Electrode reactions synthetic techniques

In the first part of the present review, new techniques of preparation of modified electrodes and their electrochemical properties are presented. The second part is devoted to applications based on electrochemical reactions of solute species at modified electrodes. Special focus is given to the general requirements for the use of modified electrodes in synthetic and analytical organic electrochemistry. The subject has been reviewed several times Besides the latest general review by Murray a number of more recent overview articles have specialized on certain aspects macro-molecular electronics theoretical aspects of electrocatalysis organic applicationssensor electrodes and applications in biological and medicinal chemistry. 

Limitations to the acceptance of organic electrochemistry, particularly as a synthetic technique, may have been connected with the fact that electrode reactions are normally two-dimensional, i.e., they are restricted to a surface and therefore require mass transport (see elsewhere in this chapter) and also because many reactions yield a complex mixture of products when the electrolyses are carried out using a constant current. However, as early as 1898, Haber had pointed out the importance of control of the electrode potential for the overall process, in his work where nitrosobenzene, phenylhy-droxylamine and aniline were isolated selectively from the reduction of nitrobenzene. However, design of suitable controlled-potential equipment proved to be a practical barrier, even in laboratory studies, until 1942, when the potentiostat—an instrument capable of automatically controlling the electrode potential—was introduced.Without question, this instrument has facilitated electro-organic syntheses, mechanistic studies, and specific electrooxidation and electroreduction processes. More modern and electronically... 

Indirect electrochemical methods have been intensively studied, especially from the viewpoint of development of innovative synthetic methods in industrial organic chemistry. The indirect procedure is required when the direct method is unsuitable because (1) the desired reaction does not proceed sufficiently because of an extremely slow reaction or a very low current efficiency (2) the electrolysis lacks product-selectivity and thus offers only a low yield (3) tar and products cover the surface of the electrode, interrupting the electrolysis. Indirect electrochemical techniques involve the recycling of mediators (or electron carriers) in a redox system, as depicted in Fig. 1 [1-24]. 

Whereas in earlier work it was usual to conduct synthetic reactions of this nature under constant current conditions, in recent years greater control and understanding of the systems have been achieved by employing potentiostatic techniques in which the potential of the working electrode (anode) is regulated precisely with respect to a third, reference electrode (e.g., Ag/Ag+ or saturated calomel, SCE). 

An interesting extension of the cpe technique is pulse electrolysis. The electrode is maintained not at one single potential, but at a series of potentials of controlled duration according to a predetermined program. Tills is done by means of a pulse generator (also commercially available). Pulse techniques have hitherto been used mainly for mechanistic studies 91,92-1 but hold great promise for synthetic applications too 90,2 65 As an example, in the anodic oxidation of aliphatic hydrocarbons in non-aqueous medium at a platinum anode, the electrode activity falls rapidly with time if the potential is kept constant, probably because of the formation of an adsorbed film of intermediates or products. However, regular, short cathodic pulses reactivate the anode and the reaction proceeds without difficulties 30 ... 

Electrochemical techniques are a convenient means of studying one-electron oxidations of amines. The reaction pattern of the anodic oxidation of amines depends greatly on the reaction conditions, including the nature of the electrode and the nucleophilicity of the solvent [1-3]. A major drawback of electrode oxidations is that unwanted secondary electron-transfer reactions can occur at the electrode surface. Also in electrochemical processes the effective reaction volume is limited at the electrode surface, thereby creating a high local concentration of reactive intermediates which can lead to dimerization and disproportionation reactions. These factors have to some extent, limited the synthetic utility of the anodic oxidation of amines. Because of this the anodic oxidation of amines has been intensively studied, although mainly from a mechanistic standpoint. 

Electrochemistry integrates analytical technique (determination of concentrations, reaction mechanisms, or properties9) and synthetic methods such as electrolysis.10 Electrons needed for redox reactions are provided by an electric current supplied through electrodes in a highly controlled and selective manner. Products can be isolated easier. It is well known that electrochemical redox reactions may result in reactive intermediates under mild conditions.11 Electrochemistry is a clean and convenient methodology even on the preparative scale. 

In this chapter the synthetic aspects of the earlier mentioned [M(bipy)2 (PVPjnCl]" polymers (where M = Os,Ru) are discussed. The main part of the chapter is devoted to the effect of electrolyte and polymer loading on the electrochemistry observed at electrodes modified with these materials. Interaction between the polymer layer and the electrolyte is investigated using electrochemical techniques such as cyclic voltammetry, potential step methods, and the electrochemical quartz crystal microbalance. Attention is also paid to mediation reactions using such modified electrodes. Finally, the implications of these observations for analytical applications of these materials are discussed. 

Certainly polarographic and cyclic voltammetric techniques " have been used effectively to elucidate the mechanisms of the electroreduction of both aromatic and aliphatic carbonyl compounds (and indeed a wide variety of electrode processes) but in synthetic studies the very different conditions, particularly with respect to the concentrations of reactants and products, can influence both the reaction kinetics and reaction pathway. 

The purpose of the present article is to provide, with the aid of selected examples, a brief survey of the electrochemical methods most commonly used in characterising the properties of polymer-modified electrodes and to illustrate the different types of behaviour which may be encountered when employing such systems to catalyse or mediate the electrochemical reactions of biomolecules. A comprehensive review on chemically-modified electrodes, which includes details of synthetic methods and appropriate electrochemical and auxiliary techniques for studying their properties, is available ... 

---