Dirigierende und sterische Effekte in disubstituierten Benzol-Derivaten

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.