Hydroquinone-quinone redox process

Scheme 7-42 Control of cycloaromatization through a hydroquinone quinone redox process (Nico-laou et al.).

The high adsorption capacity of Ag+ ions by all the activated carbons was attributed to the reduction of Ag+ ions to metallic silver by the hydroquinone groups present on the carbon surface, which in turn are oxidized to quinone groups. This redox process is supported by the standard reduction potentials of Ag+ (Ag+ + e Ag, E = 0.7996 V) and quinhydrone electrode, = 0.6995 V. The increase in adsorption of Ag+ ions by the ammonia-treated sample was attributed to the formation of silver amino complexes which are quite stable under the conditions used in these studies. 

To explain the observations, the reader s attention is to be switched to the reversible redox processes known for the pair of hydroquinone and quinone (denoted hereafter shortly QH and Q), which can be described as follows [18] ... 

Accompanying the oxidation of carbon support, the double layer capacitance of the catalyst layer increases gradually, and a new redox process may appear at ca. 0.6 V from quinone/hydroquinone couple. 

Flavins are widely recognized by their ability of participate in both one- and two-electron transfer processes, since these compounds can exist in three different redox states oxidized (quinone), one-electron reduced (semiquinone) and two-electron reduced (hydroquinone). The redox potential for the complete reduction of oxidized flavins is about -200 mV, but this value may largely vary in flavoproteins, as a consequence of the protein activity site environment, ranging from —400 mV to +60 mV (Fraaije and Mattevi 2000). Flavins may transfer single electrons, hydrogen atoms and hydride ions. In addition, N5 and C4a of the oxidized flavin molecule are susceptible sites for... 

The quinone-based processes utilize the redox cycle illustrated in Figure 9-8 to convert hydrogen sulfide to elemental sulfur. In these processes hydrogen sulfide is absorbed into an aqueous solution containing a quinone in the oxidized state. The absorbed hydrogen sulfide is then oxidized to elemental sulfur by the quinone, which is reduced to hydroquinone in the reaction. The hydroquinone is reoxidized to quinone by contact with air in a separate step to complete the cycle. 

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... 

Redox behavior of anthraquinone is shown in Scheme 4. The quinone moiety may be reduced to the hydroquinone form and converted to a leuco salt under alkali conditions. In general, the leuco salt has a strong affinity for cellulose and is soluble in water. The hydroquinone form is insoluble in water and has low affinity to cellulose. The preferred dyeing procedure depends on the structure and properties of the vat dye. The variables that are used to control the process include, e.g., strength and amount of alkali, reduction temperature, and the presence of salts. During the process of reduction, some side reactions, such as overreduction, hydrolysis,... 

Substances undergoing redox reactions (such as quinone-hydroquinone, sulphide-disulphide, metal complexes, redox couples) may serve as electron carriers and allow the coupling of oxidation-reduction processes across membranes (see, for instance, [6.44-6.46]) to cation or anion transport. 

The oxidation of carbohydrate involves the making and breaking of C-H bonds. This is a two electron process, and the most efficient way of achieving the oxidation will use a two electron oxidant. In many biological systems one of the key redox steps involves the two-electron conversion of a quinone to a hydroquinone (Fig. 10-6). 

Transfer of calcium cations (Ca2 + ) across membranes and against a thermodynamic gradient is important to biological processes, such as muscle contraction, release of neurotransmitters or biological signal transduction and immune response. The active transport can be artificially driven (switched) by photoinduced electron transfer processes (Section 6.4.4) between a photoactivatable molecule and a hydroquinone Ca2 + chelator (405) (Scheme 6.194).1210 In this example, oxidation of hydroquinone generates a quinone to release Ca2+ to the aqueous phase inside the bilayer of a liposome, followed by reduction of the quinone back to hydroquinone to complete the redox loop, which results in cyclic transport of Ca2 +. The electron donor/acceptor moiety is a carotenoid porphyrin naphthoquinone molecular triad (see Special Topic 6.26). 

The above reaction is called a redox copolymerization reaction [224]. The trivalent phosphorus in the monomer is oxidized to the pentavalent state in the process of polymerization and the quinone structure is reduced to hydroquinone. The phosphonium-phenolate zwitterion is the key intermediate ... 

The electrochemical processes involved in the quinone-hydroquinone redox couple have recently been examined by Bagotzky et In acid solutions the rate determining step was found to be ... 

The HRP-catalyzed polymerization of phenols was found to be a convenient way to produce redox polymers and conducting (electronically conducting and ionically conducting) polymers. Besides the interest in electronic conductive polyanilines [121], many efforts have been made to produce ionically conductive phenol polymers for battery applications. A classic effort is the synthesis of poly(hydroquinone) for use as a redox polymer. Typically, poly(quinone)s are prepared via chemical or electrochemical methodologies [122,123]. Both processes produce a large amount of by-products and lead to complex polymer structiues. The first alternative pathway to produce poly(hydroquinone) by peroxidase catalysis was based on a multienzymic... 

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