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 pKE 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 pKa value of acetic acid, we obtained pKKa = 26.6 for ionization of acetic acid as a carbon acid in water, and pKEa = 7.3 for ionization of the enol of acetic acid.
Bibliographical noteFunding Information:
This work was supportedb y the National Science Foundation. Discussion with Professor J. P. Richard was also helpful.
- Acetic acid
- Combined QM/MM simulations
- Keto-enol equilibrium