Nonlinear Hammett plots in pyridinolysis of 2,4-dinitrophenyl X-substituted benzoates: change in RDS versus resonance contribution

Abstract

Second-order rate constants (k_OH–) have been measured for nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzoates (1a–j) with Z-substituted pyridines in 80 mol% H₂O/20 mol% DMSO at 25.0 ± 0.1 °C. The Hammett plots for the reactions of 1a–j with pyridines consist of two intersecting straight lines, i.e., a large ρ value for the reactions of substrates (1a–c) possessing an electron-donating group (EDG) in the benzoyl moiety and a small one for substrates (1e–j) bearing an electron-withdrawing group (EWG). The nonlinear Hammett plots have been attributed to stabilization of the ground state of substrates 1a–c through resonance interactions between the electron-donating substituent and the carbonyl functionality, since the corresponding Yukawa–Tsuno plots exhibit excellent linear correlations with large r values. It has been shown that substrates 1e–j are not unusually more reactive than would be expected from the Hammett substituent constants, but rather, substrates 1a–c exhibit lower reactivity than would be predicted. The Brønsted-type plots for pyridinolysis of 1a–j are linear with β_nuc = 0.74–0.98, indicating that the reaction proceeds through a stepwise mechanism in which the second step is the RDS. It has been concluded that the electronic nature of the substituent X in the benzoyl moiety does not influence the RDS, but the degree of bond formation (or the effective charge on the nucleophilic site) in the transition state becomes more significant as the substituent X changes from a strong EDG to a strong EWG.