The Quinone-Hydroquinone Redox System

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 

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Conjugated conducting polymers have been found in some cases to present by themselves an intrinsic electrocatalytic activity. Among the few examples, polypyrrole has been shown to be an interesting catalyst for the quinone-hydroquinone redox system [34,35] or for the electro-reduction of dioxygen [36]. 

The PBPA electrodes possess an ion-sensitive surface, and the conductivity of the film depends on the degree of protonation or deprotonation as a function of pH.4 This electrochemical mechanism of PBPA films when interacting with protons of the solution is analogous to the quinone/hydroquinone redox system. ... 

In Section 2 we showed that the properties of amorphous carbon vary over a wide range. Graphite-like thin films are similar to thoroughly studied carbonaceous materials (glassy carbon and alike) in their electrode behavior. Redox reactions proceed in a quasi-reversible mode on these films [75], On the contrary, no oxidation or reduction current peaks were observed on diamondlike carbon electrodes in Ce3+/ 41, Fe(CN)63 4. and quinone/hydroquinone redox systems the measured current did not exceed the background current (see below, Section 6.5). We conventionally took the rather wide-gap DLC as a model material of the intercrystallite boundaries in the polycrystalline diamond. Note that the intercrystallite boundaries cannot consist of the conducting graphite-like carbon because undoped polycrystalline diamond films possess excellent dielectric characteristics. 

On the contrary, no oxidation or reduction current peaks were observed on the sp3-carbon-comprising wide-gap DLC (Eg 1.7 eV) electrodes in Ce3+/4+, Fe(CN)63 /4. and quinone/hydroquinone redox systems, as already mentioned in Section 6.3. Thus, we conclude that DLC is electrochemically inactive in itself. It gains electrochemical activity upon introducing a significant (ca. 10 %) admixture of platinum to the film bulk. Figure 34 shows the dependence of the Fe(CN)63 reduc-... 

Two cases are distinguishable according to Fig. 3. The conventional one (a) holds for all molecular systems, which will be treated in Section 2, e.g., quinones or disulfides. The redox partner reacts in the dissolved state this means at low concentrations for electrodes of the second kind. The reactant is positioned in the outer Helmholtz plane, or even in the inner Helmholtz plane, as shown for adsorbed molecules. One example is the quinone/hydroquinone redox reaction according to Eq. (13) ... 

Prominent examples are the redox pairs o- or p-quinones/hydroquinones, the corresponding quinoneimines, the diimines and the azobenzenes and disulfides [68]. V. Stackelberg [69] has pointed out that the exclusive formation or cleavage of 0-H, N-H, S-H, or S-S bonds is a necessary precondition for reversible organic redox partners. This can be clearly recognized in the case of the quinone/hydroquinone redox reaction (cf. Eq. (13)). Only O -H bonds are formed or cleaved. In contrast, in the case of the acetone/isopropanol redox system, 0-H and C-H bonds participate. 

Fig. 4.4. Dependence of the faradaic resistance measured at the equilibrium redox potential for polycrystaUine diamond electrodes on their resistivity for (1) Fe(CN)63 / and (2) quinone/hydroquinone redox systems [13].

Apart from these systems, which involve a metal on which is adsorbed a modifier (see above), there is another kind of experiment, although one from which data are as yet less available. One can make up a surface of a metal covered with a biolipid membrane (90% lipid and 10% proteins). There is evidence that some of the proteins in these ensembles are themselves the origin of electrons that can exchange with small redox molecules (e.g., the quinone-hydroquinone system) in solution. Such evidence (though scarce) is significant, for there are no metal underlayers in real biological systems, yet interfacial electron transfer seems to be common there. 

Redox systems other than the H2/H+ couple can be used to monitor the potential of the parent metal particles. For example, the quinone-hydroquinone system can be used to keep the electrochemical potential between 0.5 and 0.0 V/NHE by varying the pH from 0 to 7 for solutions of equal concentrations of quinone and of hydroquinonc [57], UPD clearly opens up a vast... 

Other redox systems of importance in biochemistry include the NADH/NAD system, the flavins, the pyruvate/lactate system, the oxalacetate/malate system, and the quinone/hydroquinone system. 

The quinone-hydroquinone system can be involved in redox mechanisms. So, in addition to acid-base catalysis, carbon materials can promote oxidations, such as the oxidative dehydrogenation of hydrocarbons, which we discuss in detail below. 

One of the most versatile electron-accepting molecules is the quinonoid compound, and the redox reaction of the quinone-hydroquinone couple is one of the most thoroughly studied proton-coupled electron transfer systems of organic molecules. Quinones show the reversible two-step le reduction in aprotic organic solvents (Fig. 2). One-electron addition to quinone forms the semiquinone radical with five n electrons. The stability of the semiquinone form is affected by the existence of a minute amount of proton, which appears as the large shift of the reduction potentials in the positive direction. This implies that quinonoid compoimds are representative acceptor molecules of which redox properties are influenced by external perturbation, such as protonation and solvation (Fig. 2). They are employed in covalently and noncovalently linked donor-acceptor systems of particular interest in the study of proton-coupled electron transfer and photoin-duced electron transfer. ... 

Some of the recently reported quinone-based redox polymers (Manecke, 1974) have the structural units represented in Fig. 12-1. In these polymeric quinone s, an attempt has been made to increase the hydrogen acceptor property of the quinone group by introducing electron-withdrawing substituents. Such substituents are known to increase the redox potential of the quinone-hydroquinone system (cf. 2,3-dichloro-... 

VI. According to the relationship shov/n in Fig. 3, the potential of the catalyst can be adjusted not only with hydrogen but also by means of redox systems. The electrode potentials of a number of organic redox systems have been determined (refs.14,16). The use of different substituted quinone-hydroquinone type systems is especially favoured. Unfortunately, however, by application of these systems the surface of the catalyst becomes contaminated, which renders EP investigations impossible. 

Oxidation-reduction (redox) Inert metal (normally Pt but certain other metals can act in a similar manner) in a solution containing two species that give rise to a redox system. E depends on of the system and the relative activities of the oxidised and reduced forms. Quinone-hydroquinone QH4O2 -1- 2H+ -1- 2e-CjH4(OH)2, which is thus pH dependent Fe - -/Fe + Mn04-/Mn +... 

The redox system consists of pyrene or 9,10-phenanthrene quinone as oxidant and an alkyl ester of 3,3, 3"-nitrilopropionic acid as reductant.121 This system deactivates oxidation by bisimidazole when irradiated at 380-550nm, since the quinone is reduced to hydroquinone and thus stabilizing the previously generated dye image.122,123... 

A similar mechanism could operate in the reduction of oxygen on chelate catalysts, as in the organic cathodes with air regeneration described by Alt, Binder, Kohling and Sandstede 13-40>. These cathodes contain a reversible insoluble quinone/hydroquinone system. The quinone, which is electrochemically reducible, can be obtained either by electrochemical oxidation or by purely chemical oxidation with H2O2 or oxygen (air). A cathodic current is observed in these systems only at potentials below the redox potential, and unusually hard current/ voltage characteristic curves are obtained. 

Note that the anodic peak due to the oxidation of leucoadrenochrome to adrenochrome near 0 V is not seen until the second positive-going potential sweep is made. The voltage separation between the anodic and cathodic peaks for the oxidation of adrenaline (peak B, Fig. 21.4, bottom) and the reduction of adrenalinequinone (peak C) is large when compared to most chemically reversible redox couples. However, this behavior is typical of many quinone-hydroquinone systems on a carbon paste surface at intermediate values of pH. 

In mitochondria there are two types of mechanisms for coupling the electron transport to the movement of protons across the membrane. The first is based on anisotropic reduction and oxidation of a lipid-soluble quinone inside the membrane. The quinone, coenzyme Q, becomes protonated upon reduction and diffuses to an oxidation site on the other side of the membrane where removal of electrons leads to proton release. This is essentially a proton carrier system with the hydroquinone acting as the proton carrier in the lipid phase of the membrane. A further refinement of this system in mitochondria provides for a coenzyme Q redox cycle where the movement of one electron through the chain allows for two protons to cross the... 

In this section, unlike the previous one, we deal with less heavily doped, semiconductor diamond. Quantitative studies of reaction kinetics have been performed in Fe(CN)63 -/4, quinone/hydroquinone (recall that this is an inner-sphere reaction), and Ce3+/4+ systems [94, 104, 110]. Potentiodynamic curves recorded in solutions containing only one (either reduced or oxidized) component of a redox system are shown on Figs. 22a and b the dependences of anodic and cathodic current peak po-... 

Fig. 24. Energy diagram of the boron-doped diamond/aqueous redox electrolyte solution interface (a) at the flat-band potential (b) at the equilibrium potential of Fe(CN)63, 4 system. Ec is the energy of conduction band bottom, Ev is the energy of valence band top, F is the Fermi level, Eft, is the flat-band potential. Shown are the electrochemical potential levels of the Fe(CN)63, 4 and quinone/hydroquinone (Q/H2Q) systems in solution. The electrode potential axis E is related to the standard hydrogen electrode (SHE). Reprinted from [110]. Copyright (1997), with permission from Elsevier Science.

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