Electrophilic and nucleophilic substitution in the Benzene ring and the Hammett Equation

Abstract

The Hammett equation as applied to aromatic nuclear electrophilic and nucleophilic substitution is tested for a wide range of substituents. For nucleophilic substitution quite good agreement is obtained for electron-attracting substituents using σ^* values, but large deviations occur with substituents releasing electrons by the conjugative mechanism. In electrophilic substitution, in so far as results are available, a fair measure of agreement obtains for electron-attracting substituents, but very large and to some extent irregular deviations occur for the conjugative electron- releasing substituents. The deviations occur whether σ or σ^* values are used, and new sets of σ_N^** and σ_E^** values are defined and the values recorded. A comparison is made of substituent effects in nuclear electrophilic and nucleophilic substitution.