Electrochemical reactions involving organic compounds

Most electron transfers that involve organic compounds have rates that tend to lie in the upper range of detection by present electrochemical techniques.42 In the absence of adsorption or fast follow-up chemical reactions, the effect of the medium often can be isolated by measurement of the variation of half-wave potentials for one-electron, reversible systems. For a reduction reaction... 

In the present chapter we want to look at certain electrochemical redox reactions occurring at inert electrodes not involved in the reactions stoichiometrically. The reactions to be considered are the change of charge of ions in an electrolyte solution, the evolution and ionization of hydrogen, oxygen, and chlorine, the oxidation and reduction of organic compounds, and the like. The rates of these reactions, often also their direction, depend on the catalytic properties of the electrode employed (discussed in greater detail in Chapter 28). It is for this reason that these reactions are sometimes called electrocatalytic. For each of the examples, we point out its practical value at present and in the future and provide certain kinetic and mechanistic details. Some catalytic features are also discussed. 

Principles and Characteristics Contrary to poten-tiometric methods that operate under null conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise occur spontaneously. It is thus possible to analyse ions and organic compounds that can either be reduced or oxidised electrochemi-cally. Polarography, which is a division of voltammetry, involves partial electrolysis of the analyte at the working electrode. 

Electrochemical fluorination in anhydrous hydrogen fluoride (Simons process) involves electrolysis of organic compounds (ahphatic hydrocarbons, haloalkanes, acid halides, esters, ethers, amines) at nickel electrodes. It leads mostly to perfluori-nated compounds, but is accompanied to a high extent by cleavage and rearrangement reactions. The mechanism of the formation of carbocations according to Eq. (1) and Scheme 1 is assumed... 

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. 

Figure 17.2 illustrates our model for splitting water by solar energy. I" is important that all the redox reactions involved in thf system be reversible. The quinone compound in the organic solvent combines the two photocatalytic reactions, and its function can be compared to the electron relaying molecules in thylakoid membranes of chloroplasts. Electron transfer reactions via quinone compouncs in artificia systems have been studied as a model of photosynthesis22-23 and in an electrochemical system for acid concentration.24 ... 

Microcells. Cells that employ thin layers of solution (thin-layer electrodes) have special virtures that have been detailed in Chapter 5. In addition, the dipping thin-layer cell is especially useful for the measurement of chronopo-tentiometric n values.61 It is constructed from the components of a micrometer such that the cell thickness can be varied to permit a plot of the transition time versus solution thickness. This allows the determination of n, the number of electrons involved in the electrochemical reaction. An accuracy of 4% has been demonstrated for a number of compounds in different organic solvents. 

The second item that needs to be fixed is the number of species and the reactions, including the stoichiometric coefficients and also the kinetics of the processes. In this context, in electrochemical oxidation processes it is important to discern between two types of anodes those that behaves only as electrons sinks (named nonactive) and those that suffer changes during the electrochemical oxidation which influence on the treatment (named active electrodes). In both cases, the main processes related to removal of the pollutant that involves irreversible oxidative routes. Consequently, the reductive processes are less important and it can be presumed that in the cathodic zone only hydrogen evolution occurs. Nevertheless, if some organic compound can be reduced at the cathode, the mass-transfer and the reduction processes must be included in the model scheme. 

Silyl enolates are a class of electron-rich, non aromatic compounds which can be described as masked enols or enolates since hydrolysis following their reaction yields ketones they can be purified by distillation or chromatography, and then converted back to the enolate anion. The electron-rich character of these species can be used for oxidation reactions and examples have been described in the preceding sections. In this section, additional examples of chemical, PET and electrochemical redox reactions involving silyl enolates will be discussed, for a better appreciation of these interesting species in organic synthesis. 

Electrochemical reactions serve as efficient and convenient methods for the synthesis of organoelemental compounds. There are four major methods for the formation of element (metal)-carbon bonds. The first method utilizes the anodic oxidation of organometallic compounds using reactive metal anodes. In the second method, the organic compounds are reduced using reactive metal cathodes. The third method involves the cathodic reduction of organic compounds in the presence of metal halides. The fourth one utilizes both the cathodic and the anodic processes. 

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