Homologous Series

HOMOLOGOUS SERIES

all organic compounds can be grouped into a comparatively small number of families or series of compounds known as homologous series. The simplest series of organic compound is the alkanes. 

A homologous series is a family of organic compounds which follows a regular structural pattern, in which each successive member differs in its molecular formula by a –CH₂­ –group.

From the above table you will notice that each member of the alkanes and the alkenes differ from their successive member by the –CH₂—respectively. Such a family of compounds is known as a homologous series and each individual member is referred to as a homologue.

 Homologous series have the following common characteristics.

General molecular formula

All members have the same general molecular formula, i.e. C_nH_2n+2 for the alkanes, C_nH_2n for the alkenes, and so on.

Difference between successive homologous.

 Each successive member in such a series differ in its molecular formula by the addition of a –CH₂—group, and in its relative molecular mass by an increase of 14

Physical properties.

The physical properties of the members change gradually as the member of carbon atoms per molecule increase. For example, the boiling points of the alkanes increase down the series, so that the first four members are gases at room temperature and standard pressure;  members with five to seventeen carbon atoms per molecular are volatile liquids; whole the higher members are wax-like solids. Similarly, the melting points and densities of the alkanes also increase, while their solubility in water decreases down the series.

Chemical properties

 The members show similar chemical properties. For example, the alkanes are fairly unreactive under ordinary conditions. They burn in air, forming carbon (IV) oxide and water, and undergo substitution reactions with other substances such as halogens.

General methods of preparation.

All members can usually be prepared by using the same general methods, e.g. alkanes can be prepared by the action of hot soda lime on the appropriate sodium salt of an acid.

Other homologous series include the alkenes, C_nH_2n, the alkanols, C_nH_2n+1 OH, and the carboxylic acids, C_nH_2n+1 COOH