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.
ABSTRACT
The replacement of a hydrogen in nitromethane and in phenylnitromethane by the PhCO group has a strong acidifying effect, i.e., PhCOCH(2)NO(2), 5, is 5.8, 6.6, and 8.6 pK(a) units more acidic than CH(3)NO(2) in water, 50% DMSO-50% water (v/v), and 90% DMSO-10% water (v/v), respectively, and PhCOCH(Ph)NO(2), 6, is 2.0, 3.0, and 3.2 pK(a) units more acidic than PhCH(2)NO(2) in the same solvents. A major focus of this paper is an attempt to sort out the relative contributions of resonance, inductive/field, and solvation effects that lead to the increased acidities. To this end rate constants for the reversible deprotonation of 5 by secondary alicyclic amines in water, 50% DMSO-50% water (v/v), and 90% DMSO-10% water (v/v) and for the reversible deprotonation of 6 by secondary alicyclic amines, carboxylate ions, thiolate ions, and aryloxide ions in water, by secondary alicyclic and primary aliphatic amines in 50% DMSO, and by secondary alicyclic anions in 90% DMSO were determined. From Brønsted plots based on these data the intrinsic rate constants (k(o)) for the reactions of 5 and 6 with the various buffer families were obtained and compared with previously determined k(o) values for the deprotonation of CH(3)NO(2) and PhCH(2)NO(2), respectively. An analysis of the changes in k(o) induced by the introduction of the PhCO group, coupled with a comparison of solvent transfer activity coefficients for the transfer of the anions (5(-) and 6(-)) from water to 50% and 90% DMSO with those for CH(2)=NO(2)(-) and PhCH=NO(2)(-), respectively, indicates a substantial increase in the resonance stabilization of 5(-) relative to CH(2)=NO(2)(-) and a corresponding sharp drop in the solvational stabilization by hydrogen bonding from water; the effects on 6(-) are qualitatively similar but quantitatively much smaller. Our study also shows that the intrinsic rate constant for proton transfer from 6 to thiolate ions is higher than for proton transfer to aryl oxide ions and amines. This result indicates that, in contrast to most other proton-transfer reactions, hydrogen-bonding stabilization of the transition state for deprotonation of 6 is not an important factor.
