Kinetics and mechanism of acid-catalyzed oxidation of substituted toluenes by pyridinium fluorochromate.

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The kinetics of oxidation of substituted toluenes by pyridinium fluorochromate(VI) (PFC), C5H5N\overset+HCrO3F−, in aqueous acetic acid, in the presence of perchloric acid, have been studied. The main products are the corresponding aldehydes. While each of the oxidation is first order with respect to the oxidant, the rate is almost independent of the substrate concentration. The reactions depend on the first power of the concentration of acid. A kinetic isotope effect, kH⁄kD=5.4 at 30 °C, was observed. Electron-releasing groups were found to moderately facilitate the reaction, whereas the electron-withdrawing groups were found to have a retardation effect. Correlation with σ value yielded a ρ value of −2.0. The reaction does not induce polymerization of acrylonitrile. The effects of temperature and solvent compositions were studied and activation parameters evaluated. These and related data suggest that the initial reaction involves hydrogen abstraction forming an intermediate which subsequently produces the corresponding aldehyde.
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