Effects of Amine Nature and Nonleaving Group Substituents on Rate and Mechanism in Aminolyses of 2,4-Dinitrophenyl X-Substituted Benzoates

Second-order rate constants have been measured for the reactions of 2,4-dinitrophenyl X-substituted benzoates (1a − f) with a series of primary amines in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 °C. The Brønsted-type plot for the reactions of 1d with primary amines is biphasic with slopes β1 = 0.36 at the high pK a region and β2 = 0.78 at the low pK a region and the curvature center at pK a° = 9.2, indicating that the reaction proceeds through an addition intermediate with a change in the rate-determining step as the basicity of amines increases. The corresponding Brønsted-type plot for the reactions with secondary amines is also biphasic with β1 = 0.34, β2 = 0.74, and pK a° = 9.1, indicating that the effect of amine nature on the reaction mechanism and pK a° is insignificant. However, primary amines have been found to be less reactive than isobasic secondary amines. The microscopic rate constants associated with the aminolysis have revealed that the smaller k 1 for the reactions with primary amines is fully responsible for their lower reactivity. The electron-donating substituent in the nonleaving group exhibits a negative deviation from the Hammett plots for the reactions of 1a − f with primary and secondary amines, while the corresponding Yukawa−Tsuno plots are linear. The negative deviation has been ascribed to stabilization of the ground state of the substrate through resonance interaction between the electron-donating substituent and the carbonyl functionality.