Effects of amine nature and nonleaving group substituents on rate and mechanism in aminolyses of 2,4-dinitrophenyl x-substituted benzoates

Abstract

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 pKa region and β2 = 0.78 at the low pKa region and the curvature center at pKa° = 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 Brø nsted-type plot for the reactions with secondary amines is also biphasic with β1 = 0.34, β2 = 0.74, and pKa° = 9.1, indicating that the effect of amine nature on the reaction mechanism and pKa° 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.