Hydration of phenylketene revisited: A counter-intuitive result

In previous work (Can. J. Chem. 1987, 65,1719-1723 and J. Am, Chem. Soc. 1995, 117,91659171), flash photolysis of diazoacetophenone or phenylhydroxycyclopropenone in aqueous solution was found to produce phenylketene as a short-lived transient species with absorbance at lambda congruent to 260 nm, which decayed with single-exponential kinetics. It has now been discovered that, in the acidity region [H+] = 0.000 01 to 0.06 M, this decay is preceded by a faster absorbance rise, and that the overall change conforms well to a double exponential rate law. Analysis of the new data produces rate profiles whose general shapes, as well as the numerical values of their constituent rate constants, plus the form of buffer catalysis, indicate that this newly discovered absorbance rise represents ketonization of phenylacetic acid enol, and that the subsequent absorbance decay represents addition of water to phenylketene. The chemistry of the system, however, requires ketene hydration to precede enol ketonization in a time sequence opposite from that of the absorbance changes. This seemingly counter-intuitive result is nevertheless consistent with the rate law that governs the time evolution of the central species in a two-step rise and decay, such as that observed here.