Basic concepts

Abstract

Our objective in this chapter is to familiarize organic chemists with fundamental electrochemical concepts that support several aspects of organic electrochemistry elaborated in this book, as well as many important features of electron transfer reactions. In fact, although the term electrochemistry evokes for most readers the idea of chemistry at electrodes, in our opinion, electron transfer chemistry constitutes a better definition when organic or organometallic electrochemistry is concerned. This is particularly obvious when one thinks of an electrode as a macrosized molecule whose ability to provide or accept electrons is virtually infinite (versus a one-shot molecular redox species) and may be precisely adjusted by fine-tuning of the electrode potential. Similarly, electrochemical reactions involving specific interactions with the electrode material are no more than the analogs of inner sphere, ion-pairing, or complexation reactions. Thus, in many respects, electrochemical reactions do not differ basically from their homogeneous counterparts except for the topology: An electrode is a 2D structure placed in a 3D volume, while molecular redox species are generally dispersed in the solution. This similarity is even more true insofar as it concerns the chemical reactivity of electrogenerated intermediates, since they evolve under conditions that a priori do not differ from those considered in homogeneous chemistry. Indeed, the extraordinarily large electric fields (of magnitude comparable to those at the origin of storm lightning) drop to negligible values within a few angstroms of the electrode surface, in contradiction to generally held ideas. Thus, as soon as molecules have moved over a few molecular diameters from the electrode surface, they probe no special electrical effects associated with their electrochemical origin. This is an important point, although not well recognized or publicized, since it permits an easy transposition of electrochemical results to homogeneous situations or vice versa.