29771-06-0Relevant articles and documents
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
Castro, Enrique A.,Borquez, Maria T.,Parada, Paulina M.
, p. 5072 - 5077 (1986)
Curved Broensted-type plots (log kN vs. pKa) 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 pKa value at the center of curvature (pKa0) increases with increasing inductive electron withdrawal from the nonleaving group of the substrate (pKa0 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 pKa0 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+.
