Homologous Series

Homologous series is a series of compounds with similar chemical properties and same functional group differing with the successive member by CH2. Carbon chains of varying length have been observed in organic compounds having the same general formula. Such organic compounds that vary from one another by a repeating unit and have same general formula form a series of compounds. Alkanes with general formula CnH2n+2, alkenes with general formula CnH2n and alkynes with general formula CnH2n-2 form the most basic homologous series in organic chemistry.

Homologous series

Examples of Homologous series

The successive members vary from each other by a CH2 unit. For example in CH4 and C2H6, the difference is -CH2 unit and the difference between C2H6 and C3H8   is also -CH2 unit. So CH4, C2H6, and C3H8 are homologs. The same thing can be observed in case of alkenes in which the first member is ethene and the successive members are C3H6, C4H8, and C5H10. They differ from each other by a –CH2 unit.

All the members belonging to this series have the same functional groups. They have similar physical properties that follow a fixed gradation with increasing mass. The properties of CH3OH, C2H5OH, and C3H7OH are similar and follow a gradual change with increasing molecular mass of the successive members of the series. This is because, with the increase in the molecular mass of the compounds, the number of bonds also increases. Therefore, properties such as melting and boiling point, solubility, etc. that depend on the mass and the total number of bonds in a compound show a gradual change with an increase in molecular masses of the compounds. Chemical properties of the members of a homologous series are the same due to the fact that they all have the same functional groups in them.

Homologous Series

This series has enabled the scientists and engineers to study different organic compounds in a systematic manner. They can predict the properties of organic compounds belonging to a particular homologous series based on the data available from the other members of the same series. The study of organic compounds has been simplified.

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Practise This Question

Octane and propane are members of a homologous series. then those two compounds differ by ________.