Hydroquinone

HYDROQUINONE, para-dihydroxybenzene, C₆H₄(OH)₂(1, 4), prismatic colorless crystals with a melting point of 169–170°, soluble in water (1:17), and even more easily in alcohol and ether. An alkaline solution of hydroquinone greedily absorbs O, which is the basis for its use in gas analysis. It reduces ammoniacal copper solution and Fehling's reagent; upon oxidation, it converts into quinone and quinhydrone. It possesses antiseptic and antipyretic properties, due to which it was used in the treatment of gonorrhea and febrile diseases. Now, in view of its toxicity, it is not used in therapy. In animals, it causes convulsions of central origin and sharp changes in respiration—after a significant acceleration, respiratory paralysis occurs. In warm-blooded animals, after an initial slowing (action on the center of the vagus nerve), an acceleration of the heartbeat is observed. As an energetic reducing agent, hydroquinone has found wide application in photography as one of the most important developers. It possesses a strong antioxygenic (antioxidant) effect and is therefore used for the stabilization of various easily oxidizable substances. Hydroquinone and its derivatives are widespread in the plant kingdom, where they serve as chromogens, which play an essential role in cell respiration. Due to its ability to be easily oxidized, hydroquinone in the biological oxidation-reduction system serves as an oxygen acceptor, forming quinone, which lies at the basis of many respiratory pigments and catalyzes biological oxidative reactions. In nature, hydroquinone is found in sugar cane and in the leaf buds of pears; it arises during the breakdown of the glycoside arbutin, contained in the leaves of the bearberry (see). In the animal organism, free hydroquinone is not encountered, but it is found in the urine during alkaptonuria in the form of an acetic acid compound (homogentisic acid), and in normal urine in the form of paired compounds with sulfuric and glucuronic acids. The content of hydroquinone in the urine increases after the administration of phenol or benzene preparations, imparting to the urine a so-called "carbolic" dark coloration. Detection of hydroquinone in forensic cases: Since hydroquinone does not distill with water vapor, it is extracted from the object with alcohol and transferred into an acidic aqueous solution, from where it is extracted into ether or chloroform. The residue, after evaporation of the solvent, is tested with ferric chloride: a green coloration (quinhydrone), turning into yellow (quinone), indicates the presence of hydroquinone. Hydroquinone quickly, without heating, reduces metallic silver from an ammoniacal solution of silver nitrate and turns brown from alkalies.