ANTHRAQUINONE, an important derivative of anthracene, first prepared in 1834 by A. Laurent. It is prepared commercially from anthracene by stirring a sludge of anthracene and water in horizontal cylinders with a mixture of sodium bichro mate and caustic soda. This suspension is then run through a conical mill in order to remove all grit, the cones of the mill fitting so tightly that water cannot pass through unless the mill is running; the speed of the mill when working is about 3,000 revolutions per minute. After this treatment, the mixture is run into lead-lined vats and treated with sulphuric acid, steam is blown through the mixture in order to bring it to the boil and the anthra cene is rapidly oxidized to anthraquinone. When the oxidation is complete, the anthraquinone is separated in a filter press, washed and heated to 120°C. with commercial oil of vitriol, using about 23- parts of vitriol to 1 of anthraquinone. It is then removed to lead-lined tanks and again washed with water and dried ; the product obtained contains about 95% of anthraquinone which may be purified further by sublimation. The industrial synthesis of an thraquinone is gaining ground and 75% of the American produc tion is stated to be synthetic (1927). A process, first discovered by A. Behr and W. A. v. Dorp (1874), consists in heating phthalic anhydride with benzene in the presence of aluminium chloride when orthobenzoyl-benzoic acid is produced which on treatment with sulphuric acid or other dehydrating agent yields another anthraquinone.
Dihydroxy- and tetrahydroxy-anthraquinones are obtained when metahydroxy- and dimeta-dihydroxybenzoic acids are heated with concentrated sulphuric acid in a similar way.
Anthraquinone crystallizes in yellow needles or prisms, which melt at 277° C. It sublimes easily, and is very stable towards oxidizing agents, but is readily attacked by reducing agents. With zinc dust in presence of caustic soda it yields the secondary al and with hydriodic acid at 1 so° C. or on distillation with zinc dust, the hydrocarbon, anthracene, When fused with caustic potash, it gives benzoic acid. It behaves more as a ketone than as a quinone, since with hydroxylamine it yields an oxime, and on reduction with zinc dust and caustic soda it yields a secondary alcohol, whilst it cannot be reduced by means of sulphurous acid. Various sulphonic acids of anthraquinone are known, as well as hydroxy-derivatives, for the preparation and properties of which see ALIZARIN ; ANTHRACENE.