Flash photolytic generation of two keto tautomers of 1-naphthol in aqueous solution: kinetics and equilibria of enolization.

Benzo[b]cyclohexa-2,4-dien-1-one (4) and benzo[b]cyclohexa-2,5-dien-1-one (5), the two most stable keto tautomers of 1-naphthol (1), were generated in aqueous solution by Norrish Type II fission of 4- and 2-phenacyl-1-tetralone, respectively, and the pH-rate profiles of their enolization were measured by flash photolysis. Several isotopic exchange rates of 1 were measured in aqueous acid to determine the corresponding rate constants of ketonization. The resulting equilibrium constants for enolization are pKE(4) = -7.1 and pKE(5) = -6.2. The acidity constants of the carbon acids 4 and 5, pKa(4) = 2.1 and pKa(5) = 3.0, were then obtained from a thermodynamic cycle using pKa(1) = 9.25.