Question

How do you account for the formation of ethane during chlorination of methane?

Answer 1

Chlorination of methane proceeds via a free radical chain mechanism. The whole reaction takes place in the given three steps.
Step 1: Initiation:
The reaction begins with the homolytic cleavage of $Cl - Cl$ bond as:
$Cl-Cl\;\underrightarrow{hv}\;\dot Cl+\dot C$
Chlorine free radicle

Step 2: Propagation:
In the second step, chlorine free radicals attack methane molecules and break down the $C-H$ bond to generate methyl radicals as:
$CH_4+\dot Cl \; \underrightarrow{hv}\;\dot CH_3+H-Cl$
Methane

These methyl radicals react with other chlorine free radicals to form methyl chloride along with the liberation of a chlorine free radical.
$\dot CH_3+Cl-Cl\rightarrow CH_3-Cl+\dot Cl$
Methyl chloride
Hence, methyl free radicals and chlorine free radicals set up a chain reaction. While $HCl$ and $CH_3Cl$ are the major products formed, other higher halogenated compounds are also formed as:

$CH_3Cl+\dot Cl\rightarrow \dot CH_2Cl+HCl$
$\dot CH_2Cl+Cl-Cl\rightarrow CH_2Cl_2+Cl$
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:
$\dot Cl+\dot Cl→Cl-Cl$
$H_3\dot C+\dot CH_3\rightarrow H_3C-CH_3$
(Ethane)
Hence, by this process, ethane is obtained as a by-product of chlorination of methane