18.21: 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.
