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-et hyl thiocarbonates (NPTC, DNPTC, and TNPTC, respectively) are subjecte d to a kinetic investigation in aqueous solution, 25.0 degrees C, ioni c strength 0.2 M (KCl). Pseudo-first-order rate coefficients (K-obsd) are found under amine excess. Plots of k(obsd) vs free amine concentra tion are linear and pH independent. The Bronsted-type plot for NPTC is linear whereas those for DNPTC and TNPTC are biphasic with slopes bet a(1) = 0.2 (high pK(a)) for both, and beta(2) = 0.9 and 0.8 (low pK(a) ) for DNPTC and TNPTC, respectively. The curvature center on the PKa a xis is pK(a)(o) = 8.6 (DNPTC) and 7.3 (TNPTC). The Bronsted plots are consistent with the presence of a zwitterionic tetrahedral intermediat e on the reaction path where its breakdown is rate determining for NPT C and there isa 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 ph enoxy atom by sulfur increases pK(a)(o) and the change of methyl to et hoxy as the remaining group also enlarges the pK(a)(o) and besides des tabilizes T-+/-.