Homologous series

Homologous series

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Homologous Series

A homologous series can be defined as group of compounds in which the various members have similar structural features & similar chemical properties  &  the successive members differ in their molecular formula by ‘CH2’ . Individual member of series are called homologue.

 Characteristics of Homologous Series –

1 )    All compounds in the series are composed of same elements and contain the same functional            group.


2) All compounds in the series can be represented by one general formula .

Ex. Homologous series of alcohol-

General formula =   





3) The molecular  mass of every two adjacent members differ by 14  or  CH2 .

4) All members of series have similar chemical properties because of the presence of same functional group.

5) The members of series show a gradual gradation in their physical properties like solubility, density, melting point  & boiling  points. The physical properties  generally increases as the molecular mass increases.

Primary, Secondary, Tertiary & Quaternary Carbon atoms –


Primary Carbon atom-

A carbon atom attached to  other C-atom or no other Carbon atom is called primary carbon or 10 carbon.

Secondary Carbon atom-

A carbon atom attached to two other carbon atoms is called secondary  carbon atom or 2Carbon.

Tertiary Carbon atom –

A carbon atom attached to three other carbon atoms is  termed as tertiary carbon or 3Carbon.

Quaternary Carbon atom-

   A carbon atom attached to four other carbon atoms is termed as quaternary carbon atom or 4Carbon.

(The Hydrogen atoms attached to Primary , secondary  & tertiary Carbon atom are correspondingly termed as primary,  secondary & tertiary hydrogen  atoms respectively)

Normal, Iso & Neo hydrocarbons-

Normal hydrocarbons (Or prefix n)

Prefix n is used for those paraffins in which all the carbon atoms are in  continuous chain. The prefix ‘n’ stands for normal or straight chain.

Ex.   CH3-CH2-CH2-CH3 [n-butane] ,

CH3-CH2-CH2-CH2-CH3 [n-pentane]

CH3-CH2-CH2-CH2-CH2-CH3 [n-hexane] ,

Iso hydrocarbons

Those hydrocarbons in which   group is attached to a straight chain, are called iso hydrocarbons

Neo hydrocarbons-

Those hydrocarbons in which group is attached to a straight chain are called neo hydrocarbons .