Question

The reaction between methane and chlorine to form carbon tetrachloride is a __________ type of reaction.
(A) Addition
(B) Substitution
(C) Displacement
(D) Redox

Answer 1

Hint: Write the reaction mechanism for the formation of carbon tetrachloride using methane and chlorine. Now try to understand the different types of reaction mentioned in the options. Now try to evaluate which type of reaction best suits the reaction mechanism you have written.

Complete step-by-step answer:
We will now study the reaction mechanism for the halogenation of alkanes under sunlight as a whole in order to gain a wide range of knowledge and not just about chlorination of methane.
\[{{X}_{2}}{{\xrightarrow{uv}}_{{}}}\overset{\bullet }{\mathop{X}}\,+\overset{\bullet }{\mathop{X}}\,\]
Here X= F, Cl, Br, I.
-This initial reaction is called the initiation step of the reaction mechanism. The free radicals that are formed are in a high energy state and react quickly to gain stability.
-The halogen free radical polarizes the C-H bond of alkane leading to homolytic fission.
$C{{H}_{3}}-H+\overset{\bullet }{\mathop{X}}\,{{\xrightarrow{{}}}_{{}}}\overset{\bullet }{\mathop{C{{H}_{3}}}}\,+HX$
$\overset{\bullet }{\mathop{C{{H}_{3}}}}\,+{{X}_{2}}{{\xrightarrow{{}}}_{{}}}C{{H}_{3}}X+\overset{\bullet }{\mathop{X}}\,$
-The above reaction is called the propagation step as it keeps the reaction going forward to form products as well free radicals.
As the propagation step continues, we obtain more substituted carbon compounds and finally arrive at the compound, carbon tetrachloride. The compound is mentioned below.
$C{{H}_{4}}\text{ }+\text{ }4C{{l}_{2}}\xrightarrow{{}}\text{ }CC{{l}_{4}}\text{ }+\text{ }4HCl$
-When the methyl free radical reacts either with the halogen free radical or second molecule of methyl free radical, it leads to termination of the reaction. Hence the reaction is called the chain termination step.
$\begin{align}
& \overset{\bullet }{\mathop{C}}\,{{H}_{3}}+\overset{\bullet }{\mathop{X}}\,{{\to }_{{}}}C{{H}_{3}}X \\
& \overset{\bullet }{\mathop{C}}\,{{H}_{3}}+\overset{\bullet }{\mathop{C}}\,{{H}_{3}}{{\to }_{{}}}{{C}_{2}}{{H}_{6}} \\
\end{align}$
Thus, we can conclude that the reaction between methane and chlorine to form carbon tetrachloride is a substitution reaction because chlorine atoms substitute the present hydrogen atoms in methane.

The correct answer is option (B).

Note: It is important to know that although fluorination of alkane is a feasible reaction theoretically, in practice we do not carry out this reaction as it is highly exothermic and catches fire easily hence destroying the products formed. In addition to that the C-F bond is very strong and we cannot carry out further reactions with this haloalkane. This is the reason why chlorination of alkane is preferred over fluorination to carry out further organic reactions and substitutions.