Describe substitution reaction with special reference to halogenation of alkane.

Substitution reaction: A substitution reaction is that which involves the direct replacement of hydrogen atoms of some other groups of a molecule by suitable atoms or groups without changing the structure of remaining part of the molecule. The product obtained is known as substitution product and new atom or group which enters the molecule is known as a substituent.
Halogenation: It involves the replacement of one or more hydrogen atom(s) of alkane by the corresponding number of a halogen atom(s).
(i) Chlorination: Chlorination of alkanes is carried out by treating alkane with chlorine in the presence of ultraviolet light or at 523-673 K temperature.

(ii) Bromination: Bromination takes place similarly but not so rapidly. 
(iii) Iodination: Iodination is extremely slow and reversible due to the reducing nature of HI.

Thus, iodination is carried out in the presence of some oxidising agent like iodic acid (HIO₃) of nitric acid (HNO₃) which converts HI to iodine and pushes the reaction in the forward direction.

Halogenation of higher alkanes (ethane, propane etc), yields a mixture of all possible isomerism products.