How would you explain the halogenation of benzene?

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Answer 1 · 2017-01-05T19:44:37

The halogenation of benzene is an electrophilic aromatic substitution reaction.

Electrophilic aromatic substitution

Electrophilic aromatic substitution is a reaction in which an atom on a aromatic ring is replaced by an electrophile.

A typical halogenation reaction is

The electrophile is an ion that is generated by the catalyst.

Mechanism

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Step 1. Generation of the electrophile

A Lewis acid catalyst, usually #"AlBr"_3# or #"FeBr"_3#, reacts with the halogen to form a complex that makes the halogen more electrophilic.

Step 2. Electrophilic attack on the aromatic ring

The nucleophilic π electrons of the aromatic ring attack the electrophilic #"Br"# atom .

This forms #"FeBr"_4^"-"# and generates a cyclohexadienyl cation intermediate, destroying the aromaticity of the ring.

Step 3: Loss of #"H"^"+"# and restoration of aromaticity

The #"FeBr"_4^"-"# removes the #"H"^"+"# from the ring.

This re-forms the aromatic ring, produces #"HBr"#, and regenerates the catalyst.