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:

$$\underset{\mathrm{Chlorine}\mathrm{free}\mathrm{radicals}}{\mathrm{Cl}-\mathrm{Cl}\stackrel{\mathrm{hv}}{\to }\stackrel{\bullet }{\text{Cl}}+\stackrel{\bullet }{\text{C}}}$$

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:

$$\underset{\mathrm{Methane}}{\mathrm{CH}{\phantom{A}}_4}+\stackrel{\bullet }{\text{Cl}}\stackrel{\mathrm{hv}}{\to }\stackrel{\bullet }{\text{C}}\mathrm{H}{\phantom{A}}_3+\mathrm{H}-\mathrm{Cl}$$

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

$$\stackrel{\bullet }{\text{C}}\mathrm{H}{\phantom{A}}_3+\mathrm{Cl}-\mathrm{Cl}⟶\underset{\mathrm{Methyl}\mathrm{chloride}}{\mathrm{CH}{\phantom{A}}_3-\mathrm{Cl}+}\stackrel{\bullet }{\text{C}}\mathrm{l}$$

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:

$$\mathrm{CH}{\phantom{A}}_3\mathrm{Cl}+\stackrel{\bullet }{\text{C}}\mathrm{l}⟶\stackrel{\bullet }{\text{C}}\mathrm{H}{\phantom{A}}_2\mathrm{Cl}+\mathrm{HCl}$$

$$\stackrel{\bullet }{\text{C}}\mathrm{H}{\phantom{A}}_2\mathrm{Cl}+\mathrm{Cl}-\mathrm{Cl}⟶\mathrm{CH}{\phantom{A}}_2\mathrm{Cl}{\phantom{A}}_2+\stackrel{\bullet }{\text{C}}\mathrm{l}$$

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:

$$\stackrel{\bullet }{\text{C}}\mathrm{l}+\stackrel{\bullet }{\text{C}}\mathrm{l}⟶\mathrm{Cl}-\mathrm{Cl}$$

$$\mathrm{H}{\phantom{A}}_3\stackrel{\bullet }{\text{C}}+\stackrel{\bullet }{\text{C}}\mathrm{H}{\phantom{A}}_3⟶\underset{(\mathrm{Ethane})}{\mathrm{H}{\phantom{A}}_3\mathrm{C}-\mathrm{CH}{\phantom{A}}_3}$$

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