Abstract

The title reactions are subjected to a kinetic study in water, at 25.0 °C, and an ionic strength of 0.2 M (KCl). By following the reactions spectrophotometrically two consecutive reactions are observed: the first is formation of the corresponding thionocarbamates (1-(aryloxythiocarbonyl)- pyridinium cations) and the second is their decomposition to the corresponding phenol and pyridine, and COS. Pseudo-first-order rate coefficients (k obsd1 and kobsd2, respectively) are found under excess amine. Plots of kobsd1 vs free pyridine concentration at constant pH are linear, with the slope (kN) independent of pH. The Brønsted-type plots (log kN vs pKa of the conjugate acids of the pyridines) are linear with slopes ? = 0.07 and 0.11 for the reactions of phenyl and 4-nitrophenyl chlorothionoformates, respectively. These Brønsted slopes are in agreement with those found in other stepwise reactions of the same pyridines in water, where the formation of a tetrahedral intermediate is the rate-determining step. In contrast to the stepwise mechanism of the title reactions that for the reactions of the same substrates with phenols is concerted, which means that substitution of a pyridino moiety in a tetrahedral intermediate by a phenoxy group destabilizes the intermediate. The second reaction corresponds to the pyridine-catalyzed hydrolysis of the corresponding 1-(aryloxythiocarbonyl)pyridinium cation. Plots of k obsd2 VS free pyridine concentration at constant pH are linear, with the slope (kH) independent of pH. The Brønsted plots for kH are linear with slopes ? = 0.19 and 0.26 for the reactions of the phenyl and 4-nitrophenyl derivatives, respectively. These low values are explained by the fact that as pKa increases the effect of a better pyridine catalyst is compensated by a worse leaving pyridine from the corresponding thionocarbamate.