Pyrocatechol

Pyrocatechol, more commonly known as catechol, is a benzenediol, with the formula C₆H₄(OH)₂. It is a commonly used building block in organic synthesis.

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Isolation and synthesis

Catechol was first isolated in 1839 by H. Reinsch by distilling catechin (the juice of Mimosa catechu (Acacia catechu L.f))^{[citation needed]}; it occurs free in kino and in beechwood tar; its sulfonic acid is present in the urine of horse and humans.

Catechol can 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.^{[citation needed]} 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 base 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.^{[citation needed]} It reduces silver solutions in the cold and alkaline copper on heating.^{[citation needed]} 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 (Almeida et al., 2007).

Occurrence

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.

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 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.

Uses

Pyrocatechol was used as a developing agent in black-and-white photographic developers, but except for some special purpose applications, its use is largely historical.

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.^[1]

References

• Almeida, W. L. C.; Vitor, D. N.; Pereira, M. R. G; de Sá, D. S.; Alvarez, L. D. G.; Pinheiro, A. M.; Costa, S. L.; Costa, M. F. D.; Rocha, Z. N.; El-Bachá, R. S. Redox properties of ruthenium complex with catechol are involved in toxicity to glial cells. J. Chil. Chem. Soc. 52 (3): 1240-1243, 2007.
• Barner, B. A. "Catechol" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.

See also

• Urushiol

References

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.