Kinetics and mechanism of the reactions of S‐4‐nitrophenyl 4‐methylthiobenzoate with secondary alicyclic amines and pyridines

Abstract

AbstractThe reactions of the title substrate with a series of six secondary alicyclic amines and a series of eight pyridines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0°C, and an ionic strength of 0.2 mol · dm−3. Under amine excess pseudo first‐order rate coefficients (kobs) are obtained. Plots of kobs against free amine concentration at constant pH are linear, with the slope (kN) independent of pH. The Brønsted‐type plots (log kN against pKa of the conjugate acids of the amines) are non‐linear, with the curvature center located at pKa (pK) 9.7 and 9.4, for the reactions of secondary alicyclic amines and pyridines, respectively. The plots are consistent with a zwitterionic tetrahedral intermediate (T±) on the reaction path and a change in rate‐determining step. The greater pK value for secondary alicyclic amines than pyridines is explained by the greater nucleofugality from T± of secondary alicyclic amines compared to isobasic pyridines. These pK values are lower than those found for the reactions of S‐4‐nitrophenyl 4‐Y‐substituted thiolbenzoates (Y = H, Cl, NO2) with the corresponding amine series. These results indicate that electron donation from the non‐leaving group in T± favors leaving group expulsion from T± relative to amine expulsion. Copyright © 2006 John Wiley & Sons, Ltd.