2 MECHANISMS FOR ACID-CATALYZED HYDROLYSIS OF ENOL ACETATES

Hydrolysis of α-acetoxy-p-nitrostyrene (6) in 1 M sulfuric acid shows the characteristics of normal ester hydrolysis. In D2O, the solvent isotope effect, kH2o/kD2o, is 0.75; the rate of hydrolysis is very similar to the rate of hydrolysis of isopropyl acetate. Increasing the concentration of sulfuric acid causes an increase in rate which is nonlinear with the acidity function H0, but which closely parallels the increasing rate of hydrolysis of isopropyl acetate. In sulfuric acid concentrations greater than 55 %, the rate of hydrolysis of 6 increases very rapidly. In 69% sulfuric acid, the solvent isotope effect, kH2o/kD2o, is now 3.25. Thus, the mechanism has changed to one involving initial, and rate-determining, olefin protonation. The effect of substituents in substituted α-acetoxy-styrenes further serves to substantiate the two mechanisms, and to delineate the circumstances under which each of the two mechanisms will be dominant. © 1969, American Chemical Society. All rights reserved.