The mandelamide keto-enol system in aqueous solution. Generation of the enol by hydration of phenylcarbamoylcarbene.

Flash photolysis of diazophenylacetamide in aqueous solution produced phenylcarbamoylcarbene, whose hydration generated a transient species that was identified as the enol isomer of mandelamide. This assignment is based on product identification and the shape of the rate profile for decay of the enol transient, through ketonization to its carbonyl isomer, as well as by the form of acid−base catalysis of and solvent isotope effects on the decay reaction. Rates of enolization of mandelamide were also determined, by monitoring hydrogen exchange at its benzylic position, and these, in combination with the ketonization rate measurements, gave the keto−enol equilibrium constant pKE = 15.88, the acidity constant of the enol ionizing as an oxygen acid, = 8.40, and the acidity constant of the amide ionizing as a carbon acid = 24.29. (These acidity constants are concentration quotients applicable at ionic strength = 0.10 M.) These results show the enol content and carbon acid strength of mandelamide, like those of ...