Separation of the enthalpic and entropic contributions to substituent effects for the electroreduction of organic molecules

Abstract

An analysis of substituent effects on the half-wave potentials for reversible electroreduction reactions of organic molecules is presented. The analysis is based on the linear enthalpy-entropy equation proposed earlier in an analysis of acidity constant data for aromatic acids. It is found that the change in half-wave potential with substituent prodominantly follows the corresponding standard entropy change for the reference Hammett reaction, namely, the ionization of benzoic acids. Furthermore, the entropic reaction constant is approximately equal to the classical Hammett reaction constant. It is shown that deviations from results obtained for the reference reaction can be related to a change in the relative contributions of inductive and resonance effects.