Electrodes organic electrosynthesis

Carbon electrodes are widely used in electrochemistry both in the laboratory and on the industrial scale. The latter includes production of aluminium, fluorine, and chlorine, organic electrosynthesis, electrochemical power sources, etc. Besides the use of graphite (carbons) as a virtually inert electode material, the electrochemical intercalation deserves special attention. This topic will be treated in the next paragraph. 

A synthetically very potent and unique feature of organic electrosynthesis is the oxidative or reductive Umpolung of reactivity. Reactive acceptors are anodically available as radical cations in a wide variety by the oxidative Umpolung of donors. This way two donors can be coupled in one step if one of them is converted to an acceptor at the electrode. Chemically, at least two additional... 

Today, a large number of important technologies are based on or related to electrodes reactions. Besides the chlor-alkali and aluminium industries, energy conversion in batteries and fuel cells, electrodeposition, electrorefining, organic electrosynthesis, industrial and biomedical sensors, corrosion and corrosion protection, etc. are amogst those technologies. In many of them, kinetic, catalytic or specificity aspects of electrode processes are of enormous importance. 

As discussed in Sects. 3.4 and 4.5, electrode processes coupled with homogeneous chemical reactions are very frequent and their study is of interest in many applied fields, such as organic electrosynthesis, ecotoxicity, biosciences, environmental studies, among others [15-17]. In this section, multipulse techniques (with a special focus on Cyclic Voltammetry) are applied to the study of the reaction kinetics and mechanisms of electrogenerated species. 

Griesbach U, Zollinger D, Putter H, Comninellis C (2005) Evaluation of boron doped diamond electrodes for organic electrosynthesis on a preparative scale. J Appl Electrochem 35 1265-1270... 

It is recommended that organic electrosynthesis be carried out at a constant current at first, since the setup and operation are simple. Then the product selectivity and yield can be improved by changing current density and the amoimt of electricity passed [current (A) x time (i) = electricity (C)]. However, the electrode potential changes with the consumption of the starting substrate (more positive in case of oxidation or more negative in case of reduction). Therefore the product selectivity and current efficiency sometimes decrease, particularly at the late stage of electrolysis. 

There is a trend toward chiral synthesis [132] the Englehard company is said to have piloted the electrosynthesis of chiral diols. New reactor design for the epoxidation of olefins is under development on a pilot scale [133]. Gas diffusion electrodes, developed for fuel cells and inorganic processes, are finding their first applications in organic electrosynthesis [134—136]. Another area of more than laboratory interest is bioelectrochemistry [137] (see also E. Steckhan, Chapter 27 in this volume). 

Dimensionally Stable Anodes— These anodes are composed of a base metal such as titanium, coated with a precious metal oxide (e.g., ruthenium dioxide). Such anodes can be used instead of Pt or carbon for oxygen evolution counter electrodes in an organic electrosynthesis. They have also found some applications for organic oxidation reactions [61]. 

Electrocatalysis is a heterogeneous process that involves the adsorption and the chemisorption of reactants or intermediates at the interface. These phenomena are encountered, for example, in fuel cells (FCs) or in organic electrosynthesis. The electrocatalytic activity of a given electrode for a certain reaction may be characterized by the current density at a chosen potential, which is proportional to the specific activity, when referred to the effective active surface. As shown in Figure 21.2, the role of a heterogeneous catalyst is to adsorb the electro-reactive species (reactant and intermediate) and transform it to another compound that can more readily undergo the desired... 

In this review, after a brief overview of the structural and electronic properties of metal adlayers, there are six sections describing catalytic effects on redox couples, oxidation of organic molecules, carbon monoxide, organic electrosynthesis reactions, hydrogen evolution, oxygen reduction, and metal electrodeposition. Outside the scope of this review are other UPD processes that play a role in determining the catalytic properties of electrode surfaces such as the UPD of H and OH. 

On the other hand, what are the problems which presently prevent the widespread commercial exploitation of organic electrosynthesis First we must recognize that organic electrosynthetic processes are chemically much more complex than any other processes considered in this book. Usually the overall electrode reaction is not simple electron transfer, but is a sequence of electron transfers and coupled chemical processes either on the electrode surface or in... 

Electrochemical polymerization reactions have electrochemical stoichiometry and in this regard are different from traditional polymerization reactions which are initiated either directly or indirectly [4], and which take place away from the electrode surface. In addition, the product of the electropolymerization reaction produces a film which has electroactivity and electrical conductivity [7], in contrast to many other organic electrosynthesis reactions where the electrode is covered with a product film which passivates the electrode. Moreover, many of the films are easily prepared from commercially available reagents, are stable and show little degradation in their electrical and mechanical properties in an ambient atmosphere. 

Finally, ad-atom-modified electrodes may also exhibit selectivity regarding the products obtained after electrolysis. Selective oxidation is particularly important in organic electrosynthesis. Two examples of selective oxidation of organic compounds will be discussed the oxidation of gly-colaldehyde in acid solutions on Pt and Pt/Sbajs [117] and the oxidation of gluconic acid in alkaline solutions on Pt and Pt/Pbads [118]. 

Although its history encompasses only the last fifteen years insofar as its relevance to organic electrosynthesis is concerned, the study and development of chemically modified electrodes (C.M.E. s) has stimulated a great deal of work which may eventually lead to unique s3mtheses. L. L. Miller (now at the University of Minnesota) attached covalently to oxidized surfaces of graphite a chiral moiety which, it was hoped, would induce chirality during a cathodic reduction. The results were very modest but encouraging. 

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