Quantitative voltammetry of the reduction of methyl benzoate in THF reveals strong ion pairing of the radical anion with tetra-n-butyl cations

The reduction of methyl benzoate was studied in tetrahydrofuran (THF), 0.1 M TBAP, both at platinum and glassy carbon macroelectrodes and at platinum microelectrodes. While cyclic voltammograms obtained at macroelectrodes show extensive distortion due to ohmic drop, this distortion is negligible for cyclic voltammograms obtained at microelectrodes. The system methyl benzoate/methyl benzoate radical anion was found to be chemically reversible. The fitting of chronoamperometric measurements using the Shoup and Szabo's expression obtained at microdisc electrodes allowed us to estimate the diffusion coefficient of methyl benzoate. Cyclic voltammetry was modelled allowing the determination of the formal potential, E-f(0), of the transfer coefficient, alpha, and of the standard heterogeneous kinetic constant for the electron transfer, k(0). Furthermore, it was found that the diffusion coefficient for the radical anion produced at the electrode surface has an apparent value of 2.1 x 10(-6) +/- 0.5 cm(2) s(-1) (25 degrees C) which is ca. 10 times smaller than the value obtained for the diffusion coefficient of methyl benzoate (2.1 x 10(-5) +/- 0.1 cm(2) s(-1)). Taking into account the species present in solution, this result could only be explained by a strong ion pairing between the methyl benzoate anion radical and the tetra-n-butylammonium cation used as an electrolyte ion in the solution. The ion pair is found to be stable on the time scale of cyclic voltammetry and no evidence of any following homogeneous chemical reaction was found. Further studies involved the performance of cyclic voltammetry and chronoamperometry at low temperatures (down up to -81 degrees C) and the analysis of this data allowed the determination of the activation energy, E-A, through an Arrhenius plot, both for the diffusion coefficient and for the electron transfer reaction. Copyright (C) 2008 John Wiley & Sons, Ltd.