Order in Homologous Series

Order in Homologous Series

  1. Photograph shows the first members of various homologous series.
    Order in Homologous Series 1
  2. Alkanes, alkenes, alcohols, carboxylic acids and esters belong to different homologous series. The organisation of organic compounds into the various different homologous series makes the study of organic chemistry more systematic, orderly and effective.
  3. The members of any homologous series have properties that vary in a regular and predictable manner. There is order in all homologous series with respect to the nomenclature, physical properties and chemical properties.


Systematic naming of organic compounds

All members of a homologous series are named systematically following the IUPAC rules. The IUPAC name of an organic molecule provides information about

  • which homologous series it belongs to
  • its functional group
  • the total number of carbon atoms present
  • its molecular and structural formulae

Order in physical and chemical properties

  • The physical properties of members of a homologous series show very regular pattern. These properties change gradually as the number of carbon atoms increases. Figure summarises the trend or order in the variation of physical properties of the various homologous series.
    Order in Homologous Series 2
  • Each homologous series has its own functional group. These functional groups determine the chemical reactions that an organic molecule can undergo. Hence, all members of a homologous series have very similar chemical properties.
    For example,

    • the carbon-carbon double bond allows all members of the alkene homologous series to undergo addition reactions.
    • acidic properties of carboxylic acids are due to the presence of the carboxyl group,
  • The order in the physical and chemical properties of organic molecules in a homologous series makes it easy to predict the properties and behaviour of unknown members of the series. By studying the properties of a few members of a homologous series, it is possible to deduce the properties of the other members in the same series.

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