Kinetics and Mechanism of the Oxidation of Substituted Benzyl Alcohols by Cr(VI)-4,4'-Bipyridine Complex

Cr(VI)-4,4'-bipyridine complex(4,4'-bipyridinium dichromate) was synthesized by the reaction of 4,4'-bipyridine with chromium trioxide in H2O, and characterized by IR, ICP. The oxidation of benzyl alcohol using 4,4'-bipyridinium dichromate in various solvents showed that the reactivity increased with the increase of the dielectric constant(e), in the order: cyclohexenedimethylformamide. In the presence of hydrochloric acid(HCl solution), 4,4'-bipyridinium dichromate oxidized substituted benzyl alcohols (p-CH3, H, m-Br, m-NO2) smoothly in N,N'-dimethylformamide. Electron-donating substituents accelerated the reaction, whereas electron acceptor groups retarded the reaction. The Hammett reaction constant(ρ) was -0.63(303K). The observed experimental data have been rationalized as follows; the proton transfer occurs after the prior formation of a chromate ester in the rate determining step.