Aromatic sulphonation. Part L. Sulphonation of the trimethylbenzenes: isomer distributions and hydrogen kinetic isotope effect

Abstract

Isomer distributions for the sulphonation of 1,2,3- and 1,2,4-trimethylbenzene with concentrated aqueous sulphuric acid at 25.0° have been determined. The results are discussed in terms of sulphonation by H₃SO₄⁺ and H₂S₂O₇ as sulphonating entities. The isomer distributions for the H₂S₂O₇ mechanism are 90 ± 1% 4- and 10 ± 1% 5- substitution for the 1,2,3-isomer and 75 ± 2% 5- and 25 ± 2% 6-substitution for the 1,2,4-isomer, those for the H₃SO₄⁺ mechanism are 86 ± 1% 4- and 14 ± 1% 5-substitution with the former isomer andd 89 ± 2% 5- and 11 ± 2% 6-substitution by the latter. The deviations of the observed isomer distributions and of the partial rate factors for the H₃SO₄⁺ mechanism from those calculated on the basis of the additivity principle are discussed. The sulphonation of 1,3,5-trimethylbenzene with sulphur trioxide proceeds without a primary hydrogen kinetic isotope effect, k_H/k_D being 1.15 ± 0.13 and 0.98 ± 0.09 for nitromethane at 0° and trichlorofluoromethane at –35° respectively.