Kinetics and Mechanism of the Pyridinolysis of O-Ethyl S-Aryl Thiocarbonates in Aqueous Solution

The reactions of a series of 3- and 4-substituted pyridines with S -(4-nitrophenyl), S -(2,4-dinitrophenyl), and S -(2,4,6-trinitrophenyl) O -ethyl thiocarbonates (NPTC, DNPTC, and TNPTC, respectively) are subjected to a kinetic investigation in aqueous solution, 25.0 °C, ionic strength 0.2 M (KCl). Pseudo-first-order rate coefficients ( k obsd ) are found under amine excess. Plots of k obsd vs free amine concentration are linear and pH independent. The Brönsted-type plot for NPTC is linear whereas those for DNPTC and TNPTC are biphasic with slopes β 1 = 0.2 (high p K a ) for both, and β 2 = 0.9 and 0.8 (low p K a ) for DNPTC and TNPTC, respectively. The curvature center on the p K a axis is p K a o = 8.6 (DNPTC) and 7.3 (TNPTC). The Brönsted plots are consistent with the presence of a zwitterionic tetrahedral intermediate on the reaction path where its breakdown is rate determining for NPTC and there is a change in the rate-limiting step for the reactions of the other two substrates. Comparison of the stepwise reactions of the present work with the concerted ones of DNPTC and TNPTC with secondary alicyclic amines shows that the latter amines greatly destabilize T ± relative to isobasic pyridines. Comparison of the present reactions with other pyridinolysis indicates that substitution of the oxygen phenoxy atom by sulfur increases p K a o and the change of methyl to ethoxy as the remaining group also enlarges the p K a o and besides destabilizes T ± .