Linear free energy relations for multielectron transfer kinetics: A brief look at the brønsted/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 (Brønsted plot) for mechanistic purposes is closely analogous to the well-known Tafel approach (log current versus potential) in electrochemical kinetics.