Ketonization equilibria of phenol in aqueous solution

The two keto tautomers of phenol (I), cyclohexa-2,4-dienone (2) and cyclohe xa-2,5-dienone (3), were generated by flash photolysis of appropriate precu rsors in aqueous solution, and the pH-rate profiles of their enolization re actions, 2 --> 1 and 3 --> 1, were measured. The rates bf the reverse react ions, 1 --> 2 and 1 -, 3, were determined from the rates of acid-catalyzed hydron exchange at the ortho- and para-positions of 1; the magnitude of the kinetic isotope effect was assessed by comparing the rates of hydrogenatio n of phenol-2t and -2d. The ratios of the enolization and ketonization rate constants provide the equilibrium constants of enolization, pK(E)(2, aq, 2 5 degrees C) = -12.73 +/- 0.12 and pK(E)(3, aq, 25 degrees C) = -10.98 +/- 0.15. Combination with the acidity constant of phenol also defines the acid ity constants of 2 and 3 through a thermodynamic cycle. These ketones are r emarkably strong carbon acids: pK(a)(2) = -2.89 +/- 0.12 and pK(a)(3) = -1. 14 +/- 0.15. They disappear by proton transfer to the solvent with lifetime s, tau(2) = 260 mu s and tau(3) = 13 ms, that are insensitive to pH in the range from 3-10.