The chlorination of alkanes requires energy in the form of UV light or high temperature ($523-673\text{ K}$).

• Mechanism: Free Radical Chain Reaction.

1. Initiation: Homolytic fission of $C{l}_2$ to generate $C{l}^{.}$ radicals.
2. Propagation: $C{l}^{.}$ reacts with methane to form methyl radical ($C{H}_3^{.}$) and $HCl$. Then $C{H}_3^{.}$ reacts with $C{l}_2$ to form $C{H}_{3}Cl$ and regenerate $C{l}^{.}$.
3. Termination: Collision of any two radicals stops the chain.

• Since an $H$ atom is substituted by a $Cl$ atom, it is a substitution reaction.