Kinetics and Mechanism of the Oxidation of Substituted Benzyl Alcohols by Cr(VI)-Heterocyclic Complex (2,4'-Bipyridinium Chlorochromate)

Cr(VI)-heterocyclic complex (2,4’-bipyridinium chlorochromate) was synthesized by the reaction between heterocyclic compound(2,4’-bipyridine) and chromium trioxide, and characterized by IR and ICP analysis. The oxidation of benzyl alcohol using 2,4’-bipyridinium chlorochromate in various solvents showed that the reactivity increased with the increase of the dielectric constant (e), in the order : N,N-dimet-hylformamide (DMF) > acetone > chloroform > cyclohexene. In the presence of DMF solvent with acidic catalyst such as hydrochloric acid (HCl solution), 2,4’-bipyridinium chlorochromate oxidized benzyl alcohol (H) and its derivatives (p-CH3, m-Br, m-NO2). Electron-donating substituents accelerated the reaction rate, whereas electron acceptor groups retarded the reaction rate. The Hammett reaction constant (ρ) was -0.67 (303 K). The observed experimental data have been rationalize the proton transfer occurred followed the formation of a chromate ester in the rate-determining step.