Phenols

PHENOLS, derivatives of aromatic hydrocarbons in which one or more hydrogen atoms are replaced by hydroxyls. According to the number of hydroxyl groups, a distinction is made between monohydric phenols (C_nH_2n-7OH), such as carbolic acid (see), cresols (see); dihydric phenols [C_nH_2n-8(OH)₂], such as pyrocatechol (see), resorcinol (see), hydroquinone (see); and trihydric phenols [C_nH_2n-9(OH)₃], such as pyrogallol (see). Naphthols and more complex compounds containing an OH group at the cyclic nucleus (such as oxyquinoline, morphine, and others) are also classed as phenols. Phenols are formed during the putrefaction of proteins and tyrosine. In a free state, they are found in the intestine and as traces in the urine and blood. In larger amounts, they are contained in urine in the form of paired ethereal sulfuric and glucuronic acids. Among the products of putrefaction and in urine, para-cresol is usually contained in the greatest amount, followed by smaller amounts of phenol; in addition, ortho-cresol has been found in human urine. Pyrocatechol is found in the urine of herbivores and animals on a mixed diet (including humans), but is absent in carnivores. Hydroquinone is frequently found in urine following the ingestion of phenols. Many polyhydric phenols undergo oxidation in the organism under the influence of phenolase enzymes (see Oxidizing enzymes). It is possible that polyhydric phenols play the role of catalysts in oxidation-reduction processes in the organism. Phenols are formed during the dry distillation of wood and coal. To isolate phenols from coal tar, it is treated with an aqueous alkali solution. Phenols react with alkali to give water-soluble phenolates C₆H₅NaOH -> C₆H₅ONa + H₂O, in the form of which they are separated from water-insoluble coal tar hydrocarbons. After acidification of the aqueous solution, the separated phenols are distilled with steam to separate them from resin acids. Monohydric phenols are liquids, poorly soluble in water, having a specific odor. Dihydric and trihydric phenols are crystalline substances soluble in water, alcohol, and ether. Phenols possess weakly acidic properties: with alkalis, they give water-soluble phenolates, from which they can be displaced by acids, even one as weak as carbonic acid, which indicates very insignificant acidic properties. They are neutral to litmus. Phenol solutions precipitate proteins, which is associated with their disinfecting properties. With ferric chloride, FeCl₃, they give characteristic colorations: violet with carbolic acid, resorcinol, phloroglucinol; green with pyrocatechol; yellow with hydroquinone; and red with pyrogallol. Phenols easily react with halogens, yielding trichlor-, tribrom-, and triiodophenols, which is frequently used both for the qualitative detection of phenols (precipitation of a sediment upon the addition of bromine) and for quantitative determination by adding a titrated iodine solution in the presence of soda and back-titrating the iodine that has not entered into combination with the phenol (Messinger and Vortmann method). For determining the amount of phenols in urine and blood, the Theis-Benedict method is used, based on the colorimetry of the coloration that phenols produce upon the action of diazotized para-nitroaniline, or the Folin-Denis-Tisdall method, based on the colorimetric determination of the color obtained in the reaction of phenols with phosphotungstic and phosphomolybdic acid in the presence of soda (see also Urine). Phenols and their derivatives find extensive application as medicaments for external and internal disinfection—see Carbolic acid, Cresol, Lysol, Thymol, Resorcinol, Guaiacol, Salicylic acid, Salol, Tannins.

M. A. Kuzin.