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 ࣔ H and ࣔ 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. C 2016 Wiley Periodicals, Inc. Int J Chem Kinet 48: 523-530, 2016 Scheme 2 Reaction of 2,6-dimethoxy-3,5-dinitropyridine 1 with various phenoxide anions.

Increasing the Alkaline Stability of N,N-Diaryl Carbazolium Salts Using Substituent Electronic Effects

“…Large ρ's have been reported in the literature, and they are typical in reactions involving SETs, including the Birch reduction. 43 Further analysis of the data reveals that there is a more satisfactory correlation between the modified Hammett σ p − parameters and the first-order rate constants (Figure 3). 44 This indicates that the resonance contribution has a significant impact on the reaction kinetics as well.…”

“…The high and positive reactivity parameter (ρ = 4.8) − indicates a strong development of negative charge in the transition state of the degradation reaction, and hence, the more electron donating the substituent is, the slower the degradation reaction. Large ρ’s have been reported in the literature, and they are typical in reactions involving SETs, including the Birch reduction . Further analysis of the data reveals that there is a more satisfactory correlation between the modified Hammett σ p – parameters and the first-order rate constants (Figure ).…”

Kinetic Studies on SNAr Reactions of Substituted Benzofurazan Derivatives: Quantification of the Electrophilic Reactivities and Effect of Amine Nature on Reaction Mechanism

“…In a series of papers , we have shown that the rate constants k (mol −1 L s −1 ) associated with the σ‐complexation reactions can be classified under the rubric of the original linear‐free energy relationship (LFER) proposed in 1994 by Mayr and co‐workers (Eq. ) , in which E is an electrophilicity parameter, s N is the nucleophile‐specific slope parameter, and N is a parameter characteristic of the given nucleophile in the given solvent and independent of the electrophile:…”

“…On the basis of this equation, the electrophilicity parameters E for several neutral electron‐deficient electrophiles, namely nitrobenzofuroxans , nitrotetrazolopyridines , nitrobenzofurazans , nitropyridine , and nitrobenzotriazoles have been quantified and ranked on the comprehensive electrophilicity scale defined for cationic electrophiles by Mayr and co‐workers . We noted that Mayr's approach has also been used to describe the reactivity of S N Ar substrates such as 7‐chloro‐4,6‐dinitrobenzofuroxan and ‐benzofurazan , 1‐X‐2,4‐dinitrobenzenes (X = F, Cl, Br and I) , and 2‐methoxy‐3‐X‐5‐nitrothiophenes (X = SO 2 CH 3 , CO 2 CH 3 , CONH 2 , and H) .…”

Ambident electrophilicity of 4‐nitrobenzochalcogenadiazoles: Kinetic studies and structure‐reactivity relationships

“…The ρ values are comparable to that reported for reactions, which have been suggested to proceed through a polar σ‐complexation process, for example, ρ = −2.29 for reaction of 3‐cyano‐5‐nitrothiophene with parasubstituted phenoxide anions in aqueous solution at 20°C (Ref. 1) and ρ = −1.84 to −1.75 for reactions of 2,6‐dimethoxy‐3,5‐dinitropyridine with the same series of para‐substituted phenoxide anions in aqueous solution at various temperatures 14 …”