CARBANION MECHANISM OF ESTER HYDROLYSIS .2. OMICRON-NITROPHENYL ALPHA-CYANO-AND ALPHA-DIMETHYLSULFONIOACETATE ESTERS

The pH-log rate profiles for the hydrolysis of the o-nitrophenyl esters CNCH2COO-o-NP (III), CNC-(CH3)2COO-o-NP (IV), and (CH3)2S+CH2COO-o-NP (V) have been determined at 30° in water at ionic strength = 1.0. The profile for IV is quantitatively expressed by spontaneous general-base catalysis by water- and hydroxide ion catalyzed hydrolysis. For III and V the term for spontaneous hydrolysis is obtained but at high pH the rates of alkaline hydrolysis become independent of pH. The plateau at high pH is attributed to specific base catalyzed α-proton abstraction with formation of a resonantly stabilized carbanion for III and a ylide for V. Accelerated rates of hydrolysis for III and V over those expected on the basis of linear free-energy relationships gives evidence that spontaneous collapse of carbanion to reactive ketenes occurs with III and possibly with V. Second-order rate constants for reaction of ten nucleophiles with the esters III, IV, and V have been determined and found to be independent of nucleophile concentration. This result supports carbanion formation as being specific base catalyzed. Reaction of nucleophiles other than hydroxide ion is most reasonably at the ester carbonyl carbon. Evidence is presented that p-nitrophenyl acetate does not form an unreactive carbanion in the alkaline pH range, as previously reported. The second-order rate constant for alkaline hydrolysis of p-nitrophenyl acetate is, within experimental error, invariant between pH 7 and 0.5 M KOH. © 1969, American Chemical Society. All rights reserved.