Pyrocatechol

Pyrocatechol, more commonly known as catechol, is a benzenediol, with the formula C6H4(OH)2.

Isolation, synthesis
Catechol was first isolated in 1839 by H. Reinsch by distilling catechin (the juice of Mimosa catechu (Acacia catechu L.f)); it occurs free in kino and in beechwood tar; its sulfonic acid is present in the urine of horse and man.

It results in the alkaline fusion of many resins and may be prepared by fusing ortho-phenolsulphonic acid, o-chlorphenol, o-bromophenol, 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.

Guaiacol may be obtained directly from beechwood tar, from pyrocatechin by methylation with potash and potassium methyl sulphate at 180, or from anisole 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 phosphite as guaiaco-phosphal; phosphotal is a mixture of the phosphites of creosote phenols. The valerianic ester of guaiacol is known as geosote, the henzoic as benzosol, the salicylic as guaiacolsalol, while the glycerin ether appears as guaiamar.

Ferric chloride gives a green coloration with the aqueous solution, whilst the alkaline solution rapidly changes to a green and finally to a black color on exposure to the air. It reduces silver solutions in the cold and alkaline copper on heating.

Catechol 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, respectively.

Nomenclature
Pyrocatechol is the name recommended by the International Union of Pure and Applied Chemistry (IUPAC) in its 1993 Recommendations for the Nomenclature of Organic Chemistry.

Occurrences
Urushiols are naturally existing organic compounds which have the catechol skeleton structure and diphenol functionality but with alkyl groups substituted onto the aromatic ring. Urushiols are the skin-irritating poisons found in plants like poison ivy, etc. Catecholamines are biochemically significant hormones/neurotransmitters which are phenethylamines where the phenyl group has a catechol skeleton structure. Two parts of a molecule of catechin, another natural compound which is found in tea, have the catechol skeleton structure in them.

Small amounts of catechol occur naturally in fruits and vegetables, along with the enzyme polyphenol oxidase. Upon exposure to air (as when a potato or apple is cut), the colorless catechol oxidizes to reddish-brown benzoquinone. This accounts for the browning of cut fruit and vegetables. The enzyme is inactivated by adding an acid, such as lemon juice, or by refrigeration. Excluding oxygen also prevents the browning reaction. Benzoquinone is said to be antimicrobial. which slows the spoilage of wounded fruits and other plant parts. The oxidation of catechol forms quinones and superoxide, which killed 50% of human glioblastoma cells treated with 0.23 mM of this compound in vitro (Pereira et al., 2004).

Uses
Pyrocatechol has been used as a film developing chemical.

Reference
Pereira, M.R.G.; de Oliveira, E. S.; de Villar, F. A. G. A.; Grangeiro, M. S.; Fonseca, J.; Silva, A. R.; Costa, M. F. D.; Costa, S. L.; El-Bachá, R. S., Cytotoxicity of catechol towards human glioblastoma cells via superoxide and reactive quinones generation. 2004, J. Bras. Patol. Med. Lab.,    v. 40, p. 280-285.
 * Barner, B. A. "Catechol" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
 * Barner, B. A. "Catechol" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.