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295 Hydrocarbons
Unit 9
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.
9.1 cLassi Fication
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.
Unit 9.indd 295 10/10/2022 10:37:52 AM
Rationalised 2023-24
296 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 8. 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.
9.2 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
…
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. 9.1), 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 8).
You 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.
9.2.1 Nomenclature and Isomerism
You have already read about nomenclature
of different classes of organic compounds
in Unit 8. 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. 9.1 Structure of methane
Butane (n- butane), (b.p. 273 K)
I
H H H

H—C—H
replace any H by - CH
3
H—C—C—H or C
2
H
6

H H H
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
Unit 9.indd 296 11/10/2022 15:24:01
Rationalised 2023-24
297 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
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 9.1
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
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 structures, they are known as structural
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
Unit 9.indd 297 10/10/2022 10:37:53 AM
Rationalised 2023-24
Page 4

H—C—H
replace any H by - CH
3
H—C—C—H or C
2
H
6

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 8).
Let us recall the general rules for
nomenclature already discussed in Unit 8.
Nomenclature of substituted alkanes can
further be understood by considering the
following problem:
Problem 9.2
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
–

Unit 9.indd 298 11/10/2022 15:24:36
Rationalised 2023-24
Page 5

H—C—H
replace any H by - CH
3
H—C—C—H or C
2
H
6

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
–

Unit 9.indd 298 11/10/2022 15:24:36
Rationalised 2023-24
299 Hydrocarbons
r emarks
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
t able 9.1 nomenclature of a Few o rganic 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)

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 9.3
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
Unit 9.indd 299 10/10/2022 10:37:53 AM
Rationalised 2023-24

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FAQs on NCERT Textbook: Hydrocarbons - Chemistry Class 11 - NEET

1. What are hydrocarbons?

Ans. Hydrocarbons are organic compounds that consist of hydrogen and carbon atoms. They are formed mainly from fossil fuels and are the basis for the production of various fuels and chemicals.

2. What are the different types of hydrocarbons?

Ans. Hydrocarbons can be classified into two main types: aliphatic hydrocarbons and aromatic hydrocarbons. Aliphatic hydrocarbons include alkanes, alkenes, and alkynes, while aromatic hydrocarbons include benzene and its derivatives.

3. How are hydrocarbons used in daily life?

Ans. Hydrocarbons play a significant role in our daily lives as they are used as fuels for transportation, heating, and cooking. They are also used in the production of plastics, synthetic fibers, pharmaceuticals, and various other chemicals.

4. What are the environmental impacts of hydrocarbons?

Ans. The combustion of hydrocarbons releases carbon dioxide, a greenhouse gas that contributes to climate change. Hydrocarbon spills can also contaminate water and soil, causing harm to ecosystems and human health. Additionally, the extraction and production of hydrocarbons can lead to habitat destruction and air pollution.

5. How can hydrocarbons be converted into useful products?

Ans. Hydrocarbons can be converted into useful products through various processes such as cracking, reforming, and polymerization. Cracking breaks down larger hydrocarbon molecules into smaller ones, while reforming rearranges the structure of hydrocarbons to produce desired products. Polymerization combines small hydrocarbon molecules to form polymers, which are used in the production of plastics and synthetic materials.

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