Homologous Series

What is the homologous series of hydrocarbons?

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What is the homologous series of Hydrocarbons

 

What do you mean by homologous series?

Homologous series

  1. Organic compounds are grouped into different homologous series. Alkanes form a homologous series and so do alkenes.
  2. A homologous series is a group or family of organic compounds that has certain characteristics:
    (a) Members of the series can be represented by a general formula.
    (b) Successive members differ from each other by –CH2.
    (c) Physical properties change gradually with increasing number of carbon atoms per molecule.
    (d) Members have similar chemical properties because they have the same functional group.
    (e) Members can be prepared by similar methods.
  3. A functional group is a special group of atoms attached to an organic molecule. This group of atoms determines the chemical properties of the molecule because it represents the most common site of chemical reactivity. Chemical reactions occur at the functional group.
  4. Table shows the functional groups of various homologous series.

What is the homologous series of hydrocarbons 1
There is another classification based on Homologous series.
The series of carbon compounds in which two successive compounds differ by –CH2 unit is called homologous series.
Eg: 1) CH4, C2H6, C3H8 …
2) CH3OH, C2H5OH, C3H7OH …
If you observe above series of compounds, you will notice that each compound in the series differs by –CH2 unit by its successive compound.
Homologous series of organic compounds have following characteristic features.
1) They have one general formula.
Eg: alkanes (CnH2n+2); alkynes(CnH2n-2); alcohols (CnH2n+1)OH etc.
2) Successive compounds in the series possess a difference of (-CH2) unit.
3) They possess similar chemical properties due to the same functional group
Eg: alcohols, aldehydes and carboxylic acids have functional groups C–OH, C–CHO and C-COOH respectively.
4) They show a regular gradation in their physical properties (see the table-1).
For example: we may take alkanes, alkenes, alkynes, alcohols, aldehydes, and carboxylic acids etc. as homologous series. The individual members of a homologous series are called homologs.
Observe the following tables 1, 2 and 3. They represent three different homologous series.

What is the homologous series of hydrocarbons 2
The general formula of this homologous series Alkanes is CnH2n+2, where n = 1,2,3…

What is the homologous series of hydrocarbons 3
Alkenes have general molecular formula CnH2n, where ‘n’ is 2, 3, 4, …

What is the homologous series of hydrocarbons 4
Alkynes have general molecular formula CnH2n-2, where ‘n’ is 2, 3, 4, …
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Order in Homologous Series

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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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