Kinetic Study on Aminolysis of 2-Chloro-4-nitrophenyl X-Substituted-Benzoates in Acetonitrile and in H2O Containing 20 mol % DMSO: Effects of Medium and Substituent X on Reactivity and Reaction Mechanism

Abstract

Abstract Second-order rate constants for reactions of 2-chloro-4-nitrophenyl X-substituted-benzoates 1a–1j with a series of cyclic secondary amines in MeCN have been measured. Comparison of the kinetic results with those reported previously for the corresponding reactions carried out in H2O containing 20 mol % DMSO has revealed that amines are less reactive in MeCN. The Brønsted-type plot for the aminolysis of 2-chloro-4-nitrophenyl benzoate (1f) in MeCN is linear with βnuc = 0.64, which is in contrast to the curved Brønsted-type plot reported for the reaction performed in the H2O–DMSO misxture. The Hammett plot for the reactions of 1a–1j with piperidine consists of two intersecting straight lines while the Yukawa–Tsuno plot exhibits an excellent linear correlation with ρX = 1.22 and r = 0.60, indicating that the nonlinear Hammett plot is not due to a change in rate-determining step but is caused by stabilization of substrates possessing an electron-donating substituent through resonance interactions between the substituent and the carbonyl functionality. It has been concluded that medium change from the H2O–DMSO mixture to MeCN forces the aminolysis of 1a–1j to proceed through a concerted mechanism by destabilizing the zwitterionic tetrahedral intermediate (T±).