Question

The chlorination of methane is an example of:
A.Addition
B.Elimination
C.Substitution
D.Chain reaction

Answer 1

The addition of chlorine to a molecule or a compound is known as chlorination. Chlorine can be added to a compound through addition or substitution. Methane is an organic compound having the chemical formula $C{H_4}$. It is the simplest hydrocarbon which is the compound of hydrogen and carbon.

Complete step by step answer:
In order to understand the chlorination of methane we have to identify the mechanism of chlorination. The chlorination of methane occurs in presence of light denoted by $h\nu$ in a stepwise manner. The reaction consists of a radical mechanism to produce chlorinated methane. Three steps are involved in radical mechanism, initiation, propagation and termination. Let us understand the reaction mechanism step by step.
Step$1$: Initiation:
At first the chlorine radicals are generated by cleavage of the $Cl - Cl$ bond.
$Cl - Cl\xrightarrow{{h\nu }}2C{l^ \cdot }$
Step$2$: Propagation:
$C{H_4} + C{l^ \cdot }\xrightarrow{{}}C{H_3}^ \cdot + HCl$
$C{H_3}^ \cdot + C{H_3}^ \cdot \xrightarrow{{}}C{H_3} - C{H_3}$
One hydrogen atom of methane is replaced by chlorine atom so it is a substitution reaction. The product is called chloromethane.
Step$3$: Termination:
The propagation step is stopped when a radical finds another radical. Three termination steps are possible for this reaction.
$C{l^ \cdot } + C{l^ \cdot }\xrightarrow{{}}C{l_2}$
$C{H_3}^ \cdot + C{l^ \cdot }\xrightarrow{{}}C{H_3}Cl$

The radical mechanism indicates regeneration of radicals during the course of the reaction. It allows the reaction to propagate and lead to the desired product.

Hence option C is correct i.e. substitution.

Note: The radical reaction suffers through a major drawback. If the termination of radical does not occur, then the reaction gives a mixture of products. For this reaction there are several products possible like chloromethane, dichloromethane, trichloromethane, and tetrachloromethane. That is why the termination step is very important in radical reaction.