CARBANION (E1CB) MECHANISM OF ESTER HYDROLYSIS .I. HYDROLYSIS OF MALONATE ESTERS

The pH-log rate profiles for the hydrolysis of ethyl o-nitrophenyl malonate (I), ethyl o-nitrophenyl methylmalonate (III), ethyl p-nitrophenyl methylmalonate (IV), and ethyl o-nitrophenyl dimethylmalonate (V) have been determined in water at 30°, ionic strength = 1.0. Ester V hydrolyzes via the expected spontaneous general-base water-catalyzed and hydroxide-catalyzed mechanisms. Esters I, III, and IV, which contain α-protons, have rates of water-catalyzed hydrolysis of the expected magnitude; however, the profiles for these esters in the alkaline region are unusual and are characterized by accelerated rates and one or more plateaus. The reactions of 14 nucleophiles with the various esters were quantitatively studied and in certain cases the reactions were found to become independent of nucleophile at high nucleophile concentrations. Rate accelerations (up to 104) are observed only in the hydrolysis of esters containing α-hydrogens. The improbability that kinetically detectable tetrahedral intermediate formation accounts for the results is discussed. Neighboring-nitro-group participation is dismissed since ester IV and the p-nitrophenyl analog of I both show kinetic behavior of the same form as the o-nitrophenyl derivatives. A mechanism consistent with the kinetic data is proposed and discussed. Salient features of this mechanism include general base catalyzed α-proton abstraction to form resonantly stabilized carbanion intermediates and the spontaneous collapse of the formed carbanion to reactive ketenes. © 1969, American Chemical Society. All rights reserved.