Question

Write the mechanism of Friedal-Craft's alkylation.

Answer 1

Friedal craft's alkylation:

This is an electrophilic aromatic substitution reaction.

In this reaction, the alkylation of an aromatic ring with an alkyl halide using a strong Lewis catalyst such as ZnCl₂, FeCl₃, BF₃. takes place.

The general equation:

Mechanism of the Friedel Craft's Alkylation Reaction:

Step(A):

The Alkyl Halide reacts with AlCl₃ ( a strong Lewis Acid) to form an electrophilic compound 'C', (i.e. the carbon attached to the chlorine which has a lot of positive charge ) a carbocation. { (CH₃)₃C-Cl AlCl₃}

Step(B):

After loosing the halide to the lewis acid, an Electrophilic alkyl Carbocation is formed.

The ring gains a positive charge now because the two delocalized electrons form a new bond with the CH₃⁺ ion; therefore causing a partially terminated delocalization.

Pi electrons of the aromatic benzene ring turns into a nucleophile to attack the partially positive carbon i.e the carbocation to completely eliminate the aromaticity to form a non aromatic intermediate called the cyclohexadienyl cation intermediate.

One of the Aluminium Chloride bond breaks to form the Hydrochloric acid. This regenerates the aluminium chloride catalyst in the process.