Second-order rate constants (k1) for the σ-complexation of 4-nitrobenzofurazan 1 with four 4-X-substituted phenoxide anions 2a–2d (X = OCH3, CH3, H and Cl) were measured in aqueous solution at 20 °C. Using this series of phenoxide anions as a reference, the electrophilicity parameter (E) of this electrophile 1 has been evaluated according to Mayr’s approach. With the E value of –9.42, Mayr’s equation was found to correctly predict the rate constants for the reactions of 1 with hydroxide ion in H2O and a 1:1 ratio of H2O to CH3CN. However, the large βnuc value of 1.12 obtained in the present work is clearly consistent with a single electron transfer (SET) mechanism.
The kinetics of the coupling of N1-methyl-4-nitro-2,1,3 benzothiadiazolium tetrafluoroborate 1 with a series of 4-X-substituted anilines 2a-f (X = OH, OMe, Me, H, Cl, and CN) have been investigated in acetonitrile at 20°C. The second-order rate constants result in a
The nucleophilic addition reactions of N1‐methyl‐4‐nitro‐2,1,3‐benzoselenadiazolium tetrafluoroborate 1 with aliphatic amines 2a–c (diethylamine 2a, dipropylamine 2b, and allylamine 2c) have been kinetically studied by UV–vis spectroscopy in acetonitrile solution at 20°C. The kinetic data have been analyzed, using the Mayr equation, allowing the quantification of the electrophilicity parameter (E) value of benzoselenadiazolium cation 1 (E = −14.72). The reliability of parameter E has been reasonably verified by comparison of calculated and experimental second‐order rate constants for the reactions of cation 1 with other amines 2d–f (pyrrolidine 2d, piperidine 2e, and morpholine 2f) under the same conditions as those of the amines 2a–c. A linear Brönsted plot (R2 = 0.9945) with a βnuc value of 0.55 has been obtained for the reactions of 1 with the secondary amines employed in the present work. Interestingly, satisfactory correlation between the log values of measured and calculated rate constants with a slope very close to unity has been obtained and discussed.
Through the linear free energy relationship log k = s N (N + E), the electrophilicity parameters E of 4-nitrobenzochalcogenadiazoles 1 a and 1 b have been determined, at 20°C, by kinetic investigations of their electrophilic addition reactions to a series of nitroalkyl anions 2 a-c in aqueous solution. The derived E-parameters of electrophiles 1 a (E =-12.80) and 1 b (E =-11.46) have been integrated into the electrophilicity scale established by Mayr. Interestingly the less activated compound (1 a) has an E value which is practically the same as that of the 1,3,5trinitrobenzene (E =-13.19), the common reference aromatic electrophile. Mayr's approach was found to predict the rate constants for σ-complexation reactions of our electrophiles 1 a and 1 b with aniline and 4-chloroaniline in acetonitrile with a factor of 10 À 36. Good linear correlation (r 2 = 0.9979) between the electrophilicity parameters E of 4,6-dinitrobenzochalcogenadiazoles 3 a-c and E values of their analogously 4-nitrobenzochalcogenadiazoles 1 a-c has been found and analysed. Furthermore, effect of basicity of nitronate anions 2 a-c on their nucleophilic reactivity has also been examined quantitatively on the basis of kinetic data. Imbalance effect is suggested to be responsible for the finding of an inequality between the Brønsted α elec and β nuc values.
The kinetics of the reactions of 4‐nitrobenzofurazane 1a, 4‐nitrobenzothiadiazole 1b, and 4‐nitrobenzoselenadiazole 1c with a series of 4‐Y‐substituted phenoxide anions 2a‐e (Y = OMe, Me, H, Cl, and CN) in aqueous solution at 20°C were investigated photometrically. The derived second‐order rate constants (k2) have been combined with the nucleophilicity parameters values of these series of anions 2a‐e to determine the electrophilicity parameters E of electrophiles 1a‐c according to the linear free‐energy relationship (log k2)/s versus N. General reactivity of these electrophiles 1a‐c is found to be fairly similar with E values ranging in −10.77 ± 0.61 < E < −7.53 ± 0.29. The comparison with structurally related neutral electron‐deficient heteroaromatic and aromatic compounds revealed that 1a‐c are more reactive than 1,3,5‐trinitrobenzene, as benchmark aromatic electrophile used in nucleophilic addition or substitution processes. The rate constants for the reactions of 4‐nitrobenzochalcogenadiazoles 1a‐c with some other nucleophiles were measured and found to agree with those calculated from Mayr's equation. Finally, analysis of the rate data in terms of the Brønsted approach reveals that 1a‐c exhibits especially low intrinsic reactivity in σ‐adducts 3 forming reactions.
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