Methane reacts with chlorine in the presence of light which proceeds through the free radical chain mechanism. This mechanism is characterised by three steps: initiation, propagation and termination.
1. Chain initiation:
The chain is initiated by UV light, leading to homolytic fission of chlorine molecules into free radicals (chlorine atoms).
Here we choose Cl-Cl bond for fission because C- C and C-H bonds are stronger than Cl-Cl bond.
2. Chain propagation:
It proceeds as follows – (a) Chlorine free radiais attack the methane molecule and breaks the C-H bond resulting in the generation of methyl free radicals.
(b) The methyl free radicals thus obtained attacks the second molecule of chlorine to give chloromethane (CH3Cl) and a chlorine free radical as follows –
(c) This chlorine free radical then cycles back to step (a) and both steps (a) and (b) are repeated many times and thus a chain of reaction is set up.
3. Chain termination:
Aller sometime, the reaction stops due to the consumption of reactant and the chain is terminated by the combination of free radicals.
