Determination of the Competition Parameter for Coupling or Reduction by Use of the Rotating Disk Electrode and the Ultra-microelectrode.

Abstract

A new technique is presented for measuring the competition between coupling and reduction in the reaction between aromatic anion radicals and alkyl radicals. By making use of the steady-state properties of, e.g., ultra-microelectrodes or rotating disk electrodes, it is possible to distinguish between the two reaction paths, thus allowing the determination of a competition parameter, q. The new method is shown to be applicable by comparing experimental results obtained in this way for the butyl radical with the corresponding ones obtained from chronoamperometric and linear sweep voltammetric measurements. The advantage of the method presented is that it is not limited by a direct electrochemical reduction of the substrate and is less limited by slow reactions and adsorption phenomena. These points are illustrated by extending previously measured q-values for the 2-norbornyl radical with new values, thus making the measurements complete. The reduction potential, self-exchange reorganization energy and standard potential calculated from the measurements of q-values are given for the 2-norbornyl radical. At the same time the method makes possible the determination of the ratio between the diffusion coefficients of the aromatic compound and its anion radical. This ratio is, for the investigated compounds, close to unity.