An unusual ground-state stabilization effect and origins of the alpha-effect in aminolyses of Y-substituted phenyl X-substituted benzoates

Abstract

Second-order rate constants have been measured spectrophotometrically for the reactions of X-C6H4CO2C6H4-Y with a series of primary amines in H2O containing 20 mol% DMSO at 25.0 +/- 0.1 degrees C. The reactivity increases as the substituent (X and Y) becomes a stronger electron-withdrawing group. The sigma(+) constants give better Hammett correlation than sigma constants for the reactions of 4-nitrophenyl X-substituted benzoates with glycylglycine (glygly) and hydrazine (NH2NH3, indicating that the ground-state stabilization effect is unusually significant on the reaction rates. The reactions of X-C6H4CO2C6H4-Y with glygly and NH2NH2 appear to proceed through the same mechanism, but the degree of leaving-group departure and the negative charge developed in the acyl moiety at the rate-determining TS is considered to be more significant for the glygly system than the NH2NH2 system based on beta(1g) and rho(x) values. The magnitude of the alpha-effect is observed to be not always dependent on the beta(nuc) value but dependent on the electronic nature of the substituent X and Y, i.e., an electron-donating substituent increases the alpha-effect, while an electron-withdrawing one decreases the alpha-effect. The present study has led to the conclusion that the ground-state effect is important for the reaction rates but it is not solely responsible for the alpha-effect, and the intramolecular H-bonding interactions (4) are proposed for the cause of the increasing or decreasing a-effect trends observed in the present system.