Abstract

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 44wt% 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.