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What is homologous series and it's characteristics?

Homologous Series

All organic compounds are made up of a progressively building chain of carbon atoms with a number of compounds having the same functional groups. Such a series of similarly constituted compounds are called a homologous series. Members of a homologous group are similar in structure and display similar chemical characteristics. The two consecutive members of the series differ in their molecular formula by a 'CH2' group.

Some important characteristics of the homologous series are:


    All the members conform to a general molecular formula and have a similar functional group.
    Each consecutive member differs in the molecular formula by a unit of 'CH2'.
    All the members of the series exhibit similar properties, but the extent of the reactions vary with increasing relative molecular mass.
    The physical properties, such as solubility, melting point, boiling point, specific gravity etc. show a gradual change with the increase in their relative molecular masses.
    Hydrocarbons and their major sub groups form a homologous series of organic compounds. As an illustrative example, the simplest of all hydrocarbons is methane whose molecular formula is CH4. This molecule consists of a single carbon atom linked to four hydrogen atoms by single covalent bonds. A straight line is used to represent each shared pair of electrons (bond) and the structure of methane (structural formula) can be thus written as follows:

                    structural formula of methane    
There are about 60 hydrocarbons of the methane type i.e., whose carbon atoms are linked to each other in single covalent bonds while the hydrogen atoms satisfy the remaining valencies. If their molecular formula are arranged in order of increasing number of carbon atoms in their molecules, the following series is obtained.

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Each member of this series differs from the previous one by an increment of -CH2- group. Thus, the methane family is a homologous series that can be characterised by the formula CnH2n+2. All members of this group can be prepared by the reduction of their appropriate alkyl halides (CnH2n+1X). This group is called the alkane group. Similarly the alkene and alkyne groups are characterised by the formula CnH2n-2 and CnH2n respectively.


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