Quinhydrone, formation

To a cold aqueous solution of benzoquinone, add 1 drop of sulphurous acid solution (SOj-water) the solution turns deep green-brown owing to the intermediate formation of quinhydrone, CeH402,CeIl4(0H)2. Now add excess of sulphurous acid the solution becomes colourless owing to the formation of hydroquinone. Add a few drops of FeClj solution the reaction is reversed and the deep yellow colour (distinct from that of FeCl ) is restored. 

Hg I Hg2Cl21 KCfsat, formie aeid quinhydrone, 0.05 M, sodium formate 0.25 M in formie aeid Pt... 

At this point the oxidation stage of quinonediimine has been fully reached its (very unstable) salts have scarcely any colour. The production of colour only takes place when quinonoid and benzenoid systems are present together. The molecular union of the two substances at different stages of oxidation produces the intense absorption which is a prerequisite for the formation of a dye (Willstatter and Piccard). This union need not take place in the proportion 1 1, which obtains in the present case. The relations between quinhydrone and quinone-quinol are quite similar (p. 314). 

Solid-solid complexes are usually easily and quantitatively obtained upon grinding or better, milling. This has been shown with the formation of numerous charge-transfer sandwich complexes such as quinhydrones (66) or picrates (67) [22]. Numerous one plus one combinations of essentially planar donors and acceptors provide the homogeneous complexes most easily upon milling. 

Independent support for interflavin o-contacts comes from recent chemical studies by Favaudon and Lhoste (13,14). The french authors describe, as already anticipated, a nearby diffusion-controlled dimer formation in aprotic polar medium as the first step in the interflavin contact between oxidized and reduced states, which would finally yield two flavin radicals. This dimer was shown to be not identical in any respect with the well known quinhydrone which can only be obtained in aqueous systems at high flavin concentrations. The long wave band in the absorption spectrum of the new dimer appears to be of charge transfer type, but with a highly reduced half width and better resolved shape than the flavoquinhydrone spectrum. 

Quinones are reduced to quinols by sulfur dioxide or by zinc dust in acetic acid. The ability of a quinone to act as an electron-acceptor and their reduction products (quinols) to act as electron-donors is reflected in the formation of quinhydrones which are highly coloured 1 1 charge-transfer molecular complexes. 

From the studies of the intracutaneous sensitization of guinea pigs using PPD, hydroquinone, quinhydrone and benzoquinone it has been suggested thatbenzoqui-none formation plays an important role in the allergic action of PPD [15]. 

The formation of -aminodiphenylamine is supposed to be the key intermediate in the formation of a dark green precipitate at the electrode surface during continued electrolysis of acidic aniline solutions. This has been characterized as an oligomer of aniline, for example, as the octamer emeraldine formed by a cascade of head-to-tail condensations [38,39]. Nelson, however, explained it as a mixture of mainly quinhydrone with a small amount of benzidine salt [37]. Today the electropolymerization of aniline under strongly acidic conditions is intensively studied as an important way to form the conducting polymer polyaniline [40] (see Chapters 31 and 32). 

Tests 2 and 3 illustrate the ease with which quinone is oxidized. The present test illustrates the ease of reduction. Place in a test tube a few crystals of benzoquinone and add 5 ml of water. Add about 0.1 g of sodium hydrosulfite in 2-3 portions, and heat. The solution may turn green or brown due to the intermediate formation of quinhydrone. If the solution does not decolorize, add more hydrosulfite. Cool, and observe whether the hydroquinone crystallizes. The reaction may be reversed by the addition of a few drops of ferric chloride solution. 

Carbene formation was mentioned in an earlier section. This elimination of HCl from 4-chlorophenol or elimination of other hydrogen halides from halophenols could have been inferred from earlier photochemical studies on this and other derivatives. Boule and his coworkers irradiated 4-chlorophenol under deoxygenated conditions and obtained the corresponding quinhydrone and the 2,4 -dihydroxy-5-chlorobiphenyl °. Other research demonstrated that its irradiation in neutral aqueous solutions gave the corresponding quinone " and also that de-aeration did not seem to affect the reaction. ... 

Patil AO, Curtin DY, Paul IC. Solid-state formation of quinhydrones from their components. Use of solid-solid reactions to prepare compounds not accessible from solution. J Am Chem Soc 1984 106 348-53. 

Regeimbal, J., Gleiter, S., Trumpower, B., Yu, C-A., Diwakar, M., Ballou, D., and Bardwell, J., Disulfide bond formation involves a quinhydrone-type charge-transfer complex, Proc. Nat. Acad. Set, 100, 13779-13784, 2003. 

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. 

Benzenesulfinic acid reacts easily with o- and p-quinones with the formation of sulfones (23) the hydroxy compounds formed yield crystalhne benzoyl derivatives. For special cases the condensation reaction with aniline may be important, giving rise to anils (24) and quinhydrones, i.e., molecular compoimds of quinones and phenols (25). For the detection of quinones a series of color reactions has been proposed. 

Scheme 11.21 Formation of quinhydrone 57 from benzoquinone and its hydroquinone. Subsequent collapse of the quinhydrone may give the corresponding semiquinone radical 58.

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