Page 72 - NCERT Science Class 10 English Medium

unit? Do you see any relation between the number of carbon and
hydrogen atoms in these compounds? The general formula for alkenes
can be written as C H , where n = 2, 3, 4. Can you similarly generate the
n 2n
general formula for alkanes and alkynes?
As the molecular mass increases in any homologous series, a
gradation in physical properties is seen. This is because the melting and
boiling points increase with increasing molecular mass. Other physical
properties such as solubility in a particular solvent also show a similar
gradation. But the chemical properties, which are determined solely by
the functional group, remain similar in a homologous series.

Activity
Activity 4.2
Activity 4.2
Activity 4.2
Activity 4.24.2
n Calculate the difference in the formulae and molecular masses
for (a) CH OH and C H OH (b) C H OH and C H OH, and (c) C H OH
3 2 5 2 5 3 7 3 7
and C H OH.
4 9
n Is there any similarity in these three?
n Arrange these alcohols in the order of increasing carbon atoms to
get a family. Can we call this family a homologous series?
n Generate the homologous series for compounds containing up to
four carbons for the other functional groups given in Table 4.3.

4.2.5 Nomenclature of Carbon Compounds

The names of compounds in a homologous series are based on the name
of the basic carbon chain modified by a “prefix” “phrase before” or “suffix”
“phrase after” indicating the nature of the functional group. For example,
the names of the alcohols taken in Activity 4.2 are methanol, ethanol,
propanol and butanol.
Naming a carbon compound can be done by the following method –
(i) Identify the number of carbon atoms in the compound. A compound
having three carbon atoms would have the name propane.
(ii) In case a functional group is present, it is indicated in the
name of the compound with either a prefix or a suffix (as given
in Table 4.4).
(iii) If the name of the functional group is to be given as a suffix, and the
suffix of the functional group begins with a vowel a, e, i, o, u, then
the name of the carbon chain is modified by deleting the final ‘e’ and
adding the appropriate suffix. For example, a three-carbon chain
with a ketone group would be named in the following manner –
Propane – ‘e’ = propan + ‘one’ = propanone.
(iv) If the carbon chain is unsaturated, then the final ‘ane’ in the name
of the carbon chain is substituted by ‘ene’ or ‘yne’ as given in
Table 4.4. For example, a three-carbon chain with a double bond
would be called propene and if it has a triple bond, it would be
called propyne.

Carbon and its Compounds 67

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