KINETICS AND MECHANISM OF THE PYRIDINOLYSIS OF 2,4,6-TRINITROPHENYL ACETATE AND 2,4,6-TRINITROPHENYL METHYL CARBONATE

The title reactions are subject to a kinetic study in aqueous solution at 25.0-degrees-C, ionic strength 0.2 M. The reactions are first order in both the substrate and the free base pyridine. The Bronsted-type plots obtained are nonlinear with slopes beta(1) = 0.2 and beta(2) = 0.8 at high and low basicities of the pyridines, respectively, for both substrates. The pK(a) values at the Bronsted breaks (pK(a)-degrees) are 5.0 and 6.5 for the acetate (TNPA) and the carbonate (TNPMC), respectively. The Bronsted curves can be better described by a two-step mechanism, with a tetrahedral intermediate, T+/- rather than a concerted process, although rigorously the latter mechanism cannot be ruled out. The higher pK(a)-degrees for the TNPMC reactions, relative to TNPA, is in agreement with the results found in the aminolysis of the dinitro derivatives and is explained by the increased amine nucleofugality from T+/- when Me is replaced by MeO in T+/-. Little or no effect on pK(a)-degrees is observed by substitution of the O-aryl O atom of TNPA by an S atom; this is attributed to the high instability of the intermediates T+/- involved. The larger rate constants obtained in the pyridinolysis of 2,4,6-trinitrophenyl thiolacetate compared to that of TNPA is explained by the softer character of the carbonyl center of the former substrate.