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.
