Homologous Series

All the organic compounds have been divided into different families or group depending upon their structure and chemical properties.

A homologous series is defined as a family or group of structurally similar organic compounds all the members of which contain the same functional group, Show a gradation in physical and similarity in chemical properties and any two adjacent members of which differ by a -CH₂ group. The individual members of such a series are called homologues and the phenomena is called homology.

Alkanes constitute a homologous series.

CH₄ Methane

C₂H₆ Ethane

C₃H₈ Propane

C₄H₁₀ Butane

These hydrocarbons can be represented by the same general formula C_nH_2n+2 Where n is the number of carbon atoms.

Any two adjacent members of this class differ by a CH₂ group.

If a hydrogen atom is replaced by a hydroxyl group i.e. OH , we get a new homologous series. This homologous series is called alcohols.

The general formula of this homologous series is C_nH_2n+1OH.

CH₃OH Methanol

C₂H₅OH Ethanol

C₃H₇OH Propanol

Characteristics of a Homologous series

1) Each homologous series can be represented by a general formula.

2) All the members of a given homologous series possess the same functional group.

For Example : The functional group of alcohols is the hydroxyl group i.e. -OH group.

The functional group of aldehyde is aldehydic group i.e. -CHO group.

The functional group of ketones is the ketonic group i.e -CO group.

3) The successive members of a homologous series differ by a -CH₂ group.

4) The individual members of a homologous series can be prepared by the general methods of preparation developed for that series.

5) The physical properties such as density, melting point, boiling point of the members of homologous series show a regular gradation with rise in molecular mass.

6) The chemical properties of the members of homologous series are similar though the first member may vary considerably from rest of the members.