How would you explain the halogenation of benzene?

Question

Ellie Adams · Accepted Answer

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

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.

Isaac Aronson · Answer

undefined The halogenation of benzene involves the substitution of a hydrogen atom on the benzene ring with a halogen atom (chlorine, bromine, or iodine) in the presence of a halogen carrier catalyst such as iron, aluminum chloride, or iron(III) chloride. The reaction proceeds through electrophilic aromatic substitution, where the halogen carrier catalyst helps generate an electrophile (halonium ion) by temporarily accepting a halogen atom's lone pair of electrons. This electrophile then attacks the benzene ring, displacing a hydrogen atom and forming a halogen-substituted benzene molecule.