18.17 : Directing and Steric Effects in Disubstituted Benzene Derivatives

Electrophilic substitution of a disubstituted benzene ring depends on the directing effect of the individual substituents.

If the directing effects of the substituents reinforce each other, the substitution gives a single product.

For example, bromination of p-nitrotoluene is directed at the same position—ortho to the methyl group and meta to the nitro group, giving a single product.

Alternatively, if the directing effects of the substituents compete, the more powerful activating group dominates.

For instance, nitration of p-methylphenol occurs ortho to the hydroxy group because it is a stronger activator than the methyl group.

Substituents having similar activating properties give a mixture of products.

The steric effect also plays an important role in determining product distribution.

For example, nitration of p-tert-butyltoluene occurs at the less hindered position—ortho to the methyl group.

Notably, substitution between two groups in a meta-disubstituted ring is not preferred due to the steric hindrance. For example, the nitration of m-chlorotoluene.