The solvent effect on the electrocatalytic cleavage of carbon-halogen bonds on Ag and Au

Abstract

In recent years it has been shown in detail how the electrocatalytic cleavage of carbon-halogen bonds is\ud modulated by (a) the stepwise or concerted nature of the dissociative electron-transfer mechanism,\ud which is infl uenced by the nature of the electrode surface, the type of halogen atom and the molecular\ud structure of RX as a whole, and (b) the double-layer structure (as a function of the nature and bulkiness of\ud the supporting electrolyte ions). In order to both complete and support the interpretative scheme thus\ud developed, this work is focused on the solvent role. When one compares aprotic with protic organic\ud solvents af ter appropriate intersolvental normalization, interesting peculiarities emerge, especially\ud concerning protic media. Solvent proticity deeply affects both the reaction mechanism (on both noncatalytic\ud and catalytic electrodes) and the extent of the catalytic effects. These items are discussed on the\ud basis of a complete investigation carried out with a carefully controlled experimental protocol on two\ud chloride and bromide couples, one aromatic and one aliphatic, representative of stepwise and concerted\ud mechanisms, respectively, in four aprotic and four protic solvents, on both non catalytic GC and catalytic\ud Ag and Au electrodes. The results are discussed in the framework of a recently developed interpretative\ud scheme of the carbon-halogen cleavage mechanism

Insights into Electroreductive Dehalogenation Mechanisms of Chlorinated Environmental Pollutants

“…> According to the dechlorination mechanisms, the aliphatic organic polychlorides can be generally categorized as geminal, which mainly undergo the sequential hydro‐dechlorination pathway with one chlorine elimination at each reduction step, or vicinal, which primarily follow a reductive β‐elimination route with the simultaneous removal of two chlorine ions and formation of double bond in one single step . Gennaro and co‐authors have made extensive investigations on the electroreductive dehalogenation of various organic chlorides though voltammetric studies and bulk electrolyses, including aromatic and aliphatic polychlorides and benzyl chlorides, showing the molecular structure and solvent conditions play crucial roles in determining the reductive dehalogenation mechanisms . Specifically, for the aromatic and benzyl chlorides, they normally display one irreversible reduction wave for the reductive cleavage of each C−Cl bond, following the hydro‐dechlorination mechanism.…”

“…The electrochemical strategy that exclusively uses electron as a clean reactant provides an efficient and green tool for the reductive dehalogenation of chlorinated environmental pollutants, achieving detoxification with partial transformation of their molecular structures . In this regard, this method can not only complete the effective removal of organic chlorides, meanwhile, it is also capable of recycling or producing useful substances .…”

“…The electrochemical strategy that exclusively uses electron as a clean reactant provides an efficient and green tool for the reductive dehalogenation of chlorinated environmental pollutants, achieving detoxification with partial transformation of their molecular structures . In this regard, this method can not only complete the effective removal of organic chlorides, meanwhile, it is also capable of recycling or producing useful substances . In addition, electroreductive dehalogenation offers another possibly as a preliminary treatment step for the completely removal of organic chlorides, which can be further combined with other approaches such as biodegradation and advanced oxidation processes for realizing the complete mineralization of these recalcitrant compounds .…”

“…In addition, electroreductive dehalogenation offers another possibly as a preliminary treatment step for the completely removal of organic chlorides, which can be further combined with other approaches such as biodegradation and advanced oxidation processes for realizing the complete mineralization of these recalcitrant compounds . In this context, much effort has been devoted toward achieving the electroreductive dehalogenation of organic chlorides with high dechlorination performance, and importantly, some useful results on the dehalogenation reactivity and mechanisms have been obtained accordingly …”

“…Among the extensive investigations of electroreductive dehalogenation in the last years, it is of interest to note that, all the studies can be divided into two general categories: the first one concerning mechanistic studies on the dissociative electron transfer to C−Cl bonds or, more generally, the reductive cleavage of C−Cl bonds for organic chlorides, while the second one mainly investigates the effects of critical experimental factors on the reductive dehalogenation process, in order to better understand the dechlorination mechanisms and achieve highly efficient abatement process for recalcitrant organic chloride pollutants . The available information can be found in an interesting review about the electroreductive remediation of halogenated environmental pollutants by Peters and an important monograph by Savéant, but as aforementioned, a specific investigation of the electrochemical strategies for the reductive dehalogenation mechanisms is necessary.…”