Homologous Series

Homologous series is defined as a systematic order of organic compounds having same functional group that can be represented by same general formula and prepared by same method. The difference between two consecutive members in their composition is –CH2unit.

Each member of a homologous series is known as homologue and this phenomenon is called homology.

Example:

- Alkane
CnH2n+2
n=1
CH4
n=2
CH3-CH3

- Alcohol
CnH2n+1OH
n=1
CH3-OH
n=2
CH3-CH2-OH

- Aldehyde
CnH2n+1CHO
n=0
HCHO
n=1
CH3-C-O-H

Characteristics of homologous series:

• The members of a series have same functional group.

• Two consecutive members of a homologous series differs each other in their composition by –CH2unit.
  Example:
  Alcohol (-OH)
  CH3-OH , CH3-CH2-OH , CH3-CH2-CH2-OH

• The members of a homologous series can be represented by same general formula.
  Example:
  Alcohol è CnH2n+1OH
  Aldehyde è CnH2n+1CHO
  Carboxylic acid è CnH2n+1COOH

• The members of a particular homologous series have almost same chemical properties due to presence of same functional group.

• The physical properties(like solubility, melting point, boiling point, state) of members of a homologous series either gradually increase or decrease with increase in molecular mass.

• The members of a homologous series can be prepared by same or common general method of preparation.

• The first member of homologous series generally shows certain different chemical behavior than other members of the series.