Chlorination of methane takes place following a free radical chain mechanism. The whole reaction mechanism can be represented by the following three steps below: -

Step 1: Initiation:

The reaction begins with the homolytic cleavage of Cl – Cl bond as:

Step 2: Propagation:

In the second step, chlorine free radicals attack the methane molecules and break down the C–H bond to generate methyl radicals as:-

These methyl radicals react with other chlorine free radicals to form methyl chloride along with the liberation of a chlorine free radical.

Hence, methyl free radicals and chlorine free radicals set up a chain reaction. While HCl and CH₃Cl are the major products formed, other higher halogenated compounds are also formed as:-

Step 3: Termination:

Formation of ethane is a result of the termination of chain reactions taking place as a result of the consumption of reactants as:-

Hence, by this process, ethane is obtained as a by-product of chlorination of methane.