Single-Electron Transfer in σ-Complexation Reactions of 2,6-Dimethoxy-3,5-dinitropyridine with Para-X-phenoxide Anions in Aqueous Solution

Abstract

Second-order rate constants have been measured spectrophotometrically for reactions of 2,6-dimethoxy-3,5-dinitropyridine 1 with 4-X-substituted phenoxide anions (X = OMe, Me, H, Cl, and CN) 2a–e in aqueous solution at various temperatures. The effect of phenoxide substituents on the reaction rate was examined quantitatively on the basis of kinetic measurements, leading to nonlinear correlations of Δ≠H and Δ≠S with Hammett's substituent constants (σ). Each Hammett plots exhibits two intersecting straight lines for the reactions of 1 with the phenoxide anions 2a–e, whereas the Yukawa–Tsuno plots for the same reactions are linear. The large negative ρ values (−4.03 to −3.80) obtained for the reactions of 1 with the phenoxide anions possessing an electron-donating group supports the proposal that the reactions proceed through a single-electron transfer mechanism.