Study of Electro-Organic Reactions

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. 

For the investigator who wants to study electrode processes at depth, a number of more physically oriented methods are available, such as double layer capacitance measurements19 rotating disc and ring disc techniques 25 and radio-. active tracer methods 40a Spectroscopical methods in conjunction with optically transparent electrodes can be used for the study of intermediates 40b), as can also total reflectance spectroscopy 40c). 

ESR spectroscopy has found wide-spread use for the detection of radical intermediates in electrode processes 40 For the same purpose, the newly developed technique of trapping short-lived radicals by nitrones or nitroso compounds 40d- should be of considerable interest, as should also the chemically induced nuclear spin polarization (CINP) phenomenon 40e-1 be. 

---

Polarographic Methods and Related Techniques for the Study of Electro-Organic Reactions... 

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. 

Polarography offers some possibilities for the study of reaction kinetics and mechanisms of homogeneous organic reactions. The main advantages are a rather simple and easily accessible experimental technique, the possibility to work in dilute solutions and limited requirements on the amount of substances studied. The main limitation is that some of the components of the reaction mixture must be polarographically active. But this limitation is not so restrictive as it would appear, because most substances that can be studied spectro photometrically are electro-active as well. For rapid reactions polarography seems to be most useful for a range of second-order rate constants between about 10 -10 sec M, whereas for faster reaetions the specific properties of the electrode, in particular its electrical field and adsorption, can play a role. A certain limitation is that for most systems the equilibrium constant has to be known from independent measurements. 

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... 

In this chapter, emphasis will be placed on same basic electrochemical mechanistic aspects of studies in electro-organic chemistry rather than the myriad of specific, essentially organic reaction mechanisms that involve eventual product formation from the initial, electrochemically generated intermediate. We shall, however, refer in a later section of this chapter to a selection of reactions that are mechanistically better understood and may lead, or have led, to commercially viable processes (Section 13). In other chapters in this series of volumes " will be found accounts of electroorganic reactions which are presented more specifically from the point of view of the organic chemistry involved, e.g., in follow-up reactions after electron transfer. 

In a later section (Section 8), some of the fundamental aspects of polarography will be briefly considered with particular reference to the question of reversibility of various electro-organic reactions studied by means of polarography. Finally, an attempt will be made to discuss those techniques complementary to the basic polarographic method, but essential for any deductions regarding the nature of intermediates and products in an organic electrode reaction, and therefore regarding its overall reaction mechanism. 

Apart from analytical applications, polarography is useful in electro-organic reaction studies as a means for establishing ranges of potential over which one or more reduction (or oxidation) steps occur and thus as a basis for choice of conditions for a potentiostatic preparation. Of course, information obtained at the Hg surface normally used in (cathodic) polarography cannot always be transferred to behavior at other metals where different reaction pathways may arise due to adsorption effects and catalysis. 

On the other hand, the work of Atobe and co-workers was probably the first modem example investigating electropolymerization under sonication in a complete series of papers at low frequencies. Starting from electro-organic reactions under ultrasonic fields [12], polymerization of aniline was studied both in electrochemical [13] and chemical route [14, 15] as well as synthesis of nanoparticle synthesis [16, 17]. 

Electroanalytical chemists and others are concerned not only with the application of new and classical techniques to analytical problems, but also with the fundamental theoretical principles upon which these techniques are based. Electroanalytical techniques are proving useful in such diverse fields as electro-organic synthesis, fuel cell studies, and radical ion formation, as well as with such problems as the kinetics and mechanisms of electrode reactions, and the effects of electrode surface phenomena, adsorption, and the electrical double layer on electrode reactions. 

Electro-organic chemistry is the study of the oxidation and reduction of organic molecules and ions, dissolved in a suitable solvent, at an anode and cathode respectively in an electrolysis cell, and the subsequent reactions of the species so formed. The first experiment of this type was reported in 1849 by Kolbe, who described the electrolysis of an aqueous solution of a carboxylate salt and the isolation of a hydrocarbon. The initial step involves an anodic oxidation of the carboxylate anion to a radical which then dimerises to the alkane. 

Fleischmann et al. [549] studied the electro-oxidation of a series of amines and alcohols at Cu, Co, and Ag anodes in conjunction with the previously described work for Ni anodes in base. In cyclic voltammetry experiments, conducted at low to moderate sweep rates, organic oxidation waves were observed superimposed on the peaks associated with the surface transitions, Ni(II) - Ni(III), Co(II) -> Co(III), Ag(I) - Ag(II), and Cu(II) - Cu(III). These observations are in accord with an electrogenerated higher oxide species chemically oxidizing the organic compound in a manner similar to eqns. (112) (114). For alcohol oxidation, the rate constants decreased in the order kCn > km > kAg > kCo. Fleischmann et al. [549] observed that the rate of anodic oxidations increases across the first row of the transition metals series. These authors observed that the products of their electrolysis experiments were essentially identical to those obtained in heterogeneous reactions with the corresponding bulk oxides. 

A major focus of more recent studies on adsorption at metal electrodes has been the investigation of the mechanism of electro-oxidation of organic fuels (methanol, formic acid, formaldehyde, etc. [55, 56]) and the electro-reduction of carbon dioxide. The former type of reaction is important in the context of the development of fuel cells a major problem has been the poisoning of the anode by carbon fragments and mechanistic insights are urgently needed. In the latter case, the development of C02 sensors has a high priority. 

To April 1987, the in-situ IR studies on the electro-oxidation of small organic molecules show that the strongly adsorbed fragments which act as poisons in these reactions are all CO species. There has been no spectroscopic evidence for the presence of COH, even for conditions of less than saturation coverage by (CO)ads. In addition, the adsorbed CO is very stable, requiring fairly high potentials for its oxidation to C02 [55]. The reader is referred to the reviews by Bewick and Pons [55] and Foley et al. [56, 69] and the references cited therein for a more detailed treatment. 

---