o-diphenol

Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is a toxic organic compound with the molecular formula . 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.

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

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''). 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''. 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''. In 1879, the '' Journal of the Chemical Society'' recommended that catechol be called "catechol", and in the following year, it was listed as such.

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.

Catechol is produced industrially by the hydroxylation of phenol using hydrogen peroxide.Fiegel, Helmut ''et al.'' (2002) "Phenol Derivatives" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH: Weinheim. .
:C6H5OH + H2O2 -> C6H4(OH)2 + H2O

It can be produced by reaction of salicylaldehyde with base and hydrogen peroxide (Dakin oxidation), 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 bond as promoted by hydroiodic acid (HI).

Reactions

Organic chemistry

Like other difunctional benzene derivatives, catechol readily condenses to form heterocyclic compounds. Cyclic esters are formed upon treatment with dichloro electrophiles. For example, using phosphorus trichloride or phosphorus oxychloride gives the cyclic chlorophosphonite or chloro phosphonate, respectively; sulfuryl chloride gives the sulfate; and phosgene () gives the carbonate:
:C6H4(OH)2 + XCl2 -> C6H4(O2X) + 2 HCl where X = PCl or POCl; ; CO

With metal ions

Basic solutions of catechol react with iron(III) to give the red . 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. Iron-containing dioxygenase enzymes catalyze the cleavage of catechol.

Redox chemistry

Catechols convert to the semiquinone radical. At , this conversion occurs at 100 mV:
:C6H4(OH)2 -> C6H4(O)(OH) + 1/2 H2

For the redox of the semiquinone radical to the catecholate dianion, the potential ranges from 530 to 43 mV as the pH varies from 7 to 13.5:
:C6H4(OH)2 -> C6H4O2^2- + H+

Catechol is produced by a reversible two-electron, two-proton reduction of 1,2-benzoquinone ( vs SHE; vs SHE).

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

Natural occurrences

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 citric acid contained in lemon juice. Excluding oxygen also prevents the browning reaction. However, the activity of the enzyme increases in cooler temperatures. Benzoquinone is said to be antimicrobial, a property that slows the spoilage of damaged fruits and other plant parts.

Catechol derivatives

DHSA.svg, 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione, a metabolite of cholesterol;
Catechin structure.svg, Catechin, a component of tea.
Piceatannol.svg, Piceatannol, an antioxidant found in some red wines.
Urushiol.svg, urushiols, the active agent in poison ivy (R = (CH₂)₁₄CH₃, (CH₂)₇CH=CHCH₂CH=CHCH₂CH=CH₂, and others)
catecholamine.svg, catecholamines, drugs imitating them (such as MDMA), hormones/neurotransmitters
Dopamine.svg, Dopamine, derived from the amino acid tyrosine, an adhesive used by mussels.
Quercetin.svg, Quercetin, which is found in many foods.

Catechol derivatives are found widely in nature. They often arise by hydroxylation of phenols.
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.

4-tert-Butylcatechol, which is synthetic, not natural, is used as an antioxidant and polymerisation 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. It is a common building block in organic synthesis. 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