Question

Ethane is formed during the formation of chloromethane by chlorination of methane because:
A) Higher members of the hydrocarbons are generally formed during reactions.
B) Two methyl free radicals may combine during chlorination to give ethane.
C) Two chloromethane molecules react to form ethane.
D) Chlorine free radical reacts with methane to give ethane.

Answer 1

To solve 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. Methane is an organic compound which has a molecular formula ${\text{C}}{{\text{H}}_{\text{4}}}$.

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 and 3. Termination.
We will see the mechanism of chlorination step by step.

1) Initiation:
In this step, the chlorine radicals are generated by the cleavage of chlorine-chlorine $\left( {{\text{Cl}} - {\text{Cl}}} \right)$ bond. The cleavage of chlorine-chlorine bond occurs in the presence of light denoted by $h\nu$.
The reaction is as follows:
${\text{Cl}} - {\text{Cl}}\xrightarrow{{h\nu }}{\text{C}}{{\text{l}}^*} + {\text{C}}{{\text{l}}^*}$

2) Propagation:
In this step, one hydrogen atom of methane is replaced by the chlorine atom. Thus, it is a substitution reaction. The product formed is known as chloromethane.
The reactions are as follows:
${\text{C}}{{\text{H}}_{\text{4}}} + {\text{C}}{{\text{l}}^*} \to {\text{CH}}_3^* + {\text{HCl}}$
${\text{CH}}_3^* + {\text{C}}{{\text{l}}_2} \to {\text{C}}{{\text{H}}_{\text{3}}} - {\text{Cl}} + {\text{C}}{{\text{l}}^*}$

3) Termination:
The propagation stops when the chlorine radical finds another chlorine radical. Three termination reactions are possible.
The reactions are as follows:
$2{\text{CH}}_3^* \to {\text{C}}{{\text{H}}_{\text{3}}} - {\text{C}}{{\text{H}}_3}$
${\text{CH}}_3^* + {\text{C}}{{\text{l}}^*} \to {\text{C}}{{\text{H}}_3} - {\text{Cl}}$
$2{\text{C}}{{\text{l}}^*} \to {\text{C}}{{\text{l}}_2}$
In the termination step, we can see that two methyl free radicals combine and produce ethane.
Thus, ethane is formed during the formation of chloromethane by chlorination of methane because two methyl free radicals may combine during chlorination to give ethane.

Thus, the correct option is (B) two methyl free radicals may combine during chlorination to give ethane.

Note: The mechanism of chlorination of methane 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 methane is an example of free radical substitution.