The following reaction is an example of-

${\mathrm{CH}}_4+{\mathrm{Cl}}_2\stackrel{\mathrm{h\gamma }}{\to }{\mathrm{CH}}_3\mathrm{Cl}+\mathrm{HCl}$

In the presence of sunlight, chlorine is added to hydrocarbons(alkanes). Chlorine can be replaced by hydrogen atoms. It is called a substitution reaction. 

Example: Methane reacts with chloride in the presence of sunlight from Chloromethane and Hydrochloric acid. Halogenation reaction is an example of a substitution reaction