The Electrochemical Study of Vanillin in Acetonitrile

A detailed electrochemical study of vanillin was performed in acetonitrile using a platinum electrode. At low scan rates ≤ 1 V s^-1, vanillin displayed an anodic peak at ca. 1.12 vs. (Fc/Fc⁺)/V, and a cathodic peak at ca. 0.09 vs. (Fc/Fc⁺)/V (on the reverse cyclic voltammetry (CV) scan after the initial oxidation) due to the reduction of a secondary reaction product. Both redox processes were found to involve a transfer of two electrons via controlled-potential electrolysis experiments, and it is surmised that vanillin first undergoes a -2e^-/-H⁺ oxidation, followed by a hydrolysis reaction and the loss of its methoxy substituent to generate a corresponding 1,2-benzoquinone that can subsequently be reduced via +2e^-/ + 2H⁺. When higher scan rates ≥2 V s^-1 were employed, however, the aforementioned homogeneous reactions were partially outrun; leading to the detection of an additional cathodic peak at ca. -0.28 vs. (Fc/Fc⁺)/V which is ascribed to the reduction of the transient hemiketal intermediate that is formed immediately after the reaction of the phenoxonium cation with trace water. A similar trend to the fast scan rate CVs was likewise recorded at lowered temperatures (≤-10 °C). Digital simulations were used to model the cyclic voltammetry data which enabled an estimation of the electrochemical and kinetic parameters associated with the electrode reactions. Independently, vanillin can also be electrochemically reduced at ca. -1.58 vs. (Fc/Fc⁺)/V, which is proposed to involve the formation of molecular hydrogen.