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). You
have already read that C–C and C–H σ 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