Organic Electrode Reactions

An alternative and electrochemically more significant method for classification of electroorganic reactions is based upon the type of mechanism of the electrode process. This may be described in terms of the number and sequence of electrochemical steps (involving the transfer of electrons) and chemical steps. A useful example of both types of classification is the electrooxidation of an aliphatic amide, e.g., AW-dimethylformamide  

the formation of the substituted amide (I) may be classified as a substitution reaction or, in terms of the electrode process, as an ECEC reaction. 

Based on the material given in several textbooks and reviews published in recent years, one may simply summarize the many electroorganic 

Electroreduction cathodic processes Electro-oxidation anodic processes Other electrochemical processes 

Reduction of alkenes and Oxidation of aliphatic Indirect oxidation and 

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The development and the very widespread use of the polarographic technique to record i-E curves and the more recent designing of electronic devices known as potentiostats which automatically control the potential of the working electrode at a pre-set value has led to many examples in the literature of organic electrode reactions whose products depend on the potential. Some examples are cited below ... 

While it is widely realized that pressure is a useful variable for increasing the solubility of the electroactive species and hence the rate of the electrode process, it is mostly forgotten that it is also a variable which affects several of the steps in the overall process. In fact these more subtle effects of pressure on organic electrode reactions do not seem to have been investigated although it is possible to estimate their importance by considering the known effects of pressure on chemical systems (Hamann, 1957). 

Reactive radical ions, cations and anions are frequent intermediates in organic electrode reactions and they can serve as polymerization initiators, e.g. for vinylic polymerization. The idea of electrochemically induced polymerization of monomers has been occasionally pursued and the principle has in fact been demonstrated for a number of polymers But it appears that apart from special cases with anionic initiation the heterogeneous initiation is unfavorable and thus not competitive for the production of bulk polymers A further adverse effect is the coating of electrodes... 

Fewer examples are reported for organic electrode reactions some alkyl halides were catalytically reduced at electrodes coated with tetrakis-p-aminophenylporphy-rin carboxylate ions are oxidized at a triarylamine polymer and Os(bipy)3 in a Nafion film catalytically oxidizes ascorbic acid Frequently, modified electrodes fail to give catalytic currents for catalyst substrate combinations that do work in the homogeneous case even when good permeability of the film is proven... 

Coupling an electrochemical cell to an analytical device requires that hindering technical problems be overcome. In the last years there has been a considerable improvement in the combined use of electrochemical and analytical methods. So, for instance, it is now possible to analyze on-line electrode products during the simultaneous application of different potential or current programs. A great variety of techniques are based on the use of UH V for which the emersion of the electrode from the electrolytic solution is necessary. Other methods allow the in situ analysis of the electrode surface i.e the electrode reaction may take place almost undisturbed during surface examination. In the present contribution we shall confine ourselves to the application of some of those methods which have been shown to be very valuable for the study of organic electrode reactions. 

These new experimental approaches gave renewed motivation to the study of classical organic electrode reactions for direct electrochemical energy conversion. The present contribution intends to give a survey of the recent progress in the study of methanol oxidation attained by application of the above mentioned techniques. 

Mechanisms of Enzyme Action, Use of Product Inhibition and Other Kinetic Methods in the Study of (Walter). Mechanisms of Organic Electrode Reactions (Elving Pullman). ... 

Fig. 1 Steps constituting a typical organic electrode reaction E, E educt, P, P product circles indicate adsorbed molecules.

The majority of organic electrode reactions is characterized by the generation of a reactive intermediate at the electrode by ET and subsequent reactions typical for that species. Thus, the oxidation or reduction step initiates the follow-up chemistry to the reaction products ( doing chemistry with electrodes [14]). 

The reaction mechanisms of organic electrode reactions are thus composed of at least one ET step at the electrode as well as preceding and follow-up bond-breaking, bond-forming, or structural rearrangement steps. These chemical steps may be concerted with the electron transfer [15, 16]. The instrumental techniques described in this chapter allow the investigation of the course of the reaction accompanying the overall electrolysis. 

In order to classify the various mechanisms of organic electrode reactions, a specific nomenclature has been developed [17]. It is often extended in an informal way to accommodate particular reaction features, and one may find additional or deviant symbols. 

Mechanisms of Organic Electrode Reactions (Elving Pullman). 3 1... 

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... 

The attention towards electron transfer processes involving host-guest adducts of cyclodextrins (CDs) has become important with regard to their use as modifiers of organic electrode reactions. CDs, when added to solution or to electrode surfaces, can improve the selectivity of electrochemical synthesis. To elucidate the details of electron transfer reactions of guest molecules complexed inside CDs, the redox behavior of ferrocenecarboxylic acid in presence of jff-CD was studied, and this showed that the oxidation of the complexed ferrocenecarboxilic anion, FCA", must proceed via the dissociation of the host-guest adduct to form free FCA" which then transfers an electron to the electrode [81]. 

Since the primary intermediates in organic electrode reactions are usually radical ions or neutral radicals, the combination of electrochemical equipment with an electron spin resonance (ESR) spectrometer is a desirable possibility. The major practical problem encountered in designing an adequate experimental setup arises from the physical restrictions imposed on the electrochemical cell by the shape and size of the resonance cavity. Two different approaches have been taken to meet the requirements. One involves the formation of the radical species outside the magnetic field in a streaming solution that carries the electrode products into the ESR cavity. By the other technique the radical species are formed by electrolysis in a small cell placed directly in the cavity. Both techniques have for many years been used extensively in qualitative and semiquantitative work, and the design and construction of cells have now reached a high level of sophistication [363-377]. 

The empirical representation of electrode process rates according to a relation such as Eq. (1) or its exponential form, Eq. (4), takes into account that, for many electrode processes. In i is linear in 7) over an appreciable range (> 0.2 V say) of potentials. More will be said about this later with regard to specific examples however, it must be stated here that for some processes such as rapid redox reactions (high Iq values) and some organic electrode reactions, a quadratic term in ry may also arguably appear in Eq. 

In electrochemical proton transfer, such as may occur as a primary step in the hydrogen evolution reaction (h.e.r.) or as a secondary, followup step in organic electrode reactions or O2 reduction, the possibility exists that nonclassical transfer of the H particle may occur by quantum-mechanical tunneling. In homogeneous proton transfer reactions, the consequences of this possibility were investigated quantitatively by Bernal and Fowler and Bell, while Bawn and Ogden examined the H/D kinetic isotope effect that would arise, albeit on the basis of a primitive model, in electrochemical proton discharge and transfer in the h.e.r. 

A counter electrode reaction is needed in the organic electrochemical reactor. The reactant and product(s) of this reaction must not interfere with the primary organic electrode reaction. Hence, a membrane separator is often used to divide the anode and cathode compartments of the reactor the membrane adds to the cost of the reactor and usually increases its operating voltage. 

It was previously shown that oxidation produced a radical cation which could be detected by ESR [111, 112] provided the 4 position was fully substituted. If the 4 position contained a hydrogen atom, then the initial oxidation was followed by deprotonation with the formation of a neutral radical which could undergo further oxidation or dimerisation. In-situ spin trapping using PBN demonstrated the presence of radical intermediates in the electro-oxidation of substituted 1,4-dihydropyridines. That the trapped radicals were the deprotonated neutral radical, and not the primary radical cation, was demonstrated by comparison of the ESR parameters with those of the spin adduct produced by electroreduction of A-methylpyridine ion cations, where only the neutral dihydropyridyl radical would be produced. The work of Volke and co-workers clearly demonstrates how spin trapping may be applied to the study of more complex organic electrode reactions and how comparison of ESR spectra generated from different precursors may be used to reveal the nature of the trapped radical. 

The representative applications of in-situ ESR described in Sect. 5 demonstrate the amount of information it is possible to obtain concerning electrode reactions with this method. Since the first experiments were performed over 25 years ago, the method has been used to identify and investigate radicals produced from a vast range of organic electrode reactions. Considerable study of intermediates in inorganic electrode reactions has also taken place. The information obtained from these studies, together with the development of the improved techniques outlined above, indicate that in-situ ESR will continue to make a major contribution to the study of electrode reactions. 

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