Kinetics and mechanism of oxidation of arylalkanes to carbonyl compounds with acidic pyridinium fluorochromate (PFC)

Abstract

Oxidation of diphenylmethane and fluorene by pyridinium fluorochromate, C₅H₅NHCrO₃)F, in aqueous acetic acid containing perchloric acid has been studied. The main products are benzophenone and fluorenone from diphenylmethane and fluorene respectively. While each oxidation is first order each in [oxidant] and [acid] the rate is almost independent of [substrate]. A kinetic isotope effect, <em>k</em>_H/<em>k</em>_D = 5.4 at 30&deg;C has been observed for the oxidation of fluorene. For both the oxidation processes, electron-releasing groups moderately facilitate the reactions whereas the electron-withdrawing groups have a retardation effect. Correlation with &sigma;/&sigma;⁺ value yielded a &rho; value of -1.14 for the diphenylmethane oxidation and a &rho; value of - 1.82 for the fluorene oxidation. The reaction does not induce polymerisation of acrylonitrile. The effects of temperature and different solvent compositions have been studied and activation parameters evaluated. These and related data suggest that the initial reaction involves hydrogen abstraction forming an intermediate, which subsequently undergoes hydrolysis to produce the corresponding ketones.