Reactions of O‐aryl S‐aryl dithiocarbonates with pyridines in aqueous ethanol: kinetics and mechanism

The reactions of O‐(4‐methylphenyl) S‐(4‐nitrophenyl) dithiocarbonate ( 1 ), O‐(4‐chlorophenyl) S‐(4‐nitrophenyl) dithiocarbonate ( 2 ), and O‐(4‐chlorophenyl) S‐phenyl) dithiocarbonate ( 3 ) with a series of pyridines were subjected to a kinetic investigation in 44 wt% ethanol–water, at 25.0 °C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) were determined. For the studied reactions, plots of k_obs versus free pyridine concentration at constant pH were linear, with the slope (k_N) independent of pH. The Brønsted‐type plots for ( 1 ) and ( 2 ) were biphasic, suggesting a stepwise mechanism with a change in the rate‐determining step, from breakdown to the formation of a tetrahedral intermediate (T^±), as the basicity of the pyridines increases. For the reactions of ( 3 ), at the pK_a range of the pyridines studied, only the breakdown to products of T^± was observed. Copyright © 2009 John Wiley & Sons, Ltd.