Electrode Reactions of Organic Compounds

At the end of this section, it would be worthwhile pointing out another example of the electric double layer affecting the kinetics of electrode reactions, namely, its effect on the rate of reactions of physically adsorbed organic substances. Of course, common effects are possibly involved here too, which were long ago experimentally demonstrated.In addition, since such organic molecules are polar, their adsorption changes the potential distribution, thereby influencing the ( i effect. For simplicity, in what follows, we shall not take the i i effects into consideration. 

Just as in the case of effects, the potential dependence of the adsorption 

When studying reactions of organic compounds, it is often possible to directly determine, using an appropriate method, the concentration of the adsorbed substance. In this case, one can obtain polarization curves at a constant concentration on the surface rather than in the bulk. 

The properties of large organic molecules are usually quite similar both in the oxidized and reduced forms therefore, their adsorption energy is affected almost similarly by the electric field, i.e., a, — Uf = a, whereas the maximum adsorption potentials for particles of dissimilar charges (e.g., in the case of a neutral molecule and an anion, the product of its reduction) may differ markedly. That is why Eq. (112) can be approximately written as 

the effect of expulsion of adsorbed substances by the electric field may manifest itself through a variation in the effective value of a. Estimates suggest that, for appropriate values of the parameters, this variation may amount to 10%, i.e., it lies at the limits of accuracy of experiments commonly conducted with organic compounds.  

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The role of the pH of the medium in the electrode reactions of organic compounds in aqueous solutions is well understood and has been recently reviewed in detail (Zuman, 1969). In particular, our understanding of this parameter is due to the large number of polarographic investigations where it has been found that the half-wave potential, the limiting current and the shape of the wave for an oxidation or reduction process may all be dependent on the acidity of the medium. 

The electrode reactions of organic compounds containing two atoms of P per molecule were investigated in N, N-dimethylformamide containing 0.1 M Et4NC104 by simultaneous electrochemical reduction and observation of the ESR signal23. The one-electron reduction is reversible and forms an anion radical further reduction of the anion... 

Fleischmann, M., Petrov, I.N. and Wynne-Jones, W.F.K. (1963) The investigation of the kinetics of the electrode reactions of organic compounds by potentiostatic methods. Proceedings of the 1st Australian Conference on Electrochemistry, p. 500. 

Fleischmann, M., Gara, W.B. and HiUs, G.J. (1975) Electrode reactions of organic compounds as a function of pressnre. Journal of Electroanalytical Chemistry, 60, 313. 

Fleischmann, M. and Fletcher, D. (1971) The electrode reactions of organic compounds, in Reactions of Molecules on Electrodes (ed. N. Hush), John WUey, p. 347. 

In general, the reactions of organic compounds at a voltammetric electrode are slower and more complex than those for inorganic species. Consequently, theoretical intei-pretation of the data is often more difficult or impossible. Generally, a much stricter adherence to detail is required for quantitative work. Despite these handicaps, organic polarography has proved fruitful for the determination of structure, the quantitative analysis of mixtures, and occasionally the qualitative identification of compounds. 

The diversity of radical processes during ECFs indicates that the latter do not differ much from reactions of organic compounds with metal fluorides of variable valence. However, this is not always so. Therefore, the nature of the electrodes used in ECFs was treated in detail (OOJAP(K) 160382, 00JAP(K)204492). 

Komori, T. and Nonaka, T. (1983) Stereochemical studies of the electrolytic reactions of organic compounds. Part 22. Electroorganic reactions on organic electrodes. 3. Eleetrochemieal asymmetric oxidation of phenylr -clohexylsulfide on poly-L-Valine coated Platinum. J. Amer. Chem. Soc. 105, 5690-5691. 

ACs were also modified by treatment by 2-nitro-l-naphthol to manufacture composite supercapacitor electrodes, in which EDL capacitance and pseudocapacitance of the following redox reactions of organic compounds is used o-aminonaphthol o-naphthaquinoneimine (Leitner et al., 2004) ... 

Due to the pronounced lack of reversibility of most electrode reactions of organic species, the pH effect is not always predictable, although one should always expect a cathodic (toward more negative values) shift of Ei/2 values upon increase of pH. The degree of reversibility of many electrode reactions is also affected by pH. In cases of total irreversibility (e.g., reduction of organic halides) E1/2 values are more or less pH independent. The effect of pH on E1/2 of three pairs of closely related compounds is shown in Figure 1. 

The application of ILs as solvent systems for the electrochemical study and electrosynthesis of organic molecules is discussed in Chap. 15, highlighting some particular examples, with the aim of demonstrating any similarities and differences observed in ILs respect to conventional aqueous or organic solvent systems. Chapter 14 also discusses some electrochemical reactions of organic compounds in ILs. Meanwhile, the electrode kinetics of organometallic complexes of some transition metal ions with tris(2,2 -bipyridine) have been presented and discussed in detail in Chap. 16. 

Finally in electrode reactions of organometallic compounds the similarities between inorganic ligands and organic ligands will be emphasized. 

Bagotzky, V. S. and Vasilyev, Yu. B. 1964. Some characteristics of oxidation reactions of organic compounds of platinum electrodes. M trochmL cta, 9, 869—882. 

Fig. 9.1 Dr. Alexander Borisovich Ershler (1935-1989) with his group. From right Dr. chem. A.B. Ershler, Ph.D. phys. Eduard M. Podgaetskii, Engr. Tatyana S. Orekhova, Ph.D. chem. Ida M. Levinson another member of the group, postgraduate student Vladimir Kurmaz, was drafted at this time into the Soviet army. Areas of research adsorption theory of neutral organic compounds and its influence on kinetics of electrode reactions, electrode reactions of organomercury compounds, development of new electrochemical methods (high-speed pulse chronopotentiometry, electroreflection, and optical transitions at the metal-electrolyte interface, etc.). ELAN, 1975...

The aspects and applications concerning the redox reactions of organic compounds at BDD electrodes were reviewed recently,and also by our group. Further, reviews on general electrochemical properties and surface... 

As in chemical systems, however, the requirement that the reaction is thermodynamically favourable is not sufficient to ensure that it occurs at an appreciable rate. In consequence, since the electrode reactions of most organic compounds are irreversible, i.e. slow at the reversible potential, it is necessary to supply an overpotential, >] = E — E, in order to make the reaction proceed at a conveniently high rate. Thus, secondly, the potential of the working electrode determines the kinetics of the electron transfer process. 

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. 

Polarographic studies of organic compounds are very complicated. Many of the compounds behave as surfactants, most of them exhibit multiple-electron charge transfer, and very few are soluble in water. The measurement of the capacitance of the double layer, the cell resistance, and the impedance at the electrode/solution interface presents many difficulties. To examine the versatility of the FR polarographic technique, a few simple water-soluble compounds have been chosen for the study. The results obtained are somewhat exciting because the FR polarographic studies not only help in the elucidation of the mechanism of the reaction in different stages but also enable the determination of kinetic parameters for each step of reduction. 

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... 

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