| Abstract
| - Flash photolysis of phenyldiazoacetic acid and the methyl,n-butyl, and isobutyl esters of benzoylformicacid in aqueous solution generated a transient species that wasidentified, through product determination and theform of acid−base catalysis plus solvent isotope effects on itsdecay, as the enol of mandelic acid, 1.Whenbenzoylformate esters were the flash photolysis substrates, the enolwas formed by hydration of phenylhydroxyketene,5, itself generated by Norrish type II photoelimination ofthe esters, and when phenyldiazoacetic acid was thesubstrate,the enol was formed by hydration of phenylcarboxycarbene, 6,produced by dediazotization of the diazo acid. Ratesof enolization of mandelic acid were also determined, by monitoring theincorporation of deuterium into its α-positionfrom a D2O solvent, and combination of these with rates ofketonization gave the keto−enol equilibrium constant,pKE = 16.19. The acidity constant of theenol ionizing as an oxygen acid was determined as well,p= 6.39, andcombination of that with KE led to theionization constant of the keto form of mandelic acid ionizing as acarbonacid,p= 22.57. (These acidity constants are concentrationquotients, applicable at ionic strength = 0.10M.)These results are compared with other keto−enol systems, andtheir bearing on the enzymatic racemization of mandelicacid is discussed.
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