Homologous Series

HOMOLOGOUS SERIES

Homologous series is a group or a class of organic compounds having same general formula and similar chemical properties in which the successive members differ by a - CH2 group.

Let us consider members of alkanes given in Table 11.1. Their condensed structural formulas are given below:

Methane      -        CH₄

Ethane  -     CH₃CH₃

Propane      -        CH₃CH₂CH₃

Butane  -     CH₃(CH₂)₂CH₃

Pentane       -        CH3(CH2)3CH3

If you observe the above series. you can notice that each successive member has one methylene group more than the precedent member of the series and hence they are called homologs.

Characteristics of homologous series

� Each member of the series differs from the preceding or succeeding member by one methylene group (–CH₂) and hence by a molecular mass of 14 amu.

� All members of a homologous series contain the same elements and functional group.

� They are represented by a general molecular formula. e.g. Alkanes, C_nH_{2n + 2}.

� The members in each homologous series show a regular gradation in their physical properties with respect to their increase in molecular mass.

� Chemical properties of the members of a homologous series are similar.

� All the members can be prepared by a common method.