AN UNUSUAL GROUND-STATE STABILIZATION EFFECT AND ORIGINS OF THE ALPHA-EFFECT IN AMINOLYSES OF Y-SUBSTITUTED PHENYL X-SUBSTITUTED BENZOATES

Second-order rate constants have been measured spectrophotometrically for the reactions of X-C6H4CO2C6H4-Y with a series of primary amines i n H2O containing 20 mol% DMSO at 25.0 +/- 0.1 degrees C. The reactivit y increases as the substituent (X and Y) becomes a stronger electron-w ithdrawing group. The sigma(+) constants give better Hammett correlati on than sigma constants for the reactions of 4-nitrophenyl X-substitut ed benzoates with glycylglycine (glygly) and hydrazine (NH2NH3, indica ting that the ground-state stabilization effect is unusually significa nt on the reaction rates. The reactions of X-C6H4CO2C6H4-Y with glygly and NH2NH2 appear to proceed through the same mechanism, but the degr ee of leaving-group departure and the negative charge developed in the acyl moiety at the rate-determining TS is considered to be more signi ficant 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 elec tronic nature of the substituent X and Y, i.e., an electron-donating s ubstituent increases the alpha-effect, while an electron-withdrawing o ne decreases the alpha-effect. The present study has led to the conclu sion that the ground-state effect is important for the reaction rates but it is not solely responsible for the alpha-effect, and the intramo lecular H-bonding interactions (4) are proposed for the cause of the i ncreasing or decreasing a-effect trends observed in the present system .