Aminolysis and pyridinolysis of O‐aryl S‐(4‐nitrophenyl) thiocarbonates in aqueous ethanol. Kinetics and mechanism

Abstract

AbstractThe reactions of O‐phenyl S‐(4‐nitrophenyl) thiocarbonate (PNPTC) and O‐(4‐chlorophenyl) S‐(4‐nitrophenyl) thiocarbonate (ClPNPTC) with a series of secondary alicyclic (SA) amines and pyridines are subjected to a kinetic investigation in 44 wt% ethanol–water, at 25.0°C and an ionic strength of 0.2 M. The reactions are followed spectrophotometrically at 420 nm (4‐nitrobenzenethiolate anion appearance). Under amine excess, pseudo‐first‐order rate coefficients (kobs) are found. For all these reactions, plots of kobs vs free amine concentration at constant pH are linear, the slope (kN) being independent of pH. The Brønsted‐type plots (log kN vs pKa of the conjugate acid of the amines) for the reactions of the series of SA amines with PNPTC and ClPNPTC are linear with slopes 0.59 and 0.54, respectively. The values of these slopes are in accordance with a concerted mechanism. The Brønsted‐type plots for the pyridinolysis reactions are biphasic, suggesting a stepwise mechanism with a change in the rate‐determining step, from breakdown to formation of a tetrahedral intermediate, as the basicity of the pyridines increases. Copyright © 2008 John Wiley & Sons, Ltd.