THE HALOGENATION OF BENZENE

This page gives you the facts and a simple, uncluttered mechanism for the electrophilic substitution reaction between benzene and chlorine or bromine in the presence of a catalyst such as aluminium chloride or iron. If you want this mechanism explained to you in detail, there is a link at the bottom of the page.

The facts

Benzene reacts with chlorine or bromine in an electrophilic substitution reaction, but only in the presence of a catalyst. The catalyst is either aluminium chloride (or aluminium bromide if you are reacting benzene with bromine) or iron.

Strictly speaking iron isn't a catalyst, because it gets permanently changed during the reaction. It reacts with some of the chlorine or bromine to form iron(III) chloride, FeCl₃, or iron(III) bromide, FeBr₃.

These compounds act as the catalyst and behave exactly like aluminium chloride in these reactions.

The reaction with chlorine

The reaction between benzene and chlorine in the presence of either aluminium chloride or iron gives chlorobenzene.

or:

The reaction with bromine

The reaction between benzene and bromine in the presence of either aluminium bromide or iron gives bromobenzene. Iron is usually used because it is cheaper and more readily available.

or:

The formation of the electrophile

We are going to explore the reaction using chlorine and aluminium chloride. If you want one of the other combinations, all you have to do is to replace each Cl by Br, or each Al by Fe.

As a chlorine molecule approaches the benzene ring, the delocalised electrons in the ring repel electrons in the chlorine-chlorine bond.

It is the slightly positive end of the chlorine molecule which acts as the electrophile. The presence of the aluminium chloride helps this polarisation.

The electrophilic substitution mechanism

Stage one

Stage two

The hydrogen is removed by the AlCl₄^- ion which was formed in the first stage. The aluminium chloride catalyst is re-generated in this second stage.