SOLVENT AND SECONDARY SUBSTRATE ISOTOPE EFFECTS ON THE ACID-CATALYZED KETONIZATION OF ACETOPHENONE ENOL IN AQUEOUS-SOLUTION

The enol of acetophenone was generated flash photolytically in aqueous solution by photosolvolysis of PhCBr=CH2 and photohydration of PhC=CH and PhC-CD, and rates of its ketonization were measured in dilute perchloric acid solutions in H2O and D2O at 25-degrees-C. The rate constants so obtained provide the solvent isotope effects, k(H)/k(D) = 5.02 +/- 0.08, and the secondary isotope effect for deuterium substitution at the beta-position of the enol double bond, (k(H)/k(D))beta = 0.999 +/- 0.014. Arguments are presented which show that these isotope effects require a stepwise rather than a concerted mechanism for the ketonization reaction.