Electro-organic reactions

Electroorganic reactions may be conveniently classified on the basis of products formed into the following categories  

Conversion reactions (one functional group into another one), 

Electron transfer reactions (with formation of stable radical ions or ions). 

All of these reactions types can be effected at the cathode or the anode. However, since organic electrode processes often occur via a blend of radical and ionic mechanisms, it should be stressed that the simplicity of the classification scheme above does not imply that the electrochemical oxidation or reduction of a given substrate results in only one type of reaction on the contrary, many processes give products emanating from two or more of the reaction types above, and it may often involve a lot of experimental work to find optimum conditions for a (desired) reaction to occur, if it is at all possible. 

If the reaction is instead carried out at a graphite anode 36- the formation of 1 is completely suppressed and 2 is obtained in good yield. It is a sad fact that we cannot at present interpret this difference mechanistically  

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Electrochemical processes can be made more competitive when products are made at both electrodes simultaneously. Paired electro-organic reactions are divided into three categories. 

Use of Isotopic Effects in the Determination of Electro-Organic Reaction Mechanisms. Much work has been carried out on the mechanism by which hydrocarbons can be clectrochemically oxidized. Were that easy, it might be possible to use available oil in electrochemical devices (fuel cells) to convert chemical to electrical energy 2—3 times more efficiently than do heat engines (Chapter 13). 

Electro-organic reactions generally take place in more than one electron transfer step and may involve complex proton transfer steps (chemical step) to the various possible intermediates i.e. 

For the organic chemist, product studies in the widest sense, ie., including stereochemical aspects, isotope effects, etc. fall most natural in the study of electro-organic reactions. However, there are also some simple electrochemical techniques which are extremely useful in the design of electrochemical syntheses and can be set up in any laboratory for a modest cost. These methods — which are the ones to be discussed here - include different kinds of voltammetry, controlled potential electrolysis, and coulometry, andigive information as to the nature of the electro-active species, the possible nature of intermediates involved and their reactions with reagents present, and the number of electrons involved in the process. 

However, even if a consideration of the macroscopic properties of the SSE many times is useful as a first approximation for predicting the outcome of an unknown electro-organic reaction, it must be borne in mind that the composition of the electrolyte at the electrode surface and its immediate vicinity might be completely different from that of the bulk of the solution. Current theory 19>79 assumes that the electrode surface is covered by an adsorbed layer of ions and neutral molecules during electrolysis. The thickness of this layer, the electrical... 

Finally, when it comes to electricity cost, electro-organic reactions do not have to be driven backward many desired products can be obtained by electrochemical cell reactions that have negative AG s. If these reactions can be broken up into two reactions in an electrochemical cell, they form a kind of fuel cell that is intrinsically electrogenerative, so now electricity is being made and can be used elsewhere—the electricity costs have turned into a gain (Section 13.3). 

After the preceding brief survey of electro-organic reactions, let us go back to the experimental situation and try to discern the nature of the mechanistic problems originating from the reaction conditions peculiar to electrochemical experiments. Figure 2 shows, in the direction of the arrows, the train of events involved in the electrochemical process and the associated mechanistic problems in the different layers of the electrolyte solution... 

The historical development of electro-organic chemistry is well documented by several authors, e.g. in refs. 514 and 521-527, and therefore will not be repeated here. Much of the pioneering work in the field was carried out at Pt, i.e. Pt covered with an oxide film of monolayer dimensions in the case of anodic reactions. Electro-organic reactions at Pt have been analyzed in considerable detail by Conway [517] and will not be discussed here. Rather, attention will be focussed on oxide electrocatalysts and metal anodes covered with oxide films of multilayer dimensions, e.g. Ni and Pb. However, before commencing with a discussion of such oxide catalysts, some important factors in electroorganic chemistry will be briefly reviewed. 

In aqueous electrolytes, simple exchange of one or two electrons and protons is observed at many electrode surfaces. In anodic reactions, however, radical intermediate formation may also occur, as for example in the Kolbe reaction [525], The order in which electron and proton exchange occur, whether accompanied by chemical reactions or not, can often be characterized by standard electrochemical analysis. Radical ion formation is mostly involved in electro-organic reactions in aprotic non-aqueous... 

Metal oxide electrodes have been relatively infrequently employed in electro-organic reactions and, even in those cases which have been moderately well studied, there are still some questions regarding the reaction mechanisms, e.g. whether a surface oxide species mediates the organic transformation or not in the case of oxidation reactions. The study of certain types of model organic compounds, e.g. alcohols and aldehydes, at metal oxide electrodes could lead to further insight into oxide electrocatalysis. 

This phenomenon can be explained from two aspects one from the actual course and specialty of a heterogeneous electro-organic reaction, and the other from special physico-chemical properties of SCFs. The effect of pressure on the solubility of benzaldehyde and benzyl alcohol in SC CO2 is shown Figure 16. Tlie significant effect of pressure on the solubility is one ofthe reasons for tlie variation of FE and selectivity ofbenzaldehyde. When the solubility of benzaldehyde in SC CO2 increased dramatically, more benzaldehyde produced enters SC CO2 phase, and thus the FE and selectivity ofbenzaldehyde were enhanced. When the pressure reached a certain value, the solubility of benzyl alcohol (the reactant) in SC CO2 also increased considerably, which is not favorable to producing benzaldehyde. When the pressure is lower than 9.3 MPa, the first factor is dominant and thus the FE and selectivity ofbenzaldehyde increased with pressure. However, as pressure was larger than about 9.3 MPa, the second factor became dominant. Therefore a maximum value in FE or selectivity occurred. 

The choice of anode material for electro-organic reactions is limited, due to anodic dissolution of most metals. The most common materials are... 

Another intriguing approach to electrocatalysis involves the use of underpotential-deposited monolayers and submonolayers of foreign metal adatoms on metal substrates. Such layers afford unique electronic and morphological surface properties, not usually achievable with pure metal or alloys. Underpotential-deposited layers have been found to have high catalytic activity for such reactions as H2 generation, 02 reduction, and certain electro-organic reactions. 

The electrochemist is fortunate in that the bibliography of electro-organic reactions is excellent. An early work by Fichter describes thousands of... 

R. C. Alkire and S. Swann, Bibliography of Electro-organic Reactions 1801-1975 , Electrochemical Society, Princeton, N.J., 1980. 

Different electro-organic reaction systems have been studied. The anodic reactions investigated are mainly the four-electron methoxylation of 4-methoxytoluene [6, 7, 11, 12] and the two-electron methoxylation of furan to 2,5-dimethoxy-2,5-dihydrofuran [10], but also other methoxylation and acetoxylation reactions [11]. Methoxylation reactions are performed in methanol as a solvent, whereas acetoxyla-tions are performed in acetic add. Moreover, Kiipper et al. [7] reported the anodic two-electron decarboxylation of sodium glucanate in an aqueous medium. The cathodic... 

We shall first present for reference purposes some basic aspects of electrode kinetics, as they may apply to electro-organic reactions, outlining the dependence of electrode reaction rates on potential, electrode material, adsorption behavior, etc., and the role of consecutive reaction steps. 

When adsorption is significant in an electro-organic reaction, interpretations of reaction order in terms of mechanisms must be made with care. Consider a case where the current density at a given constant potential E is... 

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