EXERCISES

Question 13.1:-

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

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:

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:

$C{H}_{4}+CI\stackrel{h\nu }{\to }C{H}_{3}+H-CI$

Methane

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

Methyl chloride

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:

$C{H}_{3}Cl+CI\to ·C{H}_{2}CI+HCI\phantom{\rule{0ex}{0ex}}C{H}_{2}CI+CI-CI\to C{H}_{2}C{I}_{2}+CI·$

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:

$CI·+CI·\to CI-CI\phantom{\rule{0ex}{0ex}}{H}_{3}C·+·C{H}_{3}\to {H}_{3}C-C{H}_{3}$

(Ethane)

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