Nonlinear Broensted-Type Plot in the Pyridinolysis of 2,4-Dinitrophenyl Benzoate in Aqueous Ethanol

Article abstract of DOI:10.1021/jo00219a029

From the kinetics of the title reaction in 44 wtpercent aqueous ethanol at 25 deg C, ionic strength 0,2 M (KCl), a curved Broensted-type plot is obtained.The curve is interpreted in terms of a zwitterionic tetrahedral intermediate in the reaction path and a change in the rate-determining step from breakdown to products of this intermediate to its formation as the pyridine becomes more basic.A semiempirical equation based on this hypothesis accounts for the experimental results.The center of the Broensted-type curvature (pK_a⁰) is located at pK_a = 9.5, lower than the value found in the pyridinolysis of 2,4-dinitrophenyl p-nitrobenzoate in the same solvent, showing that electron withdrawal from the acyl group of the substrate favors amine expulsion relative to phenoxide from the tetrahedral intermediate.It is claimed that the influence of the acyl group of the substrate on the value of pK_a⁰ cannot be quantified when comparing similar reactions in water and aqueous ethanol since the pK_a⁰ value for a given reaction should be larger in the latter solvent.The activation parameters obtained support the hypothesis that for the title reactions, pK_a⁰ > 9.