Energetics of acetic acid enol in aqueous solution. A new method for thermodynamic estimation

Abstract

A new disproportionation calculation allows the estimation of the free energy of formation of the enol of acetic acid as 65 ± 2 kcal/mol. The value of pK E derived from this free energy, pK E = 21 ± 2, is in satisfactory agreement with information from the literature about rates of exchange. Analysis of the data on rates of exchange of the C-H protons of acetic acid using Marcus theory allows an independent estimate of the enol content. Exchange in acid and in base lead to internally consistent estimates, pK E = 19.3 ± 2.2, which are within the combined uncertainties of the values from the thermodynamic estimate.