Organic electrochemical reactions, examples

Eor some common organic electrochemical reactions, for example, the Kolbe electrolysis of carboxylates [13], the adsorption of intermediates has been discussed. 

Electrons are transferred singly to any species in solution and not in pairs. Organic electrochemical reactions therefore involve radical intermediates. Electron transfer between the electrode and a n-system, leads to the formation of a radical-ion. Arenes, for example are oxidised to a radical-cation and reduced to a radical-anion and in both of these intermediates the free electron is delocalised along the 7t system. Under some conditions, where the intermediate has sufficient lifetime, these electron transfer steps are reversible and a standard electrode potential for the process can be measured. The final products from an electrochemical reaction result from a cascade of chemical and electron transfer steps. 

Organic electrochemical reactions are classified in the same way as other organic reactions [1,2]. The most important prototypes include additions (Scheme 6.1) [ 13,14], eliminations (Scheme 6.2) [15, 16], substitutions (Scheme 6.3) [17, 18], couplings and dimerisations (Scheme 6.4) [19-21], cleavages (Scheme 6.5) [22,23], and catalytic reactions (Scheme 6.6) [24,25]. Hundreds of other examples maybe found in the literature [1,2]. 

The use of preparative methods in studies of organic electrochemical reactions has been reviewed and examples of the identification of both unstable and stable intermediates and products have been presented. This review includes some interesting observations upon the fact, referred to above, that different products may be obtained from a given reaction if it is carried out at a mercury pool rather that a dropping mercury electrode. 

Preceding and subsequent homogeneous chemical reactions that occur in the bulk solution are very common. Examples include dehydration (when only a nonhydrated form of the substance is involved in the electrochemical reaction), protonation (e.g., of the anions of organic acids), and decay of complexes (in metal deposition from solutions of complex salts). 

It is hoped that the examples and the accompanying mechanistic discussions of electrochemical reactions of organosilicon compounds shown in this review will provide a guide to the potential utility of such reactions in organic synthesis and to the development of new electroorganic chemistry based on the unique properties of silicon. 

Atom economy is high. As a reagent, no compounds are needed and consequently none are produced as the electron is immaterial. This results in a greater advantage of electrochemical reactions compared to chemical conversions, namely, an effective contribution to pollution control. The direct ET from the electrode to the substrate avoids the problem of separation and waste treatment of the, frequently, toxic end products of chemical reductions or oxidations. Furthermore, by electrodialysis, organic acids or bases can be regenerated from their salts without the use of, for example, sulfuric acid or... 

The electrochemical processes involving Prussian blue and organic dyes studied above can be taken as examples of solid state redox processes involving transformation of a one solid compound into another one. This kind of electrochemical reactions are able to be used for discerning between closely related organic dyes. As previously described, the electrochemistry of solids that are in contact with aqueous electrolytes involves proton exchange between the solid and the electrolyte, so that the electrochemical reaction must in principle be confined to a narrow layer in the external surface of the solid particles. Eventually, however, partial oxidative or reductive dissolution processes can produce other species in solution able to react with the dye. 

Electrochemical reduction and oxidation processes offer several advantages over conventional methods in their application to organic synthesis. For example, selective transformations can be carried out on specific groups in a multifunctional, valuable compound under the usually mild reaction conditions. Independence of a reagent will result in drastically diminished environmental problems by spent reagents. Electrochemistry also allows the application of alternative feedstocks and better use of raw materials. Product isolation and continuous processing are simplified. 

For many inorganic and organic substances, it is rare that the electrode reaction is simply an electron transfer at the electrode surface. In most cases, the electron transfer process is accompanied by preceding and/or following reactions, which are either chemical or electrochemical. For example, for the electrode reduction of substance A, mechanisms as described below can be considered ... 

Research into chemically modified electrodes has led to a number of new ways to build chemical selectivity into films that can be coated onto electrode surfaces. Perhaps the simplest example is the use of the polymer Nafion (see Table 13.2) to make selective electrodes for basic research in neurophysiology [88]. Starting with the pioneering investigations by Ralph Adams, electrochemists have become interested in the electrochemical detection of a class of amine-based neurotransmitters in living organisms. The quintessential example of this class of neurotransmitters is the molecule dopamine, which can be electrochemically oxidized via the following redox reaction ... 

The electrochemical reduction of nitrobenzene to produce p-aminophenol has attracted industrial interest for several decades. However, some limitations may be met in this process regarding overall reaction rate, selectivity and current efficiency using a two-dimensional electrode reactor. These restrictions are due to the organic electrode reaction rate being slow and to the low solubility of nitrobenzene in an aqueous solution. In this example, a packed bed electrode reactor (PBER), which has a large surface area and good mass transfer properties, was used in order to achieve a high selectivity and a reasonable reaction rate for the production of p-aminophenol. The reaction mechanism in an acid solution can be simplified as... 

Classification by End Use Chemical reactors are typically used for the synthesis of chemical intermediates for a variety of specialty (e.g., agricultural, pharmaceutical) or commodity (e.g., raw materials for polymers) applications. Polymerization reactors convert raw materials to polymers having a specific molecular weight and functionality. The difference between polymerization and chemical reactors is artificially based on the size of the molecule produced. Bioreactors utilize (often genetically manipulated) organisms to catalyze biotransformations either aerobically (in the presence of air) or anaerobically (without air present). Electrochemical reactors use electricity to drive desired reactions. Examples include synthesis of Na metal from NaCl and Al from bauxite ore. A variety of reactor types are employed for specialty materials synthesis applications (e.g., electronic, defense, and other). 

In contrast to aliphatic amines, the anodic oxidation of aromatic amines shows a rather complex reaction pattern. Although extensive studies on the electrochemical reaction mechanism have been carried out, there are very few examples for the application of the anodic oxidation of aromatic amines to organic synthesis. 

In the kinetic interpretation of ECL data presented above the closest approach of reactants has been assumed. It seems to be fulfilled in the case of organic ECL systems (because of the Coulombic attraction between oppositely charged ions). In contrast, in ECL systems involving identically charged transition-metal complexes, Coulombic repulsion may lead to an increase in the electron transfer distance. The molybdenum(II) halide cluster ion MoeCliJ [197-199] is the one of the best-understood examples. In electrochemical reactions MogCliJ is reversibly reduced or oxidized to stable MosCl and Mo6Cl["4 respectively ... 

On the other hand, when one thinks in terms of electrochemical reductions or oxidations, special attention is devoted to the coreactant, that is, to the electrode that provides or accepts electrons. Thus, in order to discuss or compare electrochemical reactions with their organic analogs, it is of the utmost importance to use more precise terms than the so inaccurate reduction of oxidation notions. A similar problem has been addressed in the inorganic and organometallic fields. Indeed, it was early recognized that oxidation-reduction reactions at metal centers must be classified according to two types outer sphere or inner sphere reactions. A typical example of this dichotomy is given in Eqs. (14) and (15), which relate to chromium (II) oxidations by cobalt (III) complexes. 

In the following, examples of indirect electrochemical reactions (mainly redox catalysis) with an organic mediator are given. 

There are some recent reviews on the topic of organic electrochemical processes in industry [13,64-69]. Most of these reviews list several dozens of electrochemical reactions that are reputed to have reached commercial status, at least for a period of time. Many cases are not definitely confirmed some have been operated commercially for some years but are believed to be obsolete today (see Table 1). D. Degner has compiled the relevant patent literature for industrially important reactions that have been studied between the early 1970s and the late 1080s [70]. Table 1 presents many of these examples, but its message often is only These are electrochemical reactions that have or had the opportunity to achieve costs equal to those of alternatives. 

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