A group or a series of organic compounds each containing a characteristic functional group form a homologous series and the members of the series are called homologues. The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in the molecular formula by a –CH2 unit. There is a number of homologous series of organic compounds. Some of these are alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids,
amines etc.
Characteristics of homologous series:
(i) All the members of a series have similar structures and the same functional group. For example, the functional group present in alcohols is–OH group.
(ii) The members of a series can be represented by a general formula. For example, the general formula of alkanes is C
nH
2n+2.
(iii) There is a constant difference of CH
2 in the molecular formulae and a constant difference of 14 in the molecular masses of any two adjacent individual members of a series. This is clear from the formulae of alcohols given below.
(iv) Different members of a series exhibit similar chemical reactions since they have similar structures.
(v) The physical properties, such as melting point, boiling point and density show a regular gradation with the increase in molecular masses.
(vi) Different members of a series can be prepared by a similar method known as general methods of preparation.
