Linear Free-Energy Relationship in the Pyridinolysis of 2,4-Dinitrophenyl p-Chlorobenzoate in Ethanol Aqueous Solution

Article abstract of DOI:10.1021/jo00360a007

The title reactions are kinetically studied in 44 wt percent aqueous ethanol at 25 deg C, ionic strength 0.2 M (KCl).A linear Broensted-type plot (log k_N against basicity of the substituted pyridine, where k_N is the second-order rate constant for the reactions) is found.The reactions are nucleophilic as shown by both the inhibition of the rate constant observed in the presence of added excess of 2,4-dinitrophenoxide ion and the value of the Broensted-type slope (β 0.94).Comparison of Broensted-type obtained with those found in the pyridinolyses of 2,4-dinitrophenyl benzoate (curved, with the center of curvature at pK_a 9.5) and 2,4-dinitrophenyl p-nitrobenzoate (linear) under the same experimental conditions indicates that electron withdrawal from the remaining aryl group of the tetrahedral intermediate (T ^+/-) formed in these reactions shifts the center of the Broensted-type curvature toward higher pK_a values and hence favors amine expulsion from T ^+/- relative to 2,4-dinitrophenoxide ion.Activation parameters for the reactions of the title substrate with 3-methyl- and 4-(dimethylamino)pyridines are reported and their values discussed in comparison with the ones found for similar reactions.