View source for Catechol

{{Distinguish2|[[Catechin]], also sometime called catechol}}
{{chembox
| Watchedfields = changed
| verifiedrevid = 326550162
|   Name = Catechol
|   ImageFileL1 = Brenzcatechin.svg
|   ImageSizeL1 = 120px
|   ImageNameL1 = Skeletal formula
|   ImageNameL1 = Pyrocatechol
|   ImageFileR1 = Pyrocatechol-3D-balls.png
|   ImageSizeR1 = 135px 
|   ImageNameR1 = Ball-and-stick model
|   IUPACName = Benzene-1,2-diol
|   OtherNames = pyrocatechol<br />1,2-benzenediol<br />2-hydroxyphenol<br />1,2-dihydroxybenzene
| Section1 = {{Chembox Identifiers
|   SMILES = Oc1c(O)cccc1
|   ChemSpiderID = 283
|   CASNo_Ref = {{cascite}}
|   CASNo = 120-80-9
|   RTEhttp://en.wikipedia.org/w/index.php?title=Catechol&action=editCS = UX1050000
|   PubChem = 289
  }}
| Section2 = {{Chembox Properties
|   Formula = C<sub>6</sub>H<sub>6</sub>O<sub>2</sub>
|   MolarMass = 110.1 g/mol
|   ExactMass = 110.036779 u
|   Appearance = white solid
|   Density = 1.344 g/cm³, solid
|   Solubility = 43 g/100 mL
|   MeltingPtC = 105
|   BoilingPtC = 245.5
|   pKa = 9.5
  }}
| Section3 = {{Chembox Structure
|   Dipole =
  }}
| Section7 = {{Chembox Hazards
|   ExternalMSDS =
|   EUClass = Harmful ('''Xn''')
|   NFPA-H = 2
|   NFPA-F = 1
|   NFPA-R =
|   RPhrases = {{R21/22}}, {{R36/38}}
|   SPhrases = {{s2}}, {{S22}}, {{S26}}, {{S37}}
|   FlashPt = 127 °C
  }}
| Section8 = {{Chembox Related
|   Function = [[benzenediol]]s
|   OtherFunctn = [[Resorcinol]]<br />[[Hydroquinone]]
|   OtherCpds = [[1,2-Benzoquinone|1,2-benzoquinone]]
  }}
}}

'''Catechol''', also known as '''pyrocatechol''' or '''1,2-dihydroxybenzene''', is an organic compound with the molecular formula C<sub>6</sub>H<sub>4</sub>(OH)<sub>2</sub>. It is the ''ortho'' [[isomer]] of the three isomeric [[benzenediol]]s. 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 [[catechin]]s) 

==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 [[Pterocarpus marsupium|kino]] and in [[beech]]wood tar; its [[sulfonic acid]] has been detected in the [[urine]] of horse and humans.<ref>Anti-inflammatory effects of catechols in lipopolysaccharide-stimulated microglia cells: Inhibition of microglial neurotoxicity. European Journal of Pharmacology, Volume 588, Issue 1, 24 June 2008, Pages 106-113</ref>

Catechol is produced industrially by the hydroxylation of [[phenol]] using [[hydrogen peroxide]]:<ref name=Ullmann>Helmut Fiegel, Heinz-Werner Voges, Toshikazu Hamamoto, Sumio Umemura, Tadao Iwata, Hisaya Miki, Yasuhiro Fujita, Hans-Josef Buysch, Dorothea Garbe, Wilfried Paulus "Phenol Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 2002: Weinheim. DOI: 10.1002/14356007.a19_313. Article Online Posting Date: June 15, 2000</ref>
:C<sub>6</sub>H<sub>5</sub>OH  +  H<sub>2</sub>O<sub>2</sub>    →   C<sub>6</sub>H<sub>4</sub>(OH)<sub>2</sub>  +  H<sub>2</sub>O

Previously, catechol has been produced by hydrolysis of 2-substituted phenols, especially [[2-chlorophenol]], 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]].{{Fact|date=October 2007}}

==Reactions==
===Organic chemistry===
Like other difunctional benzene derivatives, catechol readily condenses to form [[heterocyclic compounds]].  Cyclic [[ester]]s are formed upon treatment with [[phosphorus trichloride]] and [[phosphorus oxychloride]], [[carbonyl chloride]], and sulphuryl chloride:
:C<sub>6</sub>H<sub>4</sub>(OH)<sub>2</sub>  +  XCl<sub>2</sub>   →  C<sub>6</sub>H<sub>4</sub>(O<sub>2</sub>X)  +  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(C<sub>6</sub>H<sub>4</sub>O<sub>2</sub>)<sub>3</sub>]<sup>3-</sup>. [[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.{{Fact|date=October 2007}} It reduces silver solutions in the cold and alkaline copper on heating.{{Fact|date=October 2007}} Catechol can also be conjugated to [[ruthenium]]. [Ru<sup>III</sup>(NH<sub>3</sub>)<sub>4</sub>(catechol)]<sup>+</sup> oxidizes faster than catechol in the presence of oxygen, but controlled potential electrolysis showed that its oxidation involves only one electron.<ref>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.</ref>

===Redox chemistry===
Catechol is produced by a reversible two-electron, two-proton reduction of [[1,2-benzoquinone]] (''E''° = +795 mV vs [[Standard hydrogen electrode|SHE]]; ''E''<sub>m</sub> (pH 7) = +380 mV vs SHE).
<ref>{{Cite journal
  | last = Horner | first = Leopold
  | last2 = Geyer | first2 = Ekkehard
  | title = Zur Kenntnis der ''o''-Chinone, XXVII: Redoxpotentiale von Brenzcatechin-Derivaten
  | journal = Chemische Berichte
  | volume = 98 | issue = 6 | pages = 2016–2045 | year = 1965
  | doi = 10.1002/cber.19650980641 }}</ref>
<ref>{{Cite journal
  | last = Nematollahi | first = D.
  | last2 = Rafiee | first2 = M.
  | title = Electrochemical oxidation of catechols in the presence of acetylacetone
  | journal = Journal of Electroanalytical Chemistry
  | volume = 566 | issue = 1 | pages = 31–37 | year = 2004 | date = 2004-05-01
  | doi = 10.1016/j.jelechem.2003.10.044 }}</ref>

:[[Image:1,2-benzoquinone_to_catechol.png|300 px|Electrochemical interconversion of 1,2-benzoquinone and catechol]]

The redox series catecholate dianion, monoanionic semiquinonate and benzoquinone are collectively called '''dioxolenes'''. Dioxolenes are used as [[ligand]]s. <ref>{{cite DOI|10.1007/BF00139966}}</ref>

==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.<ref name=Ullmann/> It is a common building block in [[organic synthesis]].<ref>Barner, B. A. "Catechol" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.</ref>  Several industrially significant flavors and fragrances are prepared starting from catechol.  [[Guaiacol]] is prepared by methylation of catechol and is 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, widely used as a replacement for [[sandalwood oil]], is prepared from catechol via guaiacol and [[camphor]].  [[Piperonal]], a flowery scent, is prepared from the methylene diether of catechol followed by condensation with [[glyoxal]] and [[decarboxylation]].<ref>Karl-Georg Fahlbusch, Franz-Josef Hammerschmidt, Johannes Panten, Wilhelm Pickenhagen, Dietmar Schatkowski, Kurt Bauer, Dorothea Garbe, Horst Surburg Flavors and Fragrances" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, 2005: Weinheim.  Published online: 15 January 2003</ref>

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

==Catechol derivatives==
The catechol skeleton occurs in a variety of natural products such as [[urushiol]]s, which are the skin-irritating [[poison]]s found in plants like [[poison ivy (plant)|poison ivy]], and [[catecholamine]]s, [[hormone]]s/[[neurotransmitter]]s, and [[catechin]], which is found in [[tea]].  Many pyrocatechin derivatives have been suggested for therapeutic applications.

==Nomenclature==

The "preferred [[International Union of Pure and Applied Chemistry|IUPAC]] name" (PIN) of catechol is ''benzene-1,2-diol''.
<ref>[http://old.iupac.org/reports/provisional/abstract04/BB-prs310305/Chapter6-Sec60-64.pdf  Preferred IUPAC Names]
September 2004, Chapter 6, Sect 60-64, p.38</ref>
The trivial name ''pyrocatechol'' is a retained IUPAC name, according to the ''1993 Recommendations for the Nomenclature of Organic Chemistry''.
<ref> IUPAC, Commission on Nomenclature of Organic Chemistry. A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993) [http://www.acdlabs.com/iupac/nomenclature/93/r93_414.htm R-5.5.1.1 Alcohols and phenols.]</ref>
<ref>{{cite book | author=Panico, R.; & Powell, W. H. (Eds.) | title=A Guide to IUPAC Nomenclature of Organic Compounds 1993 | location=Oxford | publisher=Blackwell Science | year=1994 | isbn=0-632-03488-2}}</ref>

==See also==
*[[Catechu]]
*[[Catecholamines]]

==External links==
*[http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc04/icsc0411.htm International Chemical Safety Card 0411]
*[http://www.cdc.gov/niosh/npg/npgd0109.html NIOSH Pocket Guide to Chemical Hazards]
*[http://www-cie.iarc.fr/htdocs/monographs/vol71/012-catechol.html IARC Monograph: "Catechol"]
*{{ecb}}
*[http://www.acdlabs.com/iupac/nomenclature/ IUPAC Nomenclature of Organic Chemistry] (online version of the "''Blue Book''")

==References==
{{reflist}}

{{1911}}

[[Category:Benzenediols]]
[[Category:Chelating agents]]
[[Category:Photographic chemicals]]
[[Category:IARC Group 2B carcinogens]]

[[cs:Pyrokatechol]]
[[de:Brenzcatechin]]
[[fr:Pyrocatéchol]]
[[it:Pirocatecolo]]
[[lv:Pirokatehīns]]
[[hu:Pirokatechin]]
[[nl:Catechol]]
[[ja:カテコール]]
[[pl:Pirokatechina]]
[[pt:Catecol]]
[[ru:Пирокатехин]]
[[fi:Katekoli]]
[[sv:Katekol]]
[[zh:邻苯二酚]]
[[Category:2Fix]]