5 Phenols

Phenols readily undergo sulfonation due to the electron-donating (+M, -I) property of the hydroxyl group and to the possible formation of intermediate sulfate esters which rapidly rearrange to the arylsulfonic acids.1

Phenols can either react with the sulfonating agent at the hydroxyl group by sulfation ( O-sulfonation) or by sulfonation of the aromatic ring ( C-sulfonation) or both. It is generally found3 ^,5 that sulfation is more easily achieved than sulfonation and in acidic reagents protonation of the hydroxyl moiety precedes sulfonation.5 Sulfation (esterification) is favoured by electron-withdrawing substituents in the aryl ring, but is less common in o-substituted phenols due to steric hindrance.172

Chlorosulfonic acid is especially useful in the preparation of phenol sulfonyl chlorides because, in this case, the reaction of metal sulfonates with phosphorus pentachloride cannot be used.1 Phenol 116, by treatment with chlorosulfonic acid (one equivalent) in carbon disulfide at room temperature, yields a mixture of the o- and p-sulfonic acids 117 and 118 (mainly the p-isomer 118), but at low temperature (-15 C) the products were phenyl hydrogen sulfate 119 and the p-sulfonic acid 118173 ^,180 (Equation 38). When the reaction time was increased the yield of the p-sulfonic acid 118 also increased and on standing the hydrogen sulfate 119 gradually rearranged to the sulfonic acids.1 ^,173 ^,174 Later studies175 ^,176 found that the reaction in carbon disulfide or dichloroethane at -40...