Preparation and Properties of Quinones

Introduction. The quinones are intermediate products in the oxidation of the aromatic nucleus. They may be prepared in some cases by the direct oxidation of aromatic hydrocarbons. For example, anthracene, naphthalene, and phenanthrene are oxidized to the corresponding quinones by chromic acid mixtures. Quinones are prepared more conveniently by oxidation of primary aromatic amines, particularly the p-substituted amines. p-Benzoquinone is obtained by the oxidation of aniline, p-toluidine, sulfanilic acid, p-aminophenol, and other similar compounds. Similarly the a-naph-thoquinone is obtained by oxidation of 1,4-aminonaphthol, and /9-naphthoquinone by the oxidation of 1,2-aminonaphthol. In the laboratory, although it is possible to prepare p-benzoquinone by the oxidation of aniline with acid-dichromate mixture, the method is tedious and the yield poor. Since hydroquinone is used extensively as a photographic developer and is made industrially, it is more convenient to prepare quinone by its oxidation. 

The quinones may be regarded as unsaturated diketones derived from cycloolefins, and closely related to aromatic compounds. They exhibit great reactivity with a variety of reagents. They are readily reduced and oxidized, and undergo both olefinic and carbonyl reactions. Equimolecular proportions of p-benzoquinone and hydroquinone yield a crystalline molecular compound, quinhydrone. 

Quinhydrone is utilized in a half-cell used for the measurement of hydrogen ions. 

Caution The fumes of the quinone are very irritating to the eyes and care should be exercised to protect them. 

PREPARATION OF AROMATIC HYDROCARBONS BY THE FRIEDEL-CRAFTS REACTION 

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