Aromatic sulphonation. Part XXII. Kinetic isotope effects in the reaction of [1,3,5-2H3]benzene with sulphur trioxide: competitive sulphonation of benzene and toluene

Abstract

The kinetic isotope effect of hydrogen in the sulphonation of [1,3,5-²H₃]benzene in trichlorofluoromethane has been determined as k_H/k_D= 1·23 ± 0·08 at –35°, with nitromethane as solvent k_H/k_D= 1·34 ± 0·08 at 20°. The kinetic isotope effect for the accompanying sulphonylation in trichlorofluoromethane was found to be k_H/k_D= 1·1 ± 0·1 at –35°. These results are discussed in terms of the recently established mechanism for aromatic sulphonation with sulphur trioxide in aprotic solvents. It has been determined, by competitive experiments, that the sulphonation rate ratio of toluene to benzene and the degree of para-substitution in toluene are greater with nitromethane than with trichlorofluoromethane as solvent. The apparent greater selectivity in the case of the former solvent is explained in terms of complex formation between sulphur trioxide and nitromethane.