Linear Free Energy Relations for Multlelectron Transfer Kinetics: A Brief Look at the Brcansted/Tafel Analogy

A comparison is made between electrochemical and homogeneous redox reactivity for species involving the transfer of multiple electrons at a single thermodynamic potential. In the electrochemical case the coreactant is the electrode; in the homogeneous case it is any of several members of a homologous series of one-electron reagents. In the latter case it is shown that the slope of a plot of the observed pseudo-first-order rate constant (one-electron reagent in excess) versus the thermodynamic driving force can be used to identify the cross reaction’s rate-determining step. It is noted that the proposed use of a homogeneous linear free energy relation (Brmsted plot) for mechanistic purposes is closely analogous to the well-known Tafel approach (log current versus potential) in electrochemical kinetics.