Substitution reaction involves:

A substitution reaction is a chemical reaction where an atom or a functional group in a molecule is replaced by a different atom or group of atoms. The number of bonds in the molecule usually remains the same. This reaction is a characteristic property of alkanes, a type of saturated hydrocarbons. Because all carbon valencies are already satisfied by single bonds, they cannot 'add' more atoms; they can only swap them. Example: CH4 + Cl2 → CH3Cl + HCl (Methane + Chlorine → Chloromethane + Hydrogen chloride).