Hydroquinone, redox chemistry

Brunmark and Cadenas (27A15) reviewed the major mechanisms that are involved in quinone-induced cytotoxicity in 1989. The redox chemistry of quinoid compounds was surveyed in terms of (1) reactions involving only electron transfers, such as those accomplished during the enzymatic reduction of quinones and nonenzymatic interaction with redox couples generating semiquinones, and (2) nucleophilic addition reactions. In their explanation of the mechanisms involved, quinone is reduced to the hydroquinone or semiqui-none radical by cellular reductase. The semiquinone radical then undergoes rapid autooxidation with the generation of the parent quinone and concomitant formation of superoxide. The hydroquinone reacts rapidly with superoxide to form H2O2 and the semiquinone. 

In an analogous fashion to catechols and hydroquinones, ortho- and para-alkylphenols undergo two-electron tt oxidation to form quinone-methides. Quinone-methides possess a significantly reduced propensity for redox chemistry than corresponding quinones and are therefore much more reactive... 

The close electrochemical relationship of the simple quinones, (2) and (3), with hydroquinone (1,4-benzenediol) (4) and catechol (1,2-benzenediol) (5), respectively, has proven useful in ways extending beyond their offering an attractive synthetic route. Photographic developers and dye syntheses often involve (4) or its derivatives (10). Biochemists have found much interest in the interaction of mercaptans and amino acids with various compounds related to (3). The reversible redox couple formed in many such examples and the frequendy observed quinonoid chemistry make it difficult to avoid a discussion of the aromatic reduction products of quinones (see Hydroquinone, resorcinol, and catechol). 

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... 

Based upon analogies between surface and molecular coordination chemistry outlined in Table 1, we have recently set forth to investigate the interaction of surface-active and reversibly electroactive moieties with the noble-metal electrocatalysts Ru, Rh, Pd, Ir, Pt and Au. Our interest in this class of compounds is based on the fact that chemisorption-induced changes in their redox properties yield important information concerning the coordination/organometallic chemistry of the electrode surface. For example, alteration of the reversible redox potential brought about by the chemisorption process is a measure of the surface-complex formation constant of the oxidized state relative to the reduced form such behavior is expected to be dependent upon the electrode material. In this paper, we describe results obtained when iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol (DHT), and 3,6-dihydroxypyridazine (DHPz), all reversibly electroactive... 

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