TABLE OF CONTENTS In aromatic carboxylic acids, the carboxylic acid group (CO 2H) is an electron-withdrawing moiety (-I, -M effects), hence it reduces the reactivity of the attached aromatic nucleus towards electrophilic substitution. So like aromatic carbonyl compounds (Section 7), aromatic carboxylic acids are more resistant towards sulfonation than the parent aromatic hydrocarbons. The reaction therefore generally requires rather forcing conditions, e.g. the use of a large excess of chlorosulfonic acid at temperatures of more that 120 C.3 The reactions of benzoic, cinnamic and phenylacetic acid with chlorosulfonic acid were reported by Suter.1
Smiles and Stewart266 first demonstrated that the action of excess chlorosulfonic acid on benzoic acid 270 yields 3-chlorosulfonylbenzoic acid 271 (Scheme 6). Later work by Cremlyn267 showed that a large excess of the reagent (at least five equivalents) at 130 C (1 hour) was needed to obtain the optimum yield (82%) of the sulfonyl chloride 271. The postulated reaction mechanism for the chlorosulfonation of benzoic acid involves the mixed anhydride intermediate 272, which accounts for the large excess of reagent required (Scheme 6). In agreement, it was found that the use of the carboxylic acid amide often permitted a high yield of the corresponding m-chlorosulfonyl derivative to be obtained using less chlorosulfonic acid (three to four equivalents).267 On addition of the crude reaction product to crushed ice, the intermediate 272a is rapidly cleaved to regenerate the carboxylic acid group, whereas the...
