PROTONATION AND HYDRATION OF HYDROXYKETENE

The protonation and hydration reactions of hydroxyketene have been examined by ab initio self-consistent field (SCF) calculations at the 3-21G and 6-31G* basis-set levels. This ketene serves as a model compound for phenylhydroxyketene whose hydration has been studied experimentally by laser flash photolysis. Results of the calculations on the protonation reaction reveal that the operative mechanism involves protonation at C-beta of the ketene in accordance with previous experimental results and theoretical calculations. Calculations on the hydration reaction involving the water dimer predict that the ketene C = O bond is hydrated, yielding the enol of alpha-hydroxyacetic acid as the intermediate product. This is in agreement with the observation of mandelic acid enol (Ph(HO) C = C(OH)2) as an intermediate in the hydration of phenylhydroxyketene. The negative logarithm of the equilibrium constant for the gas phase keto-enol tautomerization, pK(E) = 17.7 at 298 K (3-21G//3-21G), determined for alpha-hydroxyacetic acid, indicates that the enol content of this acid is 13 times greater than for the parent acetic acid in the gas phase.