The mandelic acid keto-enol system in aqueous solution, generation of the enol by hydration of phenylhydroxyketene and phenylcarboxycarbene

Flash photolysis of phenyldiazoacetic acid and the methyl, n-butyl, and isobutyl esters of benzoylformic acid in aqueous solution generated a transient species that was identified, through product determination and the form of acid-base catalysis plus solvent isotope effects on its decay, as the enol of mandelic acid, 1. When benzoylformate esters were the flash photolysis substrates, the enol was formed by hydration of phenylhydroxyketene, 5, itself generated by Norrish type II photoelimination of the esters, and when phenyldiazoacetic acid was the substrate, the enol was formed by hydration of phenylcarboxycarbene, 6, produced by dediazotization of the diazo acid. Rates of enolization of mandelic acid were also determined, by monitoring the incorporation of deuterium into its alpha-position from a D2O solvent, and combination of these with rates of ketonization gave the keto-enol equilibrium constant, pK(E) = 16.19. The acidity constant of the enol ionizing as an oxygen acid was determined as well, pQ(a)(E) = 6.39, and combination of that with K-E led to the ionization constant of the keto form of mandelic acid ionizing as a carbon acid, pQ(a)(K) = 22.57. (These acidity constants are concentration quotients, applicable at ionic strength = 0.10 M.) These results are compared with other keto-enol systems, and their bearing on the enzymatic racemization of mandelic acid is discussed.