Initiation : The chlorination of methane follows a free radical mechanism. Initially, by the action of heat or sunlight on Cl2, the Cl – Cl bond breaks.
Cl

↷

−

↻

Cl →

.

Cl

+C

.

l

Propagation : This produces two free radicals which abstract a hydrogen free radical from the methane molecule thereby generating a methyl free radical.

.

Cl

+H

↶

−

↺

CH3 → H−Cl+

.

CH3

methyl‌radical

Termination : Now this methyl free radical may either combine with a chlorine free radical to form the desired product – chloromethane or may combine with another methyl radical to form an ethane molecule.

.

CH3

+

.

Cl

→

CH3Cl

methyl‌chloride

or

.

CH3

+

.

CH3

→

CH3−CH3

ehtane

This is how ethane is generated during the chlorination of methane.