# 1911 Encyclopædia Britannica/Xanthone

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​XANTHONE (dibenzo-γ-pyrone, or diphenylene ketone oxide), C$_{13}$H$_8$O$_2$, in organic chemistry, a heterocyclic compound containing the ring system shown below. It is obtained by the oxidation of xanthene (methylene diphenylene oxide) with chromic acid; by the action of phosphorus oxychloride on disodium salicylate; by heating 2⋅2′-dioxybenzophenone with concentrated sulphuric acid; by distilling fluoran with lime; by the oxidation of xanthydrol (R. Meyer, Ber., 1893, 26, p. 1277); by boiling diazotized 2⋅2′-diaminobenzophenone with water (Heyl., Ber., 1898, 31, p. 3034); by heating salol with concentrated sulphuric acid (C. Graebe, Ann., 1889, 254, p. 280), and by heating potassium-ortho-chlorobenzoate with sodium phenolate and a small quantity of copper powder to 180-190° C. (F. Ullmann, Ber., 1905, 38, pp. 729, 2120, 2211). It crystallizes in needles which melt at 173-174° and boil at 349-350° C., and are volatile in steam. Its solution in concentrated sulphuric acid is of a yellow colour and shows a marked blue fluorescence. The carbonyl group is not ketonic in character since it yields neither an oxime nor hydrazone. When fused with caustic potash it yields phenol and salicylic acid. Mild reducing agents convert it into xanthydrol, the group >CO becoming >CH⋅OH, whilst a strong reducing agent like hydriodic acid converts it into xanthene, the group >CO becoming >CH$_2$. Phosphorus pentasulphide at 140-150° C. converts it into xanthion by transformation of >CO to >CS (R. Meyer, Ber., 1900, 33, p. 2580), and this latter compound condenses with hydroxylamine to form xanthone oxime.
All four mono-hydroxyxanthones are known, and are prepared by heating salicylic acid with either resorcin, pyrocatechin or hydroquinone; they are yellow crystalline solids, which act as dyestuffs. The 1⋅7-dihydroxyxanthone, known as euxanthone, is prepared by heating euxanthic acid with hydrochloric acid or by heating hydroquinone carboxylic acid with β-resorcylic acid and acetic anhydride (S. Kostanecki, Ber., 1891, 24, p. 3983; C. Graebe, Ann., 1889, 254, p. 298). It is also obtained from Indian yellow (Graebe, ibid.), formed in the urine of cows fed on mango leaves. It crystallizes in yellow needles which sublime readily. On fusion with caustic potash it decomposes with formation of tetrahydroxy-benzophenone, which then breaks up into resorcin and hydroquinone. The isomeric 1⋅6-dihydroxyxanthone, isoeuxanthone, is formed when β-resorcylic acid is heated with acetic anhydride. Gentisein, or 1⋅3⋅7-trihydroxyxanthone, is found in the form of its methyl ether (gentisin) in gentian root; it is obtained synthetically by condensing phloroglucin with hydroquinone carboxylic acid.
Xanthene, C$_{13}$H$_{10}$O, may be synthesized by condensing phenol with ortho-cresol in the presence of aluminium chloride. Its tetramethyl-diamino derivative, which is formed by condensing formaldehyde with dimethyl-meta-aminophenol and subsequent elimination of water from the resulting diphenyl methane derivative, is the leuco base of pyronine, into which it passes by oxidation.
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