Mechanism for the Oxidation Reaction of Alcohols Using Cr(VI)-Pyrazine Complex

Cr(VI)-pyrazine complex (PZCC) was synthesized by the reaction of pyrazine with chromium (VI) trioxide in 6 M HCl. The structure was characterized using IR spectroscopy and inductively coupled plasma (ICP). The oxidation of benzyl alcohol using PZCC in various solvents showed that the reactivity increased with the increase of the dielectric constant, in the order: N,N'-dimethylform- amide > acetone > chloroform > cyclohexene. In the presence of N, N'-dimethylformamide solvent with an acidic catalyst such as sulfuric acid (H2SO4 solution), PZCC oxidized benzyl alcohol (H) and its derivatives (p-OCH3, m-CH3, m-OCH3, m-Cl, m-NO2). Electron-donating substituents accelerated the reaction rate, whereas electron acceptor groups retarded the reaction rate. Hammett reaction constant (rho) was -0.70 (308 K). The observed experimental data were used to rationalize the hydride ion transfer in the rate-determining step.