Importance of Work Terms in the Free Energy Relationship for Electron Transfer

The rates of arene and alkene brominations and mercurations, Diels-Alder cycloaddition as well as alkylmetal cleavages by electrophiles, electron acceptors and free atoms can be quantitatively analyzed in terms of the driving force for electron transfer. The single, unified free energy relationship for all of these rather disparate chemical processes relates the activation free energy to the standard free energy change for electron transfer and the work term for ion-pair formation. The latter is evaluated from charge transfer spectral data of the electron donoracceptor complexes formed as transient intermediates. The large variations in the apparent Brönsted slopes from unity to even negative values in these systems can be readily attributed to changes in the work terms arising from steric effects in ion-pair formation.