Anthraquinone(CHAPTER I. The Anthraquinone Group.)

The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918

The most typical methods for the formation of anthraquinone are:

• The oxidation of anthracene
• Treatment of ο-benzoyl benzoic acid with sulphuric acid (Liebermann, Ber., 1874, 7, 805; W. H. Perkin, Chem. Soc. Trans., 1891, 59, 1012)
• Interaction of phthalylchloride and benzene in the presence of aluminium chloride (Friedel and Crafts, Ann. de Chimie, (6), i, 523
• Distillation of benzoic acid with phosphorus pentoxide (Kekule and Franchimont, Ber., 1872, 5, 908)

Of these methods (a) is the only one suitable for the commercial preparation of anthraquinone, whereas those of the type of (c) and (d) have been adapted for pure and applied syntheses of numerous hydroxyanthraquinones. The hydroxyanthraquinones, like anthraquinone itself, by distillation with zinc-dust are reduced to anthracene, and on short digestion with zinc-dust and alkali give unstable alkali salts of hydroxy oxanthranols (Graebe and Liebermann, Annalen, 1 60, 126) which in solution readily absorb oxygen from the air with regeneration of the colour of the alkali salt of the unreduced substance. These two reactions are typical of hydroxyanthraquinones and are of service for their detection in nature.

By further reduction oxanthranol passes into the dihydroxy compound, which loses a molecule of water with formation of anthranol, a compound which also exists in the isomeric form. Anthranol is best obtained by the action of tin and hydrochloric acid on anthraquinone in boiling acetic acid solution (Liebermann and Gimbel, Ber., 1887, 20, 1854).

Natural anthraquinone colouring matters are accompanied in many instances by hydroxy α or β-methylanthraquinone derivatives which possess little or no tinctorial property. Of these the hydroxy β-methyl anthraquinones appear to be the most prolific, β-methyl anthraquinone, melting-point 175-177°, may be prepared by the oxidation of β-methyl anthracene, and occurs as a by-product in the manufacture of anthraquinone from crude anthracene (Perkin and Cope, Chem. Soc. Trans., 1894, 65, 843).

For commercial purposes it is, however, more easily prepared by the interaction of phthalic anhydride and toluene, in which case ο-toluylbenzoic acid is probably an intermediate product (Bücherer, Lehrbuch der Farbenchemie, 1914, 322). By distillation with zinc-dust it yields β-methylanthracene, melting-point 207°, and by oxidation with chromic acid β-anthraquinone carboxylic acid - melting-point 290-292°. α-Methylanthraquinone, a much less wellknown substance, is obtained by the oxidation of a-methylanthracene and melts at 166-167° (Birukow, Ben, 1887, 20, 2070). The corresponding anthraquinone a-carboxylic acid melts at 293-294° (Blumenfeld, Ber., 1897, 30, 1115), whereas a-methyl anthracene itself according to Birukow melts at 199-200°. Hydroxy-methyl-anthranols frequently occur in nature, usually side by side with methyl hydroxyanthraquinones. They are devoid of tinctorial property, and on gentle oxidation, preferably in acetic acid solution with chromic acid, are converted into the corresponding methylanthraquinone compounds.