Halogenation of Benzene The Need for a Catalyst

An example of electrophihc aromatic substitution is halogenation. Benzene is normally unreactive in the presence of halogens, because halogens are not electrophihc enough to disrupt its aromaticity. However, the halogen may be activated by Lewis acidic catalysts, such as ferric halides (FeXs) or aluminum halides (AIX3), to become a much more powerful electrophile. 

How does this activation work Lewis acids have the ability to accept electron pairs. When a halogen such as bromine is exposed to FeBrs, the two molecules combine in a Lewis acid-base reaction. 

The FeBr4 formed in this step now functions as a base, abstracting a proton from the cyclo-hexadienyl cation intermediate. This transformation not only furnishes the two products of the reaction, bromobenzene and hydrogen bromide, but also regenerates the FeBr3 catalyst. 

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