Catechol

Catechol (/ˈkætɪtʃɒl/ or /ˈkætɪkɒl/), also known as pyrocatechol or 1,2-dihydroxybenzene, is an organic compound with the molecular formula C

4(OH)
2. It is the ortho isomer of the three isomeric benzenediols. This colorless compound occurs naturally in trace amounts. It was first discovered by destructive distillation of the plant extract catechin. About 20,000 tonnes of catechol are now synthetically produced annually as a commodity organic chemical, mainly as a precursor to pesticides, flavors, and fragrances. Small amounts of catechol occur in fruits and vegetables.[1]

## Contents

* 1 Isolation and synthesis
* 2 Reactions
* 2.1 Redox chemistry
* 3 Catechol derivatives
* 4 Uses
* 5 Nomenclature
* 6 See also
* 7 References
* 8 External links

## Isolation and synthesis

Catechol was first isolated in 1839 by Edgar Hugo Emil Reinsch (1809–1884) by distilling it from the solid tannic preparation catechin, which is the residuum of catechu, the boiled or concentrated juice of Mimosa catechu (Acacia catechu).[2] Upon heating catechin above its decomposition point, a substance that Reinsch first named Brenz-Katechusäure (burned catechu acid) sublimated as a white efflorescence. This was a thermal decomposition product of the flavanols in catechin. In 1841, both Wackenroder and Zwenger independently rediscovered catechol; in reporting on their findings, Philosophical Magazine coined the name pyrocatechin.[3] By 1852, Erdmann realized that catechol was benzene with two oxygen atoms added to it; in 1867, August Kekulé realized that catechol was a diol of benzene, so by 1868, catechol was listed as pyrocatechol.[4] In 1879, the Journal of the Chemical Society recommended that catechol be called "catechol", and in the following year, it was listed as such.[5]

Catechol has since been shown to occur in free form naturally in kino and in beechwood tar. Its sulfonic acid has been detected in the urine of horses and humans.[6]

Catechol is produced industrially by the hydroxylation of phenol using hydrogen peroxide.[1]

5OH + H

2 → C

4(OH)
2 \+ H

It can be produced by reaction of salicylaldehyde with base and hydrogen peroxide (Dakin oxidation),[7] as well as the 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 CH
3–O bond as promoted by hydroiodic acid (HI).[7]

## Reactions

Like some other difunctional benzene derivatives, catechol readily condenses to form heterocyclic compounds. For example, using phosphorus trichloride or phosphorus oxychloride gives the cyclic chlorophosphonite or chlorophosphonate, respectively; sulfuryl chloride gives the sulfate; and phosgene (COCl
2) gives the carbonate:[8]

4(OH)
2 \+ XCl
2 → C

4(O
2X) + 2 HCl where X = PCl or POCl; SO
2; CO

Basic solutions of catechol react with iron(III) to give the red [Fe(C

3]3−. Ferric chloride gives a green coloration with the aqueous solution, while the alkaline solution rapidly changes to a green and finally to a black color on exposure to the air.[9] Iron-containing dioxygenase enzymes catalyze the cleavage of catechol.

### Redox chemistry

Catechols convert to the semiquinone radical. At pH = 7, this conversion occurs at 100 mV:[citation needed]

4(OH)
2 → C

4(O)(OH) + ½ H

The semiquinone radical can be reduced to the catecholate dianion, the potential being dependent on pH:

4(O)(OH) + e−
→ [C

2]2-
\+ H+

Catechol is produced by a reversible two-electron, two-proton reduction of 1,2-benzoquinone (E0 = +795 mV vs SHE; Em (at pH 7) = +380 mV vs SHE).[10]

The redox series catecholate dianion, monoanionic semiquinonate, and benzoquinone are collectively called dioxolenes. Dioxolenes can function as ligands for metal ions.[11]

## Catechol derivatives

* Naturally occurring catechols
* 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione, a metabolite of cholesterol[12]

* Catechin, a component of tea.

* Piceatannol, an antioxidant found in some red wines.

* Urushiols, the active agent in poison ivy (R = (CH2)14CH3, (CH2)7CH=CHCH2CH=CHCH2CH=CH2, and others)

* Catecholamines, drugs imitating them (such as MDMA), hormones/neurotransmitters

* Dopamine, a well-known neurotransmitter with many important roles in cells

* Quercetin, which is found in many foods.

Catechol derivatives are found widely in nature. They often arise by hydroxylation of phenols.[13] Arthropod cuticle consists of chitin linked by a catechol moiety to protein. The cuticle may be strengthened by cross-linking (tanning and sclerotization), in particular, in insects, and of course by biomineralization.[14]

The synthetic derivative 4-tert-butylcatechol is used as an antioxidant and polymerization inhibitor.

## Uses

Approximately 50% of the synthetic catechol is consumed in the production of pesticides, the remainder being used as a precursor to fine chemicals such as perfumes and pharmaceuticals.[1] It is a common building block in organic synthesis.[15] 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 confectioneries. 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.[16]

Josef Maria Eder published in 1879 his findings on the use of catechol as a black-and-white photographic developer,[17][18] but, except for some special purpose applications, its use is largely historical. It is rumored to have been used briefly in Eastman Kodak's HC-110 developer and Anchell supposes it to be a component in Tetenal's Neofin Blau developer.[19] It is a key component of Tanol from Moersch Photochemie in Germany.[20] Modern catechol developing was pioneered by noted photographer Sandy King, whose "PyroCat" formulation is popular among modern black-and-white film photographers.[21] King's work has since inspired further 21st-century development by others such as Jay De Fehr with Hypercat and Obsidian Acqua developers, and others.[19]

## Nomenclature

Although rarely encountered, the officially "preferred IUPAC name" (PIN) of catechol is benzene-1,2-diol.[22] The trivial name pyrocatechol is a retained IUPAC name, according to the 1993 Recommendations for the Nomenclature of Organic Chemistry.[23] [24]

## See also

* Enol
* Pyrogallol
* Thiotimoline

## References

1. ↑ 1.0 1.1 1.2 Fiege, Helmut; Voges, Heinz-Werner; Hamamoto, Toshikazu; Umemura, Sumio; Iwata, Tadao; Miki, Hisaya; Fujita, Yasuhiro; Buysch, Hans-Josef et al. (2000), "Phenol Derivatives", Ullmann's Encyclopedia of Industrial Chemistry, doi:10.1002/14356007.a19_313, ISBN 978-3-527-30385-4
2. ↑ Hugo Reinsch (1839) "Einige Bemerkungen über Catechu" (Some observations about catechu), Repertorium für die Pharmacie, 68 : 49-58. Reinsch describes the preparation of catechol on p. 56: "Bekanntlich wird die Katechusäure bei der Destillation zerstört, während sich ein geringer Theil davon als krystallinischer Anflug sublimirt, welcher aber noch nicht näher untersucht worden ist. Diese Säure ist vielleicht dieselbe, welche ich bei der zerstörenden Destillation des Katechus erhalten; … " (As is well known, catechu acid is destroyed by distillation, while a small portion of it sublimates as a crystalline efflorescence, which however has still not been closely examined. This acid is perhaps the same one, which I obtained by destructive distillation of catechu; … ). On p. 58, Reinsch names the new compound: "Die Eigenschaften dieser Säure sind so bestimmt, dass man sie füglich als eine eigenthümliche Säure betrachten und sie mit dem Namen Brenz-Katechusäure belegen kann." (The properties of this acid are so definite, that one can regard it justifiably as a strange acid and give it the name "burned catechu acid".)
3. ↑ See:
* H. Wackenroder (1841) "Eigenschaften der Catechusäure" (Properties of catechu acid), Annalen der Chemie und Pharmacie, 37 : 306-320.
* Constantin Zwenger (1841) "Ueber Catechin" (On catechin), Annalen der Chemie und Pharmacie, 37 : 320-336.
* (Anon.) (1841) "On catechin (catechinic acid) and pyrocatechin (pyrocatechinic acid)", Philosophical Magazine, 19 : 194-195.
4. ↑ See:
* Rudolf Wagner (1852) "Ueber die Farbstoffe des Gelbholzes (Morus tinctoria.)" (On the coloring matter of Dyer's mulberry (Morus tinctoria.)), Journal für praktische Chemie, 55 : 65-76. See p. 65.
* August Kekulé (1867) "Ueber die Sulfosäuren des Phenols" (On the sulfonates of phenol) Zeitschrift für Chemie, new series, 3 : 641–646; see p. 643.
* Joseph Alfred Naquet, with William Cortis, trans. and Thomas Stevenson, ed., Principles of Chemistry, founded on Modern Theories, (London, England: Henry Renshaw, 1868), p. 657. See also p. 720.
5. ↑ See:
* In 1879, the Publication Committee of the Journal of the Chemical Society issued instructions to its abstractors to "Distinguish all alcohols, i.e., hydroxyl-derivations of hydrocarbons, by names ending in ol, e.g., quinol, catechol, … " See: Alfred H. Allen (June 20, 1879) "Nomenclature of organic bodies," English Mechanic and World of Science, 29 (743) : 369.
* William Allen Miller, ed., Elements of Chemistry: Theoretical and Practical, Part III: Chemistry of Carbon Compounds or Organic Chemistry, Section I … , 5th ed. (London, England: Longmans, Green and Co., 1880), p.524.
6. ↑ Zheng, L. T.; Ryu, G. M.; Kwon, B. M.; Lee, W. H.; Suk, K. (2008). "Anti-inflammatory effects of catechols in lipopolysaccharide-stimulated microglia cells: Inhibition of microglial neurotoxicity". European Journal of Pharmacology 588 (1): 106–13. doi:10.1016/j.ejphar.2008.04.035. PMID 18499097.
7. ↑ 7.0 7.1 H. D. Dakin, H. T. Clarke, E. R. Taylor (1923). "Catechol". Organic Syntheses 3: 28. doi:10.15227/orgsyn.003.0028.
8. ↑ R. S. Hanslick, W. F. Bruce, A. Mascitti (1953). "o-Phenylene Carbonate". Org. Synth. 33: 74. doi:10.15227/orgsyn.033.0074.
9. ↑ Anderson, Bryan F.; Buckingham, David A.; Robertson, Glen B.; Webb, John; Murray, Keith S.; Clark, Paul E. (1976). "Models for the bacterial iron-transport chelate enterochelin". Nature 262 (5570): 722–724. doi:10.1038/262722a0. PMID 134287. Bibcode: 1976Natur.262..722A.
10. ↑ Schweigert, Nina; Zehnder, Alexander J. B.; Eggen, Rik I. L. (2001). "Chemical properties of catechols and their molecular modes of toxic action in cells, from microorganisms to mammals. Minireview". Environmental Microbiology 3 (2): 81–91. doi:10.1046/j.1462-2920.2001.00176.x. PMID 11321547.
11. ↑ Griffith, W. P. (1993). "Recent Advances in Dioxolene Chemistry". Transition Metal Chemistry 18 (2): 250–256. doi:10.1007/BF00139966.
12. ↑ PDB: 2ZI8​; "Studies of a ring-cleaving dioxygenase illuminate the role of cholesterol metabolism in the pathogenesis of Mycobacterium tuberculosis". PLOS Pathog. 5 (3). March 2009. doi:10.1371/journal.ppat.1000344. PMID 19300498.
13. ↑ Bolton, Judy L.; Dunlap, Tareisha L.; Dietz, Birgit M. (2018). "Formation and Biological Targets of Botanical o-Quinones". Food and Chemical Toxicology 120: 700–707. doi:10.1016/j.fct.2018.07.050. PMID 30063944.
14. ↑ Briggs DEG (1999). "Molecular taphonomy of animal and plant cuticles: selective preservation and diagenesis". Philosophical Transactions of the Royal Society B: Biological Sciences 354 (1379): 7–17. doi:10.1098/rstb.1999.0356.
15. ↑ Encyclopedia of Reagents for Organic Synthesis, 2001, doi:10.1002/047084289X, ISBN 978-0-471-93623-7
16. ↑ Fahlbusch, Karl-Georg; Hammerschmidt, Franz-Josef; Panten, Johannes; Pickenhagen, Wilhelm; Schatkowski, Dietmar; Bauer, Kurt; Garbe, Dorothea; Surburg, Horst (2003), "Flavors and Fragrances", Ullmann's Encyclopedia of Industrial Chemistry, doi:10.1002/14356007.a11_141, ISBN 978-3-527-30385-4
17. ↑ Eder, Josef Maria (January 1880). "Ueber die chemische Zusammensetzung des Pyroxylins und die Formel der Cellulose" (in de). Berichte der deutschen chemischen Gesellschaft 13 (1): 169–186. doi:10.1002/cber.18800130151. ISSN 0365-9496.
18. ↑ Eder, Josef Maria (1880) (in de). Der neue Eisenoxalat-Entwickler: und dessen Vergleichung mit dem Pyrogallus-Entwickler. Vienna: Verlag der Photographischen Correspondenz: Hermann Vogel. OCLC 80152687.
19. ↑ 19.0 19.1 Stephen G. Anchell (2012-09-10). The Darkroom Cookbook. Taylor & Francis. ISBN 978-1-136-09277-0.
20. ↑ Wolfgang, Moersch (13 January 2025). "TANOL-AB-EN.pdf" (PDF). https://www.moersch-photochemie.de/wp-content/uploads/2025/03/TANOL-AB-EN-.pdf#page=3.
21. ↑ Stephen G. Anchell; Bill Troop (1998). The Film Developing Cookbook. ISBN 978-0-240-80277-0.
22. ↑ Preferred IUPAC Names. September 2004, Chapter 6, Sect 60–64, p. 38
23. ↑ IUPAC, Commission on Nomenclature of Organic Chemistry. A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993) R-5.5.1.1 Alcohols and phenols.
24. ↑ Panico, R., ed (1994). A Guide to IUPAC Nomenclature of Organic Compounds 1993. Oxford: Blackwell Science. ISBN 978-0-632-03488-8.

This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). "Catechu". Encyclopædia Britannica (11th ed.). Cambridge University Press.

## External links

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

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