TABLE OF CONTENTS Aromatic amines are often protected before treatment with chlorosulfonic acid to avoid decomposition; the protection may be achieved by acetylation (see p 102). However, in several instances unprotected amines have been successfully chlorosulfonated with the reagent.1 In aromatic amines, the amino group exerts a powerful electron-donor (+M, -I) effect on the attached aromatic nucleus so it would be anticipated that sulfonation would preferentially occur in the o-/p-positions of the aromatic ring. On the other hand, in strongly acidic media such as sulfuric acid and chlorosulfonic acid, the amino group is extensively protonated so the electron-donor effect is reduced and an appreciable quantity of m-sulfonation may occur. Indeed, when many amines are treated with oleum at low temperature, Suter1 claimed that the m-sulfonic acids were formed. The sulfonation of amines is complicated by the effects of N-protonation, possible complex formation between the amine and the reagent, and N-sulfonation (sulfamation).5 ,172 The relative thermodynamic stability of the various products is an important factor in the sulfonation reaction. The protonated amine and the complex will be less reactive towards electrophilic substitution than the free amine, hence a substantial amount of the product may arise from sulfonation of the free amine in spite of its relatively low concentration. The kinetics of the sulfonation of aromatic amines by chlorosulfonic acid in o-dichlorobenzene has been examined (see Chapter 2, p 14). It was found that aromatic primary amines generally reacted with...
