30.1.3—Whether halogenation will occur

A methyl side chain and an aromatic ring react by different pathways. Radical conditions such as UV light favour substitution in the side chain, while a Lewis-acid catalyst with a halogen favours electrophilic substitution on the ring.

Look first for the reagent, catalyst and light/heat conditions; the same methylbenzene can therefore give different products.

Methylbenzene with Cl₂ under UV gives side-chain chlorination, whereas Cl₂/AlCl₃ gives ring chlorination, mainly the 2- and 4-isomers.

Do not choose a product from the substrate name alone. Conditions select the mechanism and therefore the position of substitution.