Question

Discuss the mechanism of chlorination of ethane and alkylation of benzene

Answer 1

Dear Student,
Chlorination of ethane

The chlorination of ethane is a free radical substitution reaction that takes place in the presence of sunlight. The free radical chlorine is generated which attacks on the ethane and the H gets substituted by the Cl leading to the formation of a new radical .This radical has an active end and keeps the chain alive or it can collide with another same free radical leading to the termination of the chain.
$$\begin{gathered} Initiation:C{l}_2\stackrel{h\nu }{\to }C{l}^{.}+C{l}^{.} \\ Propagation:C{H}_3-C{H}_2-H+C{l}^{.}\to C{H}_3-C{H_2}^{.}+HCl \\ C{H}_3-C{H_2}^{.}+Cl-Cl\to C{H}_3-C{H}_2-Cl+C{l}^{.} \\ Termination:C{l}^{.}+C{l}^{.}\to C{l}_2\left(combination\right) \\ C{H}_3-C{H_2}^{.}+C{H}_3-C{H_2}^{.}\to C{H}_3-C{H}_2-C{H}_2-C{H}_3 \end{gathered}$$
This keeps on continuing till H are replaced by Cl

Alkylation of Benzene
The alkylation of benzene or better called as friedel Crafts alkylation is an electrophilic substitution reaction in which the elecrophile CH_​3 generated by the friedel crafts reagent (CH₃Cl+AlCl₃) attacks on the electron rich benzene. Since the pi bonds in benzene are in resonance and all positions are equivalent so the electrophile can attack on any place . In the final step the H⁺ gets removed and the product is formed. AlCl₃ is a catalyst and gets regenrated after the reaction

$$\begin{gathered} 1.GenerationofelectrophileC{H}_{3}Cl+AlC{l}_3\to C{H_3}^{+}+AlC{l_4}^{-} \\ 2.C_6{H}_6+C{H_3}^{+}\to C_6{H}_{5}C{H}_3+H^{+} \\ 3.H^{+}+AlC{l_4}^{-}\to AlC{l}_3+HCl \end{gathered}$$