Quinones quinhydrone formation

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). 

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. 

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. ... 

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. 

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