# How would you explain the halogenation of benzene?

Jan 5, 2017

The halogenation of benzene is an electrophilic aromatic substitution reaction.

#### Explanation:

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 ${\text{AlBr}}_{3}$ or ${\text{FeBr}}_{3}$, 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 $\text{Br}$ atom .

This forms $\text{FeBr"_4^"-}$ and generates a cyclohexadienyl cation intermediate, destroying the aromaticity of the ring.

Step 3: Loss of $\text{H"^"+}$ and restoration of aromaticity

The $\text{FeBr"_4^"-}$ removes the $\text{H"^"+}$ from the ring.

This re-forms the aromatic ring, produces $\text{HBr}$, and regenerates the catalyst.