Acidity inversions of α-NO2 and α-SO2CF3 activated carbon acids as a result of contrasting solvent effects on transfer from water to dimethyl sulfoxide solutions

Measurements of pK(a) values for the ionization of alpha-X-substituted ethyl acetates (1, X = NO2; 2, X = CN; 3, X = SO2CF3) in H2O-Me2SO mixtures and pure Me2SO show a unique response of the acidity of the SO2CF3 derivative to the solvent changes, thereby resulting in a remarkable inversion in the acidifying effects of the strongly electron-withdrawing NO2 and SO2CF3 groups on going from H2O to Me2SO. Overall, the results obtained provide strong evidence that the powerful electron-withdrawing effect of the SO2CF3 group is by far the result of polarization effects rather than other factors such as negative hyperconjugation.