| Abstract
| - Acidity constants and rates of reversible deprotonation of acetonyltriphenylphosphonium ion (1H+),phenacyltriphenylphosphonium ion (2H+), N-methyl-4-phenacylpyridinium ion (3H+), and N-methyl-4-(phenylsulfonylmethyl)pyridinium ion (4H+) by amines in water, 50% DMSO−50% water (v/v),and 90% DMSO−10% water (v/v) have been determined. From the respective Brønsted plots, logko values for the intrinsic rate constants of the various proton transfers were obtained. Solventtransfer activity coefficients of the carbon acids and their respective conjugate bases were alsodetermined which helped in understanding how the pKa values and intrinsic rate constants dependon the solvent. Some of the main conclusions are as follows: (1) The pKa values of 1H+, 2H+, and3H+ are significantly higher than that of 4H+ because of a stronger resonance stabilization of thecorresponding conjugate bases 1, 2 and 3, respectively. (2) The electronic effects of the PPh3+ andthe N-methyl-4-pyridylium group are similar but the mix between inductive and resonance effectis different. (3) All four acids become more acidic upon addition of DMSO to the solvent. In allcases, the main factor is the stronger solvation of H3O+ in DMSO; for 1H+, 2H+, and 3H+ but not4H+ this factor is significantly attenuated by stronger solvation of the carbon acid in DMSO. (4)The intrinsic rate constants for proton transfer are relatively high for all four carbon acids andshow little solvent dependence; this contrasts with nitroalkanes which have much lower intrinsicrate constants and show a strong solvent dependence. These results can be understood by a detailedanalysis of the interplay between inductive, resonance, and solvation effects.
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