QUINONES in organic chemistry a group of compounds derived from the aromatic hydrocarbons and containing two ketonic groups, CO. (See KETONES.) The quinones are of two fold importance, first as being intermediates in the manufacture of synthetic dyes, and secondly as forming the basis of a theory of colour among organic compounds.
Among the quinones employed in dye production the most important is anthraquinone (q.v.), but a few others have found employment, such as acenaphthenequinone (see ACENAPHTHENE) and phenanthrenequinone. (See PHENANTHRENE.) Benzoquinone and its halogenated derivatives have also come into use as dye intermediates. There are two main divisions of quinones depend ing on the relative position of the ketonic groups. The para quinones contain the two ketonic groups at opposite ends of the aromatic ring in the so-called "para-position," whereas in the ortho-quinones the two ketonic groups are adjacent to one another. So far there is no decisive indication of the existence of meta quinones.
The para-benzoquinone, or ordinary quinone (formula I.), is prepared by the oxidation of aniline with sodium dichromate and dilute sulphuric acid. It sublimes in golden yellow needles, melting at 115.7° C and dissolves in ether, alcohol or hot water. It has a characteristic pungent smell.
Benzoquinone is also formed during the aerial oxidation of benzene vapour over vanadium pentoxide ; maleic anhydride is also produced in this process owing to further oxidation of the benzene nucleus.
p-Benzoquinone is a tautomeric substance behaving as if it possessed either of the formulae (I.) or (II.). The oxidising power of benzoquinone on hydriodic acid, hydroxylamine or phenylhydrazine suggests the peroxide structure (formula II.). With hydroxylamine hydrochloride, however, it yields either the mono- or di-oxime and with as-benzoylphenylhydrazine or 2:4 :6-trinitrophenylhydrazine it condenses to form hydrazones. These reactions indicate the diketonic structure (formula I.) which is now generally used for this quinone. With potassium chlorate and hydrochloric acid it furnishes tetrachlorobenzoquinone (chloranil), a colour intermediate. Other benzenoid p-quinones resemble p-benzoquinone in their general physical and chemical properties. An interesting confirmation of formula (I.) is afforded by the synthesis of para-xyloquinone (formula V.) from diacetyl (2 mols., formula IV.) under the influence of warm, aqueous caustic alkali (H. von Pechmann, 1888).
o-Benzoquinone (formula III.) was obtained in chloroform
solution by C. L. Jackson in 1901 by the action of iodine on the lead derivative of catechol; it was isolated in 1908 by R. Will stater and Muller by silver oxide oxidation of catechol dissolved in an inert solvent. It was then obtained either in red or colour less crystals. The red variety, which is inodorous, non-volatile and decomposes below 7o° C, is most probably the truly quinonoid compound as its colour corresponds with that of the more stable tetrabromo-o-benzoquinone, which crystallises in dark red prisms first obtained by Stenhouse in 1875.
From the hydrocarbon toluene, all three possible quinones have been obtained, namely two ortho-toluquinones and one para toluquinone ; the first two are unstable red substances, odourless and non-volatile like o-benzoquinone, the last resembles p benzoquinone in its odour, volatility and yellow colour.
The three quinones of naphthalene are discussed under that hydrocarbon (q.v.).
Cedriret or Coerulignone is a quinone of interesting type (formula VI.) obtained by treating with alkali bichromate the acetic acid solution of pyrogallol dimethyl ether which is present in beechwood or birchwood tars (C. Liebermann, 1873).
The crude quinone which slowly separates is dissolved in phenol below 3o° C and reprecipitated by alcohol in the form of dark blue needles. Other heteronuclear quinones of the diphenyl series have since been described by R. Willstatter and others.
Quinones on reduction yield colourless dihydro derivatives termed quinols. The combination of quinone and quinol gives rise to a quinhydrone which is more coloured than the quinone itself. Quinol and p-benzoquinone give a green quinhydrone. Quinone chloroimide and dichloroimide, 0 and C1N are yellow compounds volatile in steam and produced by the action of bleaching powder on p-aminophenol and p-phenylenediamine respectively.
Quinone-diamine, :NH, produced by oxidising p phenylenediamine in ethereal solution with silver oxide, is colour less, m.p. 124° C (R. Willstatter, 1904).
The foregoing quinones all contain unsaturated aromatic hydro carbon rings, but camphorquinone (formula VII.), obtained by the hydrolysis of isonitrosocamphor, is a bright yellow substance melting at 198° C.
The quinonoid theory of colour is dealt with in the article COLOUR : Influence of Chemical Constitution.
E. Thorpe, Dictionary of Applied Chemistry, vol. v., 1924, contains a comprehensive article on Quinones and their derivatives. (G. T. M.)