Reactions of 2,4‐Dinitrophenyl 5‐substituted‐2‐thiophenecarboxylates with R 2 NH/R 2 NH 2 + in 20 Mol % DMSO(aq). Effects of 5‐Thienyl Substituent and Leaving Group on the Reaction Mechanism

Reactions of 2,4‐dinitrophenyl 2‐thiophenecarboxylate ( 2a–d ) with R 2 NH/R 2 NH 2 + in 20 mol % DMSO(aq) have been studied. The reactions are overall second order, first order to the substrates, and first order to the nucleophiles. The Brönsted plots showed downward curves with p K a 0 = 9.5, β 1 = 0.22–0.34, and β 2 = 0.85–0.92. The k 1 values increased with a stronger electron‐withdrawing 5‐thienyl substituent and a stronger nucleophile, whereas the k 2 / k −1 values remained nearly the same for all 5‐thienyl substituents. The influence of 5‐thienyl substituent on the reaction rates showed excellent correlations on the Yukawa‐Tsuno plots with ρ = 1.28–2.16 and r = 0.20–0.60. The ρ value increased and r value decreased with a stronger nucleophile, indicating an increase in the electron density at the carbonyl carbon and a decrease in the resonance demand. From these results, a stepwise mechanism with a change in the rate‐determining step has been proposed.