Effect of Acyl Substituents on the Reaction Mechanism for Aminolyses of 4-Nitrophenyl X-Substituted Benzoates

Second-order rate constants (k N) have been measured spectrophotometrically for the reaction of 4-nitrophenyl X-substituted benzoates with a series of alicyclic secondary amines in H2O containing 20 mol % dimethyl sulfoxide at 25.0 °C. The magnitude of the k N values increases with increasing the basicity of amines and with increasing the electron-withdrawing ability of the acyl substituent X. The Hammett plots obtained are not linear but show a break or curvature as the acyl substituent X becomes electron withdrawing for all the amines studied, while the Brønsted-type plots are linear with large βnuc values for all the substrates investigated. The nonlinear Hammett plots suggest a change in the rate-determining step upon changing the acyl substituent X, whereas the linear Brønsted-type plots indicate that the rate-determining step does not change upon changing amine basicity. The Yukawa−Tsuno plots obtained are also linear with positive ρX and large r values, suggesting that the nonlinear Hammett plots are not due to a change in the rate-determining step upon changing the acyl substituent X, but due to resonance demand of the π-electron donor substituent on the acyl moiety. The magnitude of the ρX and βnuc values increases with increasing the basicity of amines and with increasing the electron-withdrawing ability of the acyl substituent X, respectively, while that of the r values decreases with increasing ρX values and amine basicity.