1911 Encyclopædia Britannica/Pyrocatechin
PYROCATECHIN, or Pyrocatechol, ortho-dioxybenzene, C6H4(OH)2, first prepared in 1839 by H. Reinsch on distilling catechin (the juice of Mimosa catechu); occurs free in kino and in beechwood tar; its sulphonic acid is present in the urine of the horse and man. It results in the alkaline fusion of many resins, and may be prepared by fusing ortho-phenolsulphonic acid, o-chlorphenol, o-bromphenol, and o-phenoldisulphonic acid with potash, or, better, by heating its methyl ether, guaiacol, C6H4(OH)(OCH3), a constituent of beechwood tar, with hydriodic acid.
Pyrocatechin crystallizes in white rhombic prisms, which melt at 104° and boil at 245°; it is readily soluble in water, alcohol and ether. Ferric chloride gives a green coloration with the aqueous solution, whilst the alkaline solution rapidly changes to a green and finally to a black colour on exposure to the air. It reduces silver solutions in the cold and alkaline copper on heating.
Guaiacol may be obtained directly from beechwood tar, from pyrocatechin by methylation with potash and potassium methyl sulphate at 180°, or from anisol by nitration, reduction of the ortho-nitroanisol to amino-anisol, which is then diazotized and boiled with water. It melts at 28° and boils at 250°. It is employed in medicine as an expectorant. The dimethyl ether or veratrol is also used in medicine. Many other pyrocatechin derivatives have been suggested for therapeutic application. Guaiacol carbonate is known as duotal, the phosphate as phosphatol, the phosphate as guaiaco-phosphal; phosphotal is a mixture of the phosphates of crcosote phenols. The valerianic ester of guaiacol is known as geosote, the benzoic as benzosol, the salicylic as guaiacolsalol, while the glycerin ether appears as guaiamar.
Pyrocatechin readily condenses to form heterocyclic compounds; cyclic esters are formed by phosphorus trichloride and oxychloride, carbonyl chloride, sulphuryl chloride, &c.; whilst ortho-phenylenediamine, o-aminophenol, and o-aminothiophenol give phenazine, phenoxazine and thiodiphenylamine.