Aminolysis of 2,4‐Dinitrophenyl X‐Substituted Benzoates and Y‐Substituted Phenyl Benzoates in MeCN: Effect of the Reaction Medium on Rate and Mechanism

Abstract

Second-order rate constants (k(N)) have been determined spectrophotometrically for the reactions of 2,4-dinitrophenyl X-substituted benzoates (1 a-f) and Y-substituted phenyl benzoates (2 a-h) with a series of alicyclic secondary amines in MeCN at 25.0 +/- 0.1 degrees C. The k(N) values are only slightly larger in MeCN than in H2O, although the amines studied are approximately 8 pK(a) units more basic in the aprotic solvent than in H2O. The Yukawa-Tsuno plot for the aminolysis of 1 a-f is linear, indicating that the electronic nature of the substituent X in the nonleaving group does not affect the rate-determining step (RDS) or reaction mechanism. The Hammett correlation with sigma- constants also exhibits good linearity with a large slope (rho(Y) = 3.54) for the reactions of 2 a-h with piperidine, implying that the leaving-group departure occurs at the rate-determining step. Aminolysis of 2,4-dinitrophenyl benzoate (1 c) results in a linear Brønsted-type plot with a beta(nuc) value of 0.40, suggesting that bond formation between the attacking amine and the carbonyl carbon atom of 1 c is little advanced in the transition state (TS). A concerted mechanism is proposed for the aminolysis of 1 a-f in MeCN. The medium change from H2O to MeCN appears to force the reaction to proceed concertedly by decreasing the stability of the zwitterionic tetrahedral intermediate (T+/-) in aprotic solvent.