2,2,6,6-Tetramethyl piperidine-1-oxyl (TEMPO)-mediated catalytic oxidation of benzyl alcohol in acetonitrile and ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate [BMIm][PF₆]: Kinetic analysis

TEMPO (2,2,6,6-tetramethyl piperidine-1-oxyl) is electrochemically oxidized to a stable form of the cation (TEMPO⁺) in acetonitrile (CH₃CN) or 1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIm][PF₆]) media. Cyclic voltammograms were characterized by a well-defined one-electron reversible redox couple in both media at low scan rates. The reduced form of TEMPO⁺ is catalytically regenerated in a follow-up chemical reaction with benzyl alcohol (BA) in the presence of 2,6-lutidine. It was observed that in [BMIm][PF₆], the redox currents are largely suppressed compared to that in CH₃CN. The apparent heterogeneous electron-transfer rate constant (k_app⁰) of the quasi-reversible redox reaction of TEMPO was determined at a Pt electrode and found to be 1.9 × 10^-3 cm s^-1 and 4.5 × 10^-2 cm s^-1 in [BMIm][PF₆] and CH₃CN, respectively. With the aid of chronoamperometry (CA), the homogeneous rate constant for the catalytic oxidation of benzyl alcohol by TEMPO, in the presence of 2,6-lutidine in CH₃CN was estimated to be 5.53 × 10¹ M^-1 s^-1 which is approximately double, relative to the value of 2.91 × 10¹ M^-1 s^-1 determined in [BMIm][PF₆].