The enormous acidifying effect of the supersubstituent NSO2CF3 on the acidity of derivatives of benzenesulfonamide and toluene-p-sulfonamide in the gas phase and in dimethyl sulfoxide

Abstract

The effect of stepwise replacement of O oxygen atoms by NSO₂CF₃ fragments in the sulfonyl group of toluene-p-sulfonamide and benzenesulfonamide on their acidity has been studied in the gas phase and dimethyl sulfoxide (DMSO). Incorporation of the first NSO₂CF₃ group into 4-MeC₆H₄SO₂NH₂ increases its gas-phase acidity by 23.6 kcal mol⁻¹. Substituting the second O by the NSO₂CF₃ group leads to an additional acidity increase of 10.7 kcal mol⁻¹; the total acidity increase is thus 34.3 kcal mol⁻¹ (25 powers of ten!). In DMSO solution the total acidity increase is 13 pK_a units (17.7 kcal mol⁻¹). These findings are also supported by computational studies using DFT B3LYP at the 6-31+G* level and the semiempirical PM3 method. The results of this work have potentially important implications for the design of new strongly acidic catalytic materials.