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Catechol

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Pyrocatechol

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Pyrocatechol, more commonly known as catechol, is the organic compound with the formula C₆H₄(OH)₂. It is one of three isomeric benzenediols. This colourless compound occurs naturally, but about 20000 tons are manufactured each year, mainly as precursors to pesticides, flavors, and fragrances.

Isolation and 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 humans. It is produced industrially by the hydroxylation of phenol using hydrogen peroxide:

C₆H₅OH + H₂O₂ → C₆H₄(OH)₂ + H₂O

Catechol has been prepared in many ways from phenols, e.g. by fusing 2-phenolsulfonic acid, 2-chlorphenol, 2-bromophenol, or 2,6-phenoldisulfonic acid with potash, or, better, by heating its methyl ether, guaiacol, a constituent of beechwood tar, with hydriodic acid. Many pyrocatechin derivatives have been suggested for therapeutic applications.

Reactions

Organic chemistry

Like other difunctional benzene derivatives, catechol readily condenses to form heterocyclic compounds. Cyclic esters are formed upon treatment with phosphorus trichloride and phosphorus oxychloride, carbonyl chloride, and sulphuryl chloride:

C₆H₄(OH)₂ + XCl₂ → C₆H₄(O₂X) + 2 HCl

where X = CO, SO2, PCl, P(O)Cl.

Catechols produce quinones with the addition of Ceric ammonium nitrate (CAN).

With metal ions

Catechol is the conjugate acid of a chelating agent used widely in coordination chemistry. Basic solutions of catechol react with iron(III) to give the red [Fe(C₆H₄O₂)₃]^3-. 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 can also be conjugated to ruthenium. [Ru^III(NH₃)₄(catechol)]⁺ oxidizes faster than catechol in the presence of oxygen, but controlled potential electrolysis showed that its oxidation involves only one electron.

Occurrence

Small amounts of catechol occur naturally in fruits and vegetables, along with the enzyme polyphenol oxidase. Upon mixing the enzyme with the substrate and exposure to oxygen (as when a potato or apple is cut), the colorless catechol oxidizes to reddish-brown benzoquinone derivatives. 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 catechol skeleton occurs in a variety of natural products such as urushiols, which are the skin-irritating poisons found in plants like poison ivy, and catecholamines, hormones/neurotransmitters, and catechin, which is found in tea.

Uses

Approximately 50% of synthetic catechol is consumed in the production of pesticides, the remainder being used as a precursor to fine chemicals such as perfumes and pharmaceuticals. It is a common building block in organic synthesis. Several industrially significant flavors and fragrances are prepared starting from catechol. Guaiacol, 2-methoxyphenol, is prepared by methylation of catechol and then converted to vanillin on a scale of about 10,000 tons/year (1990). The related monoethyl ether of catechol, guethol, is converted to "ethylvanillin," which is a component of chocolate confectionaries. 3-Trans-Isocamphylcyclohexanol (CAS#3407-42-9), widely used as a replacement for sandelwood oil, is prepared from catechol via guaiacol and camphor. Piperonal (CAS#120-57-0), a flowery scent, is prepared from the methylene diether of catechol followed by condensation with glyoxal and decarboxylation.

Pyrocatechol is used as a black-and-white photographic developers, but except for some special purpose applications, its use until recently was largely historical. Hobbyist-mixed developers use the "Pyrocat" family of developers.

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.