Organic compounds, electrode reduction

Many organic compounds undergo reduction or oxidation at a DME. Consequently, polarographic techniques have been used extensively for determinations of organic compounds and for studying the mechanisms of their electrode reactions. In aqueous solution, the reduction of organic compounds is frequently a 2e process accompanied by protonation as in Equation 3.32 ... 

Interesting reactions occur when the charge transfer at the electrode is associated with homogeneous reactions in solution that can precede or follow the electron transfer reaction at the electrode. A selection of possible schemes is shown in Table 6.1. Note the presence of many organic compounds the reduction or oxidation of these compounds involves, in many cases, the addition or removal of hydrogen, which... 

Controlled-potential coulometry also can be applied to the quantitative analysis of organic compounds, although the number of applications is significantly less than that for inorganic analytes. One example is the six-electron reduction of a nitro group, -NO2, to a primary amine, -NH2, at a mercury electrode. Solutions of picric acid, for instance, can be analyzed by reducing to triaminophenol. 

The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

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. 

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. 

Haapakka and Kankare have studied this phenomenon and used it to determine various analytes that are active at the electrode surface [44-46], Some metal ions have been shown to catalyze ECL at oxide-covered aluminum electrodes during the reduction of hydrogen peroxide in particular. These include mercu-ry(I), mercury(II), copper(II), silver , and thallium , the latter determined to a detection limit of <10 10 M. The emission is enhanced by organic compounds that are themselves fluorescent or that form fluorescent chelates with the aluminum ion. Both salicylic acid and micelle solubilized polyaromatic hydrocarbons have been determined in this way to a limit of detection in the order of 10 8M. 

ET to disulfides leads, as a rule, to the cleavage of the S—S bond by a stepwise mechanism. For the purposes of this review, we will focus only on the data obtained at inert electrodes, particularly glassy carbon electrodes. The reduction of the disulfide bond, in both simple organic compounds and... 

Kuwana T, Prench WG (1964) Electrooxidation or reduction of organic compounds into aqueous solutions using carbon paste electrode. Anal Chem 36 241-242. 

As described in Section 4.1.2, electrophilic organic compounds are reducible at the electrode. Some reducible organic compounds are listed in Table 8.5 with the potentials of the first reduction step in dipolar aprotic solvents. As described in Ref. [47], organic compotinds undergo various complicated electrode reductions. Here, however, only simple but typical cases are considered they are reductions of the outer sphere type and the dissociative electron transfer reactions. 

Tab. 8.6 Standard rate constants for electrode reductions of organic compounds determined by...

Relation between the LUMO and the half-wave potential of the first reduction wave When an organic compound, Q, is reduced, it accepts an electron from the electrode to its lowest unoccupied molecular orbital (LUMO). Here, the energy of the LUMO of Q corresponds to its electron affinity (EA). If the energies of LUMO ( lu) for a series of analogous compounds are obtained by the molecular orbital method, there should be a relationship ... 

Bransted acids (proton donors, HA) have significant influences on the reduction of organic compounds in aprotic solvents. If a weak Bronsted acid like water is added step-wise to the electrolytic solution, the height of the first polarographic wave increases at the expense of that of the second wave (Fig. 8.12). By the addition of a weak acid, the following reactions occur at or near the electrode ... 

The reductions of halogenated organic compounds (RX) involve the cleavage of carbon-halogen bonds [62]. Depending on the solvent, supporting electrolyte, electrode material and potential, it is possible to electrogenerate either alkyl radicals (R ) or carbanions (R ), which then can lead to the fonnation of dimers (R-R), alkanes (RH) and olefins [R(-H)] ... 

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... 

In nonaqueous solutions, the usable potential range is large, on the order of 5 V. This potential range allows a wide variety of oxidations and reductions to be examined. Some typical values on different types of film electrodes are reported in Table 11.2 [75]. The electrolysis of several organic compounds in these solvents has been characterized. A general observation is that the tin oxide electrode appears more suited to studies of reduction processes than of oxida-... 

Although the one-electron reduction of nitrobenzene to its radical anion in dipolar aprotic solvents is a classical example of a chemically reversible redox couple, the reductions of many organic compounds are chemically irreversible. The redox behavior of /7-chlorobenzonitrile is typical of those systems in which the initial electrode product undergoes rapid, irreversible chemical reaction to give another reducible species. 

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