Structure-reactivity correlations in the reaction of 2,4-dinitrophenyl X-substituted benzoates with alicyclic secondary amines

Abstract

Apparent second-order rate constants (kapp) have been measured spectrophotometrically for the reaction of 2,4-dinitrophenyl X-substituted benzoates with a series of alicyclic secondary amines in H2O containing 20 mol% DMSO at 25°C. The microconstants involved in the reaction (k-1/k2, k1, and k1k2/k-1) have also been calculated. The magnitude of kapp, k1, and k1k2/k-1 values increases with increasing amine basicity and with increasing acid strengthening ability of the acyl substituent X. The k-1/k2 value decreases from ca. 6.5 to 0.3 with increasing the amine basicity, but remains almost constant upon changing the acyl substituent X for a given amine, indicating that the rate-determining step is governed by the basicity of amine but not by the electronic nature of the acyl substituent X. The Brønsted-type plots for kapp show a break at pKa = 9.1, supporting the assumption that a change in the rate-determining step occurs from rate-limiting breakdown to formation of the addition intermediate as amine basicity increases. The corresponding Brønsted-type plots for k-1/k2, k1, and k1k2/k-1 are linear but their β values are different. σ+ constants show better correlation with log kapp, log k1 and log k1k2/k-1 for the reaction with low basic amines (pKa < 9.1), while σ constants exhibit better correlation for the reaction with highly basic amines (pKa > 9.1). The magnitude of ρ1 is identical to that of ρapp and ρeq for a given amine.Key words: aminolysis, Brønsted-type plot, structure-reactivity correlations, rate-determining step.