E. Castro, M. Gazitúa, José G. Santos
Oct 1, 2009
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Journal
Journal of Physical Organic Chemistry
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 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 (kobs) were determined. For the studied reactions, plots of kobsversus free pyridine concentration at constant pH were linear, with the slope (kN) independent of pH. The Bronsted-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 pKa range of the pyridines studied, only the breakdown to products of T± was observed. Copyright © 2009 John Wiley & Sons, Ltd.