Functional groups and homologous series

Functional groups
When an atom or group of atoms bonded to a carbon atom in the chain or ring of an organic compound, shown some characteristic properties of their own, they are termed as a functional group. Thus, a functional group is an atom or group of atoms which determine the chemical behavior of an organic compound. All the compounds having a particular functional group behave alike. For example all compounds containing -OH (hydroxyl) group undergo similar reactions.

The functional group present in the following molecules are encircled.

functional group of alcohols
functional group of acids
functional group of amines

functional group of carbonyls

Common functional groups and their families of compound

Common functional groups and their families of compound

Homologous series

A homologous series is a family of organic compounds containing a particular characteristic group and exhibiting similar properties. For example, the compounds given below belong to the alcohol family.

CH3OH methyl alcohol (methanol)

CH3CH2CH2OH propyl alcohol (1-propanol)

CH3CH2.CH2.CH2OH butyl alcohol (1-butanol)

Characteristics of a Homologous Series

All members of a homologous series exhibit some common characteristics. They are:

  • All the members of a homologous series can be represented by a common general formula, as they have the same functional group. For example, alkanes can be represented by the formula CnH2n+2.

CH4 C2H5 C3H8 C4H10

Methane Ethane Propane Butane
  • Each member of a homologous series has a common difference of -CH2 from the next higher or lower member.
  • Common general methods of preparation exist for all members of the series.
  • All members exhibit similar chemical behavior.
  • An increase in molecular mass of members within a homologous series show a similar regular gradation of the physical properties, such as, physical state, melting and boiling points etc.