A kinetic study is reported for the reactions of 2-methoxy-3-nitropyridine 1a and 2methoxy-5-nitropyridine 1b with three secondary amines 2a-c (morpholine, piperidine, and pyrrolidine) in aqueous solution at 20 • C. The Brønsted-type plots are linear with nuc = 0.52 and 0.55 for pyridines 1a and 1b, respectively, indicating that the reaction proceeds through a S N Ar mechanism in which the first step is the ratedetermining step. Additional theoretical calculations using the DFT/B3LYP method confirm that the C-2 carbon being the most electrophilic center for the both pyridines 1a and 1b. The second-order rate constants have been used to evaluate the electrophilicity parameters E of 1a and 1b according to the linear free energy relationship log k (20 • C) = s N (N + E). The E parameters thus derived are compared with the electrophilic reactivities of a large variety of anisoles. The validity of these E values has been satisfactorily verified by comparison of calculated and experimental second-order rate constants for the reactions of pyridines 1a and 1b with anion of ethyl benzylacetate. K E Y W O R D S density functional theory, electrophilicity parameter (E), equation of Mayr, kinetics, S N Ar, substituted pyridine Int J Chem Kinet. 2019;51:249-257.
The second‐order rate constants (k) for reaction of 7‐chloro‐4‐nitrobenzofurazan 1 and 7‐methoxy‐4‐nitrobenzofurazan 2 with a series of nitroalkyl anions and several of para‐substituted phenoxide anions in aqueous solution at 20 °C have been reported. On the basis of the linear novel approach recently designed by Mayr and coworkers, the electrophilicity parameters E at the C‐5 position of the two nitrobenzofurazans 1 and 2 have been quantified and ranked on the comprehensive electrophilicity scale. Mayr's approach was found to correctly predict the rate constants for the addition of phenoxide anions at the C‐5 position of 1 and 2 witting a factor of <2. Analysis of the kinetic measurements using Brønsted's model shows that βnuc values remain remarkably constant for changes in the nature of the substituent and that the σ‐complexation process is associated with high Marcus intrinsic barriers. In addition, satisfactory correlations between the log kexp (kexp values measured in this work for reactions of benzofurazans 1 and 2 with a series of phenoxide anions in aqueous solution at 20 °C) and log kcalcd (kcalcd values calculated from equation 1 using the electrophilicity parameters E of benzofurazans 1 and 2 and the previously published nucleophilicity parameters N and sN of the phenoxide anions) with a slope very close to unity have been obtained and discussed.
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.
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