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.

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 C_nH_2n+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.