Chapter 31 extends the treatment of halogen compounds from AS Level to include halogenoarenes, and develops an understanding of how the attachment of a halogen atom to an aromatic ring fundamentally alters its reactivity compared to a halogenoalkane.

The preparation of halogenoarenes by electrophilic substitution of an arene with chlorine or bromine in the presence of a Lewis acid catalyst is described, with chlorobenzene and the chloromethylbenzenes as specific examples. The chapter then addresses the marked difference in reactivity between halogenoalkanes and halogenoarenes towards nucleophilic substitution, using chloroethane and chlorobenzene as a direct comparison. This difference is accounted for in terms of the delocalisation of the halogen lone pair into the aromatic pi system, which strengthens the carbon-halogen bond and renders it far less susceptible to nucleophilic attack.