Bromination [Br2 plus catalyst]

Bromination [Br₂ plus catalyst] Definition:

Treatment with an aromatic such as benzene with bromine (Br₂) and a Lewis acid (such as AlCl₃ or FeCl₃) leads to formation the bromobenzene.

Bromination [Br₂ plus catalyst] Explained:

Benzene is normally unreactive in the presence of halogens, however, when added a Lewis base such as ferric halides (FeX₃) or aluminum halides (AlX₃) it comes to halogenation by electrophilic aromatic substitution.

In bromination of benzene, the Lewis acid FeBr₃ react with Br₂ to form bromobenzene in the following steps:

STEP 1. Generation of electrophile

STEP 2. Addition to an electrophile to form carbocation – the aromatic ring functions as a nucleophile, forming a positively charged intermediate called a sigma complex, or arenium ion, which is resonance stabilized. Here comes to the temporary loss of aromatic stabilization. The FeBr₄‾ also formed in this reaction which will the base in the next step.

STEP 3. Deprotonation of the sigma complex by the FeBr₄‾ with restoring aromaticity. Regeneration of catalyst FeBr₃ which is ready for another reaction cycle.