Glassy Carbon Surface Morphology as a Semiquantitative Probe for Slow Electron Transfer in Mechanistic Studies

The electrochemical reduction of organic halides is a typical case of dissociative electron transfer (DET), proceeding either along a concerted pathway (with concurrent electron uptake and bond cleavage) or a stepwise pathway (with a stable radical anion intermediate). In the first case, which is typical of many aliphatic halides, the reaction is kinetically controlled by the electron transfer, whereas in the second one, typical of aryl halides, two energy barriers must be taken into account, an “electrochemical” barrier associated with heterogeneous electron transfer and a “chemical” one, corresponding to bond cleavage. Recently, we performed an exhaustive voltammetric investigation on the reduction of aryl bromides and chlorides on glassy carbon (GC) electrodes, choosing a series of molecular structures affording a regular, well-defined sequence of increasingly more stable radical anion intermediates, i.e. following a stepwise DET pathway with the second energy barrier gradually becoming more determining with respect to the first one, as indicated by the aapp parameter. In such investigation, while using different GC electrodes (both different electrodes from the same producer and electrodes from different producers), and treating them with a carefully optimised protocol, we realised that each electrode gave a very reproducible performance, even after years of continuous use, but different electrodes gave different, albeit reproducible, performances. XPS investigation revealed that such different activity, at constant surface activation procedure, depends on different distribution of surface active groups, which, in turn, should result from different bulk compositions and/or defects in the active material. In particular, a more functionalized surface results in significant peak potential anticipation (even > 0.1 V). The results obtained for the reduction of both types of organic halides, undergoing either DETs mechanisms, clearly show that the peak potential differences between different GC electrodes are not constant, but increase very regularly with decreasing aapp. In other words, the slower the electron transfer, the higher the potential difference obtained on differently functionalised GC surface. This very interesting feature not only implies that mechanistic investigations on GC require careful surface morphology normalisation, but also provides a convenient and effective semiquantitative probe of the kinetic influence of the first electron transfer step in the overall DET process.