Question

The chlorination of ethane is an example for which type of the following reactions?
A) Nucleophilic substitution
B) Electrophilic substitution
C) Free radical substitution
D) Rearrangement

Answer 1

Hint:To answer this we must know that chlorination is the addition of chlorine to a molecule or a compound. Chlorine can be added by the method of addition or substitution. Ethane is an organic compound which has a molecular formula ${\text{C}}{{\text{H}}_{\text{3}}} - {\text{C}}{{\text{H}}_{\text{3}}}$.

Complete step-by-step answer:
We know that chlorination is the addition of chlorine to a molecule or a compound. The chlorination of ethane occurs in the presence of light denoted by $h\nu $. The chlorination of ethane occurs in a stepwise manner.

The three steps involved in chlorination of ethane are
1. Initiation
2. Propagation
3. Termination

We will see the mechanism of chlorination step by step.
Initiation:
In this step, the chlorine radicals are generated by the cleavage of chlorine-chlorine bonds. The cleavage of chlorine-chlorine bond occurs in the presence of light denoted by .
The reaction is as follows:
\[{\text{Cl}} - {\text{Cl}}\xrightarrow{{h\nu }}{\text{C}}{{\text{l}}^*} + {\text{C}}{{\text{l}}^*}\]

Propagation:
In this step, one hydrogen atom of ethane is replaced by the chlorine atom. Thus, it is a substitution reaction. The product formed is known as chloroethane.
The reactions are as follows:
${\text{C}}{{\text{H}}_{\text{3}}} - {\text{C}}{{\text{H}}_{\text{3}}} + {\text{C}}{{\text{l}}^*} \to {\text{C}}{{\text{H}}_{\text{3}}} - {\text{CH}}_2^* + {\text{HCl}}$
${\text{C}}{{\text{H}}_{\text{3}}} - {\text{CH}}_2^* + {\text{C}}{{\text{l}}_2} \to {\text{C}}{{\text{H}}_{\text{3}}} - {\text{C}}{{\text{H}}_2} - {\text{Cl}} + {\text{C}}{{\text{l}}^*}$

Termination:
The propagation stops when the chlorine radical finds another chlorine radical. Three termination reactions are possible.
The reactions are as follows:
$2{\text{C}}{{\text{H}}_{\text{3}}} - {\text{CH}}_2^* \to {\text{C}}{{\text{H}}_{\text{3}}} - {\text{C}}{{\text{H}}_2} - {\text{C}}{{\text{H}}_2} - {\text{C}}{{\text{H}}_3}$
${\text{C}}{{\text{H}}_{\text{3}}} - {\text{CH}}_2^* + {\text{C}}{{\text{l}}^*} \to {\text{C}}{{\text{H}}_3} - {\text{C}}{{\text{H}}_2} - {\text{Cl}}$
$2{\text{C}}{{\text{l}}^*} \to {\text{C}}{{\text{l}}_2}$

The mechanism of chlorination of ethane indicates regeneration of free radicals during the course of the reaction. The regeneration of free radicals allows the propagation of reaction and thus, desired product can be obtained.Thus, the chlorination of ethane is an example of free radical substitution.

Hence, the correct answer is option (C) free radical substitution.

Note: If the termination reaction does not occur then the reaction yields a mixture of products. This is the major drawback of the free radical substitution reactions. Several products like chloroethane, dichloroethane, trichloroethane, etc. are possible. Thus, the termination step is very important.