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