18.17: Directing and Steric Effects in Disubstituted Benzene Derivatives

When disubstituted benzenes undergo electrophilic substitution, the product distribution depends on the directing effect of both substituents. When the directing effects of both substituents reinforce each other, a single product is obtained. For example, bromination of p-nitrotoluene occurs ortho to the methyl group and meta to the nitro group, which is the same position, resulting in a single product. However, if the directing effects of the two groups oppose each other, the more strongly activating group directs the substituent position. For instance, in the nitration of p-methylphenol, the stronger activator—the hydroxyl group—directs the substitution ortho to it. Substituents with similar activating properties furnish a mixture of products. The steric effect is also instrumental in determining product distribution. For instance, nitration of p-tert-butyltoluene occurs at the less hindered position—ortho to the methyl group. Similarly, substitution does not usually occur between two groups in a meta-disubstituted ring.

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.