Bronsted Plots in the Reactions of 2,4-Dinitrophenyl Acetate and Methyl Phenyl Carbonate with Substituted Pyridines

Article abstract of DOI:10.1021/jo01293a027

Rate constants are reported for nucleophilic attack of a series of 3- or 4-substituted pyridines on 2,4-dinitrophenyl acetate and methyl phenyl carbonate at 25 deg C, and ionic strength 0.2 M.The Bronsted plot obtained is curved for the acetate and linear for the carbonate.The first shows two linear regions (at low and high pK_a values with slopes 0.85 and 0.2, respectively) and a curvature in between.The Bronsted slope for the carbonate is 1.3.The Bronsted curve can be accounted for in terms of a tetrahedral intermediate in the reaction path and a change in the rate-determining step from breakdown of the intermediate to its formation, as the substituted pyridine becomes more basic.A semiempirical equation based on these assumptions fits the experimental data.From the shape of the curved Bronsted plot, an equal leaving ability from the tetrahedral intermediate for 2,4-dinitrophenolate and a (hypothetical) pyridine of pK_a = 7.3 is deduced.The influence of the group that does not leave on the relative leaving abilities of phenolates and pyridines is discussed.