Catechol

Catechol
IUPAC name	Pyrocatechol
Other names	pyrocatechol benzene-1,2-diol 2-hydroxyphenol α-hydroxyphenol o-benzenediol o-hydroxyphenol 1,2-dihydroxybenzene pyrocatechin
Identifiers
CAS number	120-80-9 Y
SMILES	Oc1c(O)cccc1
ChemSpider ID	283
Properties
Molecular formula	C6H6O2
Molar mass	110.1 g/mol
Appearance	white solid
Density	1.344 g/cm³, solid
Melting point	105 °C, 378 K, 221 °F
Boiling point	245.5 °C, 519 K, 474 °F
Solubility in water	43 g/100 mL
Acidity (pKa)	9.5
Hazards
EU classification	Harmful (Xn)
R-phrases	R21/22, R36/38
S-phrases	(S2), S22, S26, S37
NFPA 704	1 2 0
Flash point	127 °C
Related compounds
Related benzenediols	Resorcinol Hydroquinone
Related compounds	1,2-benzoquinone
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Catechol, formerly known as pyrocatechol, is 1,2-dihydroxybenzene, an organic compound with the semi-empirical formula C₆H₄(OH)₂. It is the ortho isomer, one of three isomeric benzenediols. This colourless compound occurs naturally in trace amounts. About 20M kg are produced annually, mainly as a precursor to pesticides, flavors and fragrances.

Catechol occurs as feathery white crystals which are very rapidly soluble in water.

(The name "catechol" has also been used as a chemical class name, where it refers to the catechins)

Isolation and synthesis

Catechol was first isolated in 1839 by H. Reinsch by distilling catechin (from catechu, the juice of Mimosa catechu (Acacia catechu L.f)). Upon heating catechin above its decomposition point, "pyrocatechol" forms. Catechol occurs free in kino and in beechwood tar; its sulfonic acid has been detected in the urine of horse and humans.^[1]

Catechol is produced industrially by the hydroxylation of phenol using hydrogen peroxide:^[2]

C₆H₅OH + H₂O₂ → C₆H₄(OH)₂ + H₂O

Previously, catechol has been produced by hydrolysis of 2-substituted phenols, especially 2-chlorphenol, with hot aqueous solutions containing alkali metal hydroxides. Its methyl ether derivative, guaiacol, converts to catechol via hydrolysis of the CH3-O bond as promoted by hydriodic acid.^{[citation needed]}

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.^{[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.^[3]

Redox chemistry

Catechol is produced by a reversible two-electron, two-proton reduction of 1,2-benzoquinone (E° = +795 mV vs SHE; E_m (pH 7) = +380 mV vs SHE). ^{[4] [5]}

Natural Occurrence

Small amounts of catechol occur naturally in fruits and vegetables, along with the enzyme polyphenol oxidase(also known as catecholase, or catechol oxidase). Upon mixing the enzyme with the substrate and exposure to oxygen (as when a potato or apple is cut and left out), the colorless catechol oxidizes to reddish-brown melanoid pigments, derivatives of benzoquinone. The enzyme is inactivated by adding an acid, such as lemon juice, and slowed with cooling. 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

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.^[2] It is a common building block in organic synthesis.^[6] 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 10M kg per year (1990). The related monoethyl ether of catechol, guethol, is converted to "ethylvanillin", a component of chocolate confectionaries. 3-trans-isocamphylcyclohexanol (CAS#3407-42-9), widely used as a replacement for sandalwood 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.^[7]

Pyrocatechol is used as a black-and-white photographic developer, but except for some special purpose applications, its use until recently was largely historical. Modern pyrocatechol developing was pioneered by noted photographer Sandy King. His "PyroCat" formulation enjoys widespread popularity among modern B&W film photographers.

Catechol derivatives

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. Many pyrocatechin derivatives have been suggested for therapeutic applications.

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.^[8]

See also

• Catechu
• Catecholamines

References

External links

• International Chemical Safety Card 0411
• NIOSH Pocket Guide to Chemical Hazards
• IARC Monograph: "Catechol"
• European Chemicals Bureau
• IUPAC Nomenclature of Organic Chemistry (online version of the "Blue Book")

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