Quinones redox systems

Cosgrove et al. in 1985 (828a) showed that it was possible to detect hydroxyl radicals in an aqueous extract of the particulate phase of cigarette MSS under metal-mediated conditions. An unidentified alkyl radical and the carbon dioxide anion radical were also observed nnder their experimental conditions. They concluded that the results of their experiment indicated that the major particnlate-phase free radical was a quinone/hydro-quinone redox system in a polymeric matrix and that the radical signal conld become associated with DNA. Unequivocal proof was not provided for these claims. 

In addition to CuCfi, some other compounds such as Cu(OAc)2, Cu(N03)2-FeCl.i, dichromate, HNO3, potassium peroxodisulfate, and Mn02 are used as oxidants of Pd(0). Also heteropoly acid salts comtaining P, Mo, V, Si, and Ge are used with PdS04 as the redox system[2]. Organic oxidants such as benzo-quinone (BQ), hydrogen peroxide and some organic peroxides are used for oxidation. Alkyl nitrites are unique oxidants which are used in some industrial... 

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 +... 

Reactions of partial electrochemical oxidation are of considerable interest in the electrosynthesis of various organic compounds. Thus, at gold electrodes in acidic solutions, olefins can be oxidized to aldehydes, acids, oxides, and other compounds. A good deal of work was invested in the oxidation of aromatic compounds (benzene, anthracene, etc.) to the corresponding quinones. To this end, various mediating redox systems (e.g., the Ce /Ce system) are employed (see Section 13.6). 

Illustrated in Scheme 7.8 are the mechanisms that give rise to the products shown in Scheme 7.7. These mechanisms involve either electrophilic attack or an internal redox reaction. The internal redox reaction shown in Scheme 7.8 involves proton trapping from the solvent or from the hydroquinone hydroxyl group as shown. This process has been documented for the mitomycin system50 and also occurs in many quinone methide systems.25,30,31... 

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... 

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... 

In the reduction or oxidation of quinone/ quinol systems, free radicals also appear as intermediate steps, but these are less reactive than flavin radicals. Vitamin E, another qui-none-type redox system (see p.l04), even functions as a radical scavenger, by delocalizing unpaired electrons so effectively that they can no longer react with other molecules. 

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. 

The redox properties of the quinone-pyrocatecholate system are shown in Scheme 23. If the species NiL2+, NiL and Nil4 are taken into account, they can be assumed to contain nickel(II) ion and coordinated neutral benzoquinone, mononegative semiquinone and dinegative pyrocatecholate, respectively. 

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. 

Another approach to a combined system is the connection of the two systems through a quinone redox couple dissolved in an oil phase, as shown in Fig. 17.2. This system is analogous to the combination of photosystems I and II in the photosynthesis of green plants. Fig. 17.10 illustrates the structure of our model system, in which the oil-phase corresponds to the lipid bilayer membrane of chloroplast. Such a system is structurally identical to a liposome and has the possibility of development for use in a batch reactor. 

The key to the construction of the system is the choice of the quinone redox couple in the oil phase and the oil itself. The quinone compound must be reduced by Fe(II) ions, and the reduced form must be oxidized by bromine. These requirements indicate that the redox potential must be in the range between 0.77 and 1.07 V vs. NHE. After investigating of many redox compounds, we found that 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) dissolved in n-butyronitrile may be a good candidate for the system. DDQ has a largely positive redox potential because of its strong electron withdrawing substituents. 

Another example of a photoredox molecular switch is based on a ferrocene-ruthenium trisbipyridyl conjugate, in which the luminescent form 4 switches to the non-luminescent form 5 upon electrochemical oxidation (Figure 2/bottom)171. Biological systems exploit the interplay of redox and molecular recognition to regulate a wide variety of processes and transformations. In an attempt to mimic such redox systems, Deans et al. have reported a three-component, two-pole molecular switch, in which noncovalent molecular recognition can be controlled electrochemically. x Willner et al. have reported on their research activities in developing novel means to achieve reversible photostimulation of the activities of biomaterials (see Chapter 6).[91 Recently, we have shown that it is possible to switch the luminescence in benzodi-furan quinone 6 electrochemically. 101 The reduction in THF of the quinone moiety... 

Dehydroascorbic Acid and Ascorbic Acid. The oxidized and reduced forms of vitamin C (dehydroascorbic acid and ascorbic acid, respectively) have redox characteristics that are similar to those for o-quinone/catechol systems.13 Al-... 

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. 

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. 25. Dependence of faradaic resistance measured at the equilibrium redox potential on the polycrystalline film resistivity for (1) Fe(CN)63, 4 and (2) quinone/hydroquinone systems. Reprinted from [110], Copyright (1997), with permission from Elsevier Science.

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-... 

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... 

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