In substitution reaction, one functional group in reactant molecule is replaced by another functional group.
The reaction of chlorine with methane:
Hydrogen atoms of methane are successively replaced with chlorine atoms in presence of sunlight ( or in dark at high temperature (250−400oC)).
CH4 Methane Cl2−−−−−→− HClCH3−Cl methyl chloride Cl2−−−−−→− HClCH2Cl2 Methylene chloride Cl2−−−−−→− HClCHCl3 Chloroform Cl2−−−−−→− HClCCl4carbon tetrachloride