Pyridiniumchlorochromate oxidations: Studies on the kinetics and substituent effect in the oxidation of aniline in nonaqueous systems

Abstract

Pyridiniumchlorochromate (PCC) oxidizes aniline and substituted anilines except nitro anilines smoothly in chlorobenzene-nitrobenzene mixtures in the presence of dichloroacetic acid. The reaction has unit dependence on each of the aniline, PCC, and dichloroacetic acid concentrations. Electron-releasing substituents accelerate the reaction, whereas electronwithdrawing groups retard the reaction, and the rate data obey Hammett's relationship. The reaction constant p is -3.75. Azobenzene and p-benzoquinone have been obtained as products. The observed experimental data have been rationalized in terms of the formation of an intermediate complex involving PCC-amine undergoing a rapid decomposition to products.

Substituent and Solvent Effects on the Electrochemical Oxidation of para- and meta-Substituted Anilines

“…The values of the reaction constant, ρ, were found to be negative, which confirms the formation of a positively charged transition state. The reaction constants obtained in the present study are comparable to those of previously reported oxidation of anilines by various oxidants (percarbonate, ρ = −2.7 [28]; perborate, ρ = −3.0 [29]; pyridinium chlorochromate, [30], and imidazolium fluorochromate, ρ = −2.22 [31]). …”

Correlation analysis of reactivity in the oxidation of anilines by nicotinium dichromate in nonaqueous media

“…The negative ρ value is indicative of the presence of a positive nitrogen center, which would mean depletion of lone‐pair electron density, and this can be facilitated only when the oxidant forms a complex with the substrate in which the nitrogen lone‐pair can be used up in coordinating with an electron‐deficient center, preferably a metal ion. Similar mechanism has been postulated for the oxidation of anilines by pyridinium chlorochromate in chlorobenzene–nitrobenzene mixture 16 and by nicotinium dichromate in benzene‐ tert ‐butanol mixtures 22. Further, such a complex formation is supported by the negative entropy of activation observed.…”

“…The negative reaction constant indicates the formation of a positively charged activated complex with extensive bond formation between the reactants 6. The reaction constant is comparable to that of the percarbonate oxidation of molecular aniline in glacial acetic acid (ρ = −2.7 ± 0.1 14), perborate oxidation of molecular aniline in glacial acetic acid (ρ = −3.0 15), pyridinium chlorochromate oxidation of anilines in chlorobenzene‐nitrobenzene mixtures (ρ = −3.75 16), and imidazolium fluorochromate oxidation of anilines in DMF (ρ = −2.22 17). The specific rate constants for the meta‐ and para‐substituted anilines correlate satisfactorily with σ + (0.841 < r < 0.936, 0.31 < SD < 0.62, −1.33 ± 0.21 < slope < −2.03 ± 0.41) and σ and σ m (0.900 < r < 0.966, 0.26 < SD < 0.47, −1.51 ± 0.19 < slope < −2.28 ± 0.32) also.…”

Studies on the kinetics of imidazolium fluorochromate oxidation of some meta‐ and para‐substituted anilines in nonaqueous media

“…The correlations of log k obs versus pK a [22] and log k obs versus pK a (= pK a -substituted aniline -pK a aniline) [23] are nonlinear. This observation along with the results of structure-reactivity correlation indicates that molecular anilines are the reactive species [24].…”