Description: Treatment of an aromatic (such as benzene) with chlorine (Cl2) and a Lewis acid such as AlCl3 or FeCl3 leads to formation of the chlorinated aromatic.
Notes: AlCl3 is a catalyst for this reaction. The byproduct is hydrochloric acid (HCl). FeCl3 (among others) can also be used as a catalyst for this reaction.
Notes: Note that the byproduct for each of these reactions is HCl
Mechanism: This is an electrophilic aromatic substitution reaction and proceeds in two phases, an activation phase (Steps 1 and 2) and a substitution phase (Steps 3 and 4). Attack of chlorine on the Lewis acid (Step 1, arrow A) makes Cl2 more electron poor and thus a better electrophile. Cleavage of Cl–Cl gives Cl(+) which is very reactive (Step 2, arrrow B). Attack of an aromatic PI bond on Cl(+) leads to a carbocation (Step 3, arrow C) after which deprotonation (Step 4, arrows D and E) regenerates aromaticity.
Notes: It’s also reasonable to show arrow B happening at the same time as arrow C
It’s also reasonable to show cleavage of the Cl–Al bond to give Cl(–) followed by deprotonation of the carbocation with Cl(–).