Reversible redox systems

Formation of the ethano-dimer of a-tocopherol (12) by reduction of spiro dimer (9) proceeds readily almost independently of the reductant used. This reduction step can also be performed by tocopheroxyl radicals as occurring upon treatment of tocopherol with high concentrations of radical initiators (see Fig. 6.10). The ready reduction can be explained by the energy gain upon rearomatization of the cyclohexadienone system. Since the reverse process, oxidation from 12 to 9 by various oxidants, proceeds also quantitatively, spiro dimer 9 and ethano-dimer 12 can be regarded as a reversible redox system (Fig. 6.22). 

Irreversibility versus reversibility inpolarography. Previously in this chapter we dealt only with reversible redox systems, i.e., with truly Nemstian behaviour and merely diffusion control. This also applies to combined processess of electron transfer and chemical reaction (e.g., complexation) provided that both take place instantly. For instance, in EC such as... 

This method K4 yields a reduction or cathodic additional current owing to the pulse increase with a current maximum around E analogously, K3 yields an oxidation or anodic additional current owing to the pulse decrease with a current minimum around E. Both additional currents are depicted in Fig. 3.52 for a completely reversible redox system the cathodic maximum and the anodic... 

Many handbooks like the CRC Handbook of Chemistry and Physics provide, on behalf of electrochemistry investigation, values of standard reduction potentials, listed either in alphabetical order and/or in potential order. These must be considered as potentials of completely reversible redox systems. In current analytical practice one is interested in half-wave potentials of voltammetric, mostly polarographic analysis in various specific media, also in the case of irreversible systems. Apart from data such as those recently provided by Rach and Seiler (Spurenanalyse mit Polarographischen und Voltammetrischen Methoden, Hiithig, Heidelberg, 1984), these half-wave potentials are given in the following table (Application Note N-l, EG G Princeton Applied Research, Princeton, NJ, 1980). 

The quinone-hydroquinone system represents a classic example of a fast, reversible redox system. This type of reversible redox reaction is characteristic of many inorganic systems, such as the interchange between oxidation states in transition metal ions, but it is relatively uncommon in organic chemistry. The reduction of benzoquinone to hydroquinone... 

Electrochromism is observed in reversible redox systems, which exhibit significant color changes in different oxidation states. Violenes, whose general structure is represented in Figure 1, are typical examples that exhibit electrochromism (1). 

Two Step Reversible Redox Systems of the Weitz Type... 

With 47 such a reversible redox system has been realized. From electrochemical data again a two step electron transfer has to be derived although the concentration of 7sem is estimated to be as small as 10 %. 

Hi n,M. B. Determination of Molecular Weights by Light Scattering. 77, 141-232(1978). Hiinig, S. H., Berneth, H. Two Step Reversible Redox Systems of the Weitz Type. 92,1-44... 

Cyclic voltammetry has perhaps become the most popular electroanalytical, electrochemical technique [23, 27], and many reports have appeared in which E° values were determined in this way. However, reliable formal potentials can be determined only for electrochemically reversible systems [28]. For any reversible redox system - provided that the electrode applied is perfectly inert, that is, there are... 

Deprotonation of 113 generates 114 which is the medium oxidation level of a four-step, reversible redox system enabling the electrochemical transformation of [4]radialene 116 cyclobutadiene (118) via intermediates 115 and 117, if R = CO Et (83LA658) (Scheme 20). 

Investigations of the redox behaviour of cyclotetrasilenyl rings have revealed a reversible redox system for the cation, radical and anion (Scheme 10.11). ... 

In addition, electrode reactions are frequently characterized by an irreversible, i.e., slow, electron transfer. Therefore, overpotentials have to be applied in preparative-scale electrolyses to a smaller or larger extent. This means not only a higher energy consumption but also a loss in selectivity as other functions within the molecule can already be attacked. In the case of indirect electrolyses, no overpotentials are encountered as long as reversible redox systems are used as mediators. It is very exciting that not only overpotentials can be eliminated but frequently redox catalysts can be applied with potentials which are 600 mV or in some cases even up to 1 Volt lower than the electrode potentials of the substrates. These so-called redox reactions opposite to the standard potential gradient can take place in two different ways. In the first place, a thermodynamically unfavorable electron-transfer equilibrium (Eq. (3)) may be followed by a fast and irreversible step (Eq. (4)) which will shift the electron-transfer equilibrium to the product side. In this case the reaction rate (Eq. (5)) is not only controlled by the equilibrium constant K, i.e., by the standard potential difference be-... 

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