Aminolysis of 2,4-Dinitrophenyl and 3,4-Dinitrophenyl 2-Thiophenecarboxylates: Effect of ortho-Nitro Group on Reactivity and Mechanism

Abstract

Second-order rate constants (kN) have been measured spectrophotometrically for reactions of 3,4-dinitrophenyl 2-thiophenecarboxylate (2) with a series of alicyclic secondary amines in 80 mol % <TEX>$H_2O$</TEX>/20 mol % dimethyl sulfoxide at 25.0 <TEX>${^{\circ}C}$</TEX>. The Br&#x00F8;nsted-type plot exhibits a downward curvature for the aminolysis of 2. The curved Brønsted-type plot is similar to that reported for the corresponding reactions of 2,4-dinitrophenyl 2- thiophenecarboxylate (1). The reactions of 1 and 2 have been suggested to proceed through the same mechanism, i.e., through a zwitterionic tetrahedral intermediate (<TEX>$T^{\pm}$</TEX>) with a change in the rate-determining step. Substrate 2 is less reactive than 1 toward weakly basic amines (e.g., <TEX>$pK_a$</TEX> < 10.4) but becomes more reactive as the basicity of amines increases further. Dissection of kN into the microscopic rate constants has revealed that the reaction of 2 results in a smaller <TEX>$k_2/k_{-1}$</TEX> ratio but larger <TEX>$k_1$</TEX> than the corresponding reaction of 1. Steric hindrance exerted by the ortho-nitro group has been suggested to be responsible for the smaller <TEX>$k_1$</TEX> value found for the reactions of 1.