A theoretical investigation of the enol content of acetic acid and the acetate ion in aqueous solution

Ab initio molecular orbital calculations and statistical Monte Carlo simulations employing a combined quantum and molecular mechanical potential were used to determine the enol contents of acetic acid and the acetate ion in aqueous solution. A pK_E of 19.3 ± 0.3 was predicted for the keto-enol equilibrium of acetic acid, and 21.8 ± 0.8 for the acetate ion in water. The results are found to be in good accord with Guthrie's calculations based on disproportionation reactions and kinetic data. Combining with the experimental pK_a value of acetic acid, we obtained pK_a^k = 26.6 for ionization of acetic acid as a carbon acid in water, and pK_a^E = 7.3 for ionization of the enol of acetic acid.