Kinetics and Mechanism of the Pyridinolysis of 2,4,6-Trinitrophenyl Acetate and 2,4,6-Trinitrophenyl Methyl Carbonate

Article abstract of DOI:10.1021/jo00035a028

The title reactions are subject to a kinetic study in aqueous solution at 25.0 deg C, ionic strength 0.2 M.The reactions are first order in both the substrate and the free pyridine.The Broensted-type plots obtained are nonlinear with slopes β₁ = 0.2 and β₂ = 0.8 at high and low basicities of the pyridines, respectively, for both substrates.The pK_a values at the Broensted breaks (pK_a⁰) are 5.0 and 6.5 for the acetate (TNPA) and the carbonate (TNPMC), respectively.The Broensted 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_a⁰ 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_a⁰ 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.