Generation of the enol of methyl mandelate by flash photolysis of methyl phenyldiazoacetate in aqueous solution and study of rates of ketonization of this enol in that medium.

Flash photolysis of methyl phenyldiazoacetate in aqueous solution produced phenylcarbomethoxycarbene, whose hydration generated a short-lived transient species that was identified as the enol isomer of methyl mandelate. This assignment is supported by the shape of the rate profile for decay of the enol transient, through ketonization to its carbonyl isomer, as well as by solvent isotope effects and the form of acid-base catalysis of the ketonization reaction. Comparison of the present results with previously published information on the enol of mandelic acid shows some interesting and readily understandable similarities and differences.