Question

Role of Lewis acid, $AlCl_3$ in electrophilic aromatic substitution is:
A.To produce electrophile
B.Increase rate of reaction
C.To produce nucleophile
D.Electron acceptor

Answer 1

In the electrophilic substitution reaction alkyl halide reacts with an aromatic ring. There is no electrophile already present in the reactant, so the electrophile must have to be generated.

Complete step by step answer:
A Lewis acid is that substance which can accept electrons. Aluminium has a group number 13 is a 3rd period element and hence has vacant d orbital. Hence the aluminium can accept electrons in the vacant orbital and is a Lewis acid.
Electrophilic aromatic substitution reactions are those reactions in which an electrophile substitutes a group on the aromatic ring. An electrophile is electron deficient species which are in need of electrons. The reactant used in this reaction is alkyl halide. Alkyl halide reacts with aluminium chloride that is ${\text{AlC}}{{\text{l}}_3}$ and forms carbocation which acts as electrophile. The reacts occurs as follow:
${\text{AlC}}{{\text{l}}_3} + {\text{R}} - {\text{Cl}} \to {\text{AlC}}{{\text{l}}_4}^ - + {{\text{R}}^ + }$
The carbotation sur to positive charge is electron deficient. So the role of ${\text{AlC}}{{\text{l}}_3}$ is to generate the electrophile.

The correct option is A.

Note:
The above reaction belongs to the class of Friedel Craft reaction. There are two types of FC or Friedel Craft reaction, Freidel craft alkylation and Freidel craft acylation. In the former one alkyl group substitutes at aromatic ring and in the later one the acyl group makes substitution at aromatic ring. Aluminium chloride is white solid in chlorine and oxygen is present in the ration one is two 2.