What are the Homologous Series and its properties?

Homologous Series

Definition

Homologous series refers to a series of compounds with the same functional group and also similar chemical properties.

Meaning of Homologous Series

A homologous series refers to a series of carbon compounds which have a different number of carbon atoms. However, they contain the same functional group.

In this series, a group of compounds share similar basic chemical makeup. Also, in such a series, the compounds show a difference in the number of iterations of a certain aspect of their structure.

A frequent reference occurs in organic chemistry to homologous series. Here, compounds can differ by the length of their carbon chain. Moreover, such differences can have a significant effect on the physical properties of the chemicals.

The presence of a functional group such as alcohol decides the properties of the carbon compound, regardless of the length of the carbon chain. For example, the chemical properties of CH3OH, C2H5OH, C3H7OH and C4H9OH are all very similar.

Hence, such a series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called a homologous series.

Some examples of the homologous series . If we look at the formulae of successive compounds, say –
CH4 and C2H6 — these differ by a –CH2 unit

C2H6 and C3H8 — these differ by a –CH2 unit

Similarly, take the homologous series for alkenes. The first member of the series is ethene. The succeeding members have the formula C3H6 , C4H8 and C5H10. These also differ by a –CH2 unit and Molecular mass 14u . The general formula for alkenes can be written as CnH2n, where n = 2, 3, 4. Similar generate the general formula for alkanes and alkynes.


Physical properties of homologous series

The molecular mass increases in any homologous series,. 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.

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