Kinetics and Mechanism of the Pyridinolysis of 2,4-Dinitrophenyl and 2,4,6-Trinitrophenyl O-Ethyl Dithiocarbonates.

Abstract

The title reactions are subjected to a kinetic study in water, 25.0 degrees C, ionic strength 0.2 M (KCl). Under amine excess, pseudo-first-order rate coefficients are found, which are linearly related to the free amine concentration. No dependence of the slopes of these plots (k(N)) on the pH values was observed. The Brönsted-type plots (log k(N) vs pK(a) of the pyridines) are biphasic with slopes beta(1) = 0.2 (high pK(a)) for both reaction series and beta(2) = 1.0 and 0.9 (low pK(a)) for the dinitro and the trinitro derivatives, respectively, and with the curvature center at pK(a) = pK(a)(0) = 6.9 and 5.6 for the dinitro and the trinitro compounds, respectively. These results can be explained by the formation of a zwitterionic tetrahedral intermediate (T(+/-)) in a stepwise reaction. Comparison of these Brönsted-type plots with those in the reactions of the same substrates with secondary alicyclic amines shows that the latter amines are better nucleofuges from T(+/-) than isobasic pyridines. Comparison of the Brönsted-type plots for the dinitro and trinitro derivatives obtained in this work with those for the pyridinolysis of S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl) O-ethyl thiocarbonates indicates that substitution of S(-) by O(-) in T(+/-) increases the amine/ArS(-) nucleofugality ratio from T(+/-).