoms.
13.2.1 Nomenclature and Isomerism
You have already read about nomenclature
of different classes of organic compounds in
Unit 12. Nomenclature and isomerism in
alkanes can further be understood with the
help of a few more examples. Common names
are given in parenthesis. First three alkanes
– methane, ethane and propane have only
one structure but higher alkanes can have
more than one structure. Let us write
structures for C
4
H
10
. Four carbon atoms of
C
4
H
10 
can be joined either in a continuous
chain or with a branched chain in the
following two ways :
Fig. 13.1  Structure of methane
Butane (n- butane), (b.p. 273 K)
I
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375 HYDROCARBONS
In how many ways, you can join five
carbon atoms and twelve hydrogen atoms of
C
5
H
12
? They can be arranged in three ways as
shown in structures III–V
structures, they are known as structural
isomers. It is also clear that structures I and
III have continuous chain of carbon atoms but
structures II, IV and V have a branched chain.
Such structural isomers which differ in chain
of carbon atoms are known as chain isomers.
Thus, you have seen that C
4
H
10
 and C
5
H
12
have two and three chain isomers respectively.
Problem 13.1
Write structures of different chain isomers
of alkanes corresponding to the molecular
formula C
6
H
14
. Also write their IUPAC
names.
Solution
(i) CH
3
 – CH
2
 – CH
2
 – CH
2
– CH
2
– CH
3
n-Hexane
2-Methylpentane
3-Methylpentane
2,3-Dimethylbutane
2,2 - Dimethylbutane
Based upon the number of carbon atoms
attached to a carbon atom, the carbon atom is
termed as primary (1°), secondary (2°), tertiary
(3°) or quaternary (4°). Carbon atom attached
to no other carbon atom as in methane or to
only one carbon atom as in ethane is called
primary carbon atom. Terminal carbon atoms
are always primary. Carbon atom attached to
two carbon atoms is known as secondary.
Tertiary carbon is attached to three carbon
atoms and neo or quaternary carbon is
attached to four carbon atoms. Can you identify
1°, 2°, 3° and 4° carbon atoms in structures I
II
2-Methylpropane (isobutane)
(b.p.261 K)
Structures I and II possess same
molecular formula but differ in their boiling
points and other properties. Similarly
structures III, IV and V possess the same
molecular formula but have different
properties. Structures I and II are isomers of
butane, whereas structures III, IV and V are
isomers of pentane. Since difference in
properties is due to difference in their
III
Pentane (n-pentane)
(b.p. 309 K)
2-Methylbutane (isopentane)
(b.p. 301 K)
IV
2,2-Dimethylpropane (neopentane)
(b.p. 282.5 K)
V
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376 CHEMISTRY
to V ? If you go on constructing structures for
higher alkanes, you will be getting still larger
number of isomers. C
6
H
14
 has got five isomers
and C
7
H
16
 has nine. As many as 75 isomers
are possible for C
10
H
22
.
In structures II, IV and V, you observed
that –CH
3
 group is attached to carbon atom
numbered as 2. You will come across groups
like –CH
3
, –C
2
H
5
, –C
3
H
7
 etc. attached to carbon
atoms in alkanes or other classes of
compounds.  These groups or substituents are
known as alkyl groups as they are derived from
alkanes by removal of one hydrogen atom.
General formula for alkyl groups is C
n
H
2n+1
(Unit 12).
Let us recall the general rules for
nomenclature already discussed in Unit 12.
Nomenclature of substituted alkanes can
further be understood by considering the
following problem:
Problem 13.2
Write structures of different isomeric alkyl groups corresponding to the molecular formula
C
5
H
11
. Write IUPAC names of alcohols obtained by attachment of  –OH groups at different
carbons of the chain.
Solution
Structures of – C
5
H
11
 group Corresponding alcohols Name of alcohol
 (i) CH
3
 – CH
2
 – CH
2
 – CH
2
– CH
2 
– CH
3
 – CH
2
 – CH
2
 – CH
2
– CH
2 
– OH Pentan-1-ol
(ii) CH
3
 – CH – CH
2
 – CH
2
 – CH
3
CH
3
 – CH – CH
2
 – CH
2
– CH
3
Pentan-2-ol
| |
OH
(iii) CH
3
 – CH
2
 – CH – CH
2 
– CH
3
CH
3
 – CH
2
 – CH – CH
2
– CH
3
Pentan-3-ol
           | |
OH
CH
3
CH
3
3-Methyl-
| | butan-1-ol
(iv) CH
3
 – CH – CH
2
 – CH
2 
– CH
3
 – CH – CH
2
 – CH
2
– OH
 CH
3
CH
3              
2-Methyl-
 | | butan-1-ol
(v) CH
3
 – CH
2
 – CH – CH
2 
– CH
3
 – CH
2
 – CH – CH
2
– OH
CH
3
CH
3
2-Methyl-
| | butan-2-ol
(vi) CH
3
 – C – CH
2
 – CH
3
CH
3
 – C – CH
2
 – CH
3
| |
OH
CH
3
CH
3
2,2- Dimethyl-
| | propan-1-ol
(vii) CH
3
 – C – CH
2
 – CH
3
 – C – CH
2
OH
                 | |
  CH
3
CH
3
CH
3
CH
3
OH 3-Methyl-
|       | | |       butan-2-ol
(viii) CH
3
 – CH – CH –CH
3
CH
3
 – CH – CH –CH
3
2019-20
377 HYDROCARBONS
Remarks
Lowest sum and
alphabetical
arrangement
Lowest sum and
alphabetical
arrangement
sec is not considered
while arranging
alphabetically;
isopropyl is taken
as one word
Further numbering
to the substituents
of the side chain
Alphabetical
priority order
Table 13.1  Nomenclature of a Few Organic Compounds
important to write the correct structure from
the given IUPAC name. To do this, first of all,
the longest chain of carbon atoms
corresponding to the parent alkane is written.
Then after numbering it, the substituents are
attached to the correct carbon atoms and finally
valence of each carbon atom is satisfied by
putting the correct number of hydrogen atoms.
This can be clarified by writing the structure
of 3-ethyl-2, 2–dimethylpentane in the
following steps :
i) Draw the chain of five carbon atoms:
C – C – C – C – C
ii) Give number to carbon atoms:
C
1
– C
2
– C
3
– C
4
– C
5
Structure and IUPAC Name
 
(a)
1
CH
3
–
2
CH – 
3
CH
2
 – 
4
CH – 
5
CH
2
 – 
6
CH
3
(4 – Ethyl – 2 – methylhexane)
          
(b)
8
CH
3 
–
 7
CH
2 
–
 6
CH
2 
–
 5
CH –
 4
CH –
    3
C   –
 2
CH
2 
–
 1
CH
3
 
(3,3-Diethyl-5-isopropyl-4-methyloctane)
 
(c)
1
CH
3
–
2
CH
2
–
3
CH
2
–
4
CH–
5
CH–
6
CH
2
–
7
CH
2
–
8
CH
2
–
9
CH
2
–
10
CH
3
5-sec– Butyl-4-isopropyldecane
(d)
1
CH
3
–
2
CH
2
–
3
CH
2
–
4
CH
2
–
5
CH–
6
CH
2
–
7
CH
2
–
8
CH
2
–
9
CH
3
5-(2,2– Dimethylpropyl)nonane
(e)
1
CH
3 
–
 2
CH
2 
–
 3
CH –
 4
CH
2 
–
 5
CH –
 6
CH
2 
–
 7
CH
3
3–Ethyl–5–methylheptane
Problem 13.3
Write IUPAC names of the following
compounds :
(i) (CH
3
)
3
 C CH
2
C(CH
3
)
3
(ii) (CH
3
)
2
 C(C
2
H
5
)
2
(iii) tetra – tert-butylmethane
Solution
(i) 2, 2, 4, 4-Tetramethylpentane
(ii) 3, 3-Dimethylpentane
(iii) 3,3-Di-tert-butyl -2, 2, 4, 4 -
tetramethylpentane
If it is important to write the correct IUPAC
name for a given structure, it is equally
2019-20