Question

The function of $AlC{l_3}$ in Friedel- Crafts reaction is:
A. to absorb $HCl$
B. to absorb water
C. to produce nucleophile
D. to produce electrophile

Answer 1

The Friedel- Crafts reaction is a special type of acylation/ alkylation reaction which is helpful in attaching an alkyl or an acyl group to the benzene ring with the help of an electrophile. In this type of reaction, the reactant includes alkyl/ acyl halide along with benzene in the presence of a Lewis acid which accepts a halogen atom temporarily so that it can release an electrophile.

Complete step by step answer:

The function of $AlC{l_3}$ ​in Friedel-Crafts reaction is to produce an electrophile, which later adds to the benzene as benzene is an electron rich complex and is attracted towards the electrophile. This electrophilic aromatic substitution allows the synthesis of monoacetylated products from the reaction between arenes (such as benzene) and acyl/ alkyl chlorides or anhydrides. The products are deactivated and do not undergo a second substitution as there the halide ions cannot remain stable along with the Lewis acid and detaches itself from the aluminium chloride. The halide ion is accepted by the aluminium chloride to form $AlCl_4^ -$ and thus it releases a ${R^ + }$which is next attacked by the benzene ring as it acts as a nucleophile.

Note:
Generally, a stoichiometric quantity of the Lewis acid catalyst is required (which means that the Lewis acid should be taken in a limited amount and not in excess) for both the substrate and the product to form coordination complexes. $AlC{l_3}$​ (and the other Lewis acids just like it) will coordinate to halogens, and facilitate the breaking of these bonds. In doing so, it increases the electrophilicity of its counter joining partner, increasing its reactivity.