The key role of the medium in the reductive cleavage of RX bonds at catalytic and non-catalytic electrodes: from aprotic and protic traditional solvents to ionic liquids

In the last years we have shown how the electrocatalytic cleavage of carbonhalogen bonds is modulated by (a) the stepwise or concerted nature of the DET mechanism (as a function of the electrode surface, of the nature of the halogen atom, and of the molecular structure of RX) and (b) the double layer structure (as a function of the nature and bulkiness of the supporting electrolyte ions). Recently, in order to both complete and support our interpretative scheme, we are concentrating on the solvent role, carrying out a systematic investigation on model chlorides and bromides in four aprotic and four protic traditional solvents (VOCs) of increasing polarity and proticity on glassy carbon GC, Au and Ag electrodes (representing non-catalytic, moderately catalytic, and highly catalytic electrodes, respectively, in traditional solvents). Comparing aprotic with protic organic solvents (after appropriate intersolvental normalization) interesting peculiarities emerge, especially concerning protic media: solvent proticity deeply affects both the reaction mechanism (on both non-catalytic and catalytic electrode surfaces) and the extent of the catalytic effects, which regularly and remarkably increase with increasing solvent proticity; in particular, water appears to provide the limiting case of the alcohol series, granting the highest catalytic effects observed for each molecule in the entire series of traditional media. These observations have attracting applicative implications. We will also present a first extension of this study to room temperature ionic liquids (RTILs), which are currently arising considerable interest on account of their advantages over traditional organic solvents, including negligible vapour pressure, high intrinsic conductivity and easy recyclability, and which look particularly attractive as media for electrochemical processes on account of their two-fold role as both solvent and supporting electrolyte. This first study, carried out in BMIMPF6, shows that RTILs, consistently with their high polarity, strongly enhance the catalytic effects of Ag and Au electrodes, performing comparably to the most efficient protic cases (MeOH and water).