Solvent Effect on the Relative Nucleofugalities of Pyridines and Phenoxide Ions: Broensted-Type Plots in the Pyridinolysis of 2,4-dinitrophenyl Methyl Carbonate and 2,4-Dinitrophenyl Acetate in Aqueous Ethanol

Article abstract of DOI:10.1021/jo00376a005

Curved Broensted-type plots (log k_N vs. pK_a) are obtained for the nucleophilic reactions of the title in 44 wt percent aqueous ethanol, at 25 deg C, ionic strength 0.2 M (KCl).The curves are described by a semiempirical equation based on the existence of a tetrahedral intermediate (T^+/-) in the reaction path and a change of the rate-determining step from breakdown to formation of T^+/- as the pyridine is more basic.The pK_a value at the center of curvature (pK_a⁰) increases with increasing inductive electron withdrawal from the nonleaving group of the substrate (pK_a⁰ 7.6 and 8.0 for the acetate and carbonate esters, respectively), favoring amine expulsion, relative to that of 2,4-dinitrophenoxide ion, from T^+/-.The pK_a⁰ values for the present reactions are larger than those for the same reactions in water, which shows that amine expulsion from T^+/- is fovored over 2,4-dinitrophenoxide expulsion by the change to a less polar solvent.Specific acid catalysis of aminolysis is found in the reaction of both substrates with 4-cyanopyridine at low pH. The most likely mechanism involves amine attack to the substrate to form T^+/-, followed by protonation equilibrium with T⁺ and breakdown to products of T^+/- and T⁺.