KINETICS AND MECHANISM OF THE PYRIDINOLYSIS OF 2,4-DINITROPHENYL AND 2,4,6-TRINITROPHENYL O-ETHYL DITHIOCARBONATES

The title reactions are subjected to a kinetic study in wa ter, 25.0 d egrees 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 Bronsted-type plots (log k(N) vs pK(e) 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)) f or the dinitro and the trinitro derivatives, respectively, and with th e curvature center at pK(a)=pK(a)(0)=6.9 and 5.6 for he dinitro and th e trinitro compounds, respectively. These results can be explained by the formation afa zwitterionic tetrahedral intermediate (T-+/-) in a s tepwise reaction. Comparison of these Bronsted-type plots with those i n the reactions of the same substrates with secondary alicyclic amines shows that the latter amines are better nucleofuges from T-+/- than i sobasic pyridines. Comparison of the Bronsted-type plots for the dinit ro and trinitro derivatives obtained in this work with those for the p yridinolysis of S-(2,4-dinitrophenyl) and S-(2,4,6-trinitrophenyl)-O-e thyl thiocarbonates indicates that substitution of S- by O- in T-+/- i ncreases the amine/ARS(-) nucleofugality ratio from T-+/-.