Bromination of Aromatics to give Bromoarenes
Description: Treatment of an aromatic such as benzene (Br2) and a Lewis acid (such as AlBr3 or FeBr3) leads to formation of the bromobenzene.
Notes: AlBr3 is a catalyst for this reaction. FeBr3 can be productively used as well. FeCl3 and AlCl3 will also promote the reaction but are slightly inferior due to the fact they will form small amounts of the brominated product.
Notes: The byproduct of each of these reactions is HBr.
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 bromine on the Lewis acid (Step 1, arrow A) makes Br2 more electron poor and thus a better electrophile. Cleavage of Br–Br gives Br(+) which is very reactive (Step 2, arrrow B). Attack of an aromatic PI bond on Br(+) 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 Br–Al bond to give Br(–) followed by deprotonation of the carbocation with Br(–).