Kinetics of Proton Transfer from Cationic Carbon Acids in Water and Aqueous DMSO. Effect of Activating Groups and Solvent on Intrinsic Rate Constants

Abstract

[reaction: see text] 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, log k(o) values for the intrinsic rate constants of the various proton transfers were obtained. Solvent transfer activity coefficients of the carbon acids and their respective conjugate bases were also determined which helped in understanding how the pKa values and intrinsic rate constants depend on the solvent. Some of the main conclusions are as follows: (1) The pK(a) values of 1H+, 2H+, and 3H+ are significantly higher than that of 4H+ because of a stronger resonance stabilization of the corresponding conjugate bases 1, 2 and 3, respectively. (2) The electronic effects of the PPh3+ and the N-methyl-4-pyridylium group are similar but the mix between inductive and resonance effect is different. (3) All four acids become more acidic upon addition of DMSO to the solvent. In all cases, the main factor is the stronger solvation of H3O+ in DMSO; for 1H+, 2H+, and 3H+ but not 4H+ 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 and show little solvent dependence; this contrasts with nitroalkanes which have much lower intrinsic rate constants and show a strong solvent dependence. These results can be understood by a detailed analysis of the interplay between inductive, resonance, and solvation effects.