On the Theory of Oxidation-Reduction Reactions Involving Electron Transfer. III. Applications to Data on the Rates of Organic Redox Reactions

A recently developed theory of oxidation-reduction reactions (Part I) is used to calculate the rates of organic redox reactions whose mechanism involves the transfer of an electron from one reactant to the other. The theory can be used to discuss factors affecting the rates of these reactions. These factors include a standard free energy of reaction, the Coulombic interaction of the ionic charges of the reactants, and the solvation of the charged reactants. Attention is focused on the relation between the rate of the redox step in the over-all process and the standard free energy change, ΔF0, of this step, rather than on the more usual but less fundamental one between the corresponding quantities of the over-all process itself. An approximate method is described for applying the theory to a molecule whose charge is not located at its center. Essentially all organic molecules lie in this category. As an example of these considerations, several typical reactions are discussed. These reactions involve the oxidation of a series of hydroquinones by ferric ions and of a series of leucoindophenols by dissolved oxygen. They are assumed to possess electron transfer, rather than atom transfer, mechanisms. Free energies and entropies of activation of the redox step are calculated using the theoretical equation. The calculated results are considered to be in reasonable agreement with the experimental data, no adjustable parameters being employed.