Catechol, also known as catechol or 1,2-dihydroxybenzene, is a toxic organic compound with a molecular formula of C6H4(OH)2. It is the ortho isomer of the three quinone isomers. This colorless compound occurs naturally in trace amounts. It was first discovered by the decomposition and distillation of plant extracts catechins. About 20,000 tons of catechols are now synthetically produced annually as commodity organic chemicals, mainly as precursors for pesticides, flavors and fragrances.

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

Edgar Hugo Emil Reinsch (1809-1884) was the first to distill and isolate catechins in 1839 from solid tannins to prepare catechins, the residue of catechins, boiled or juice concentrate. [4] When the catechins are heated above their decomposition point, what Reinsch originally named Brenz-Katechusäure (burned catechins) sublimates into a white efflorescence. This is a thermal breakdown product of flavanols in catechins. In 1841, Wackenroder and Zwenger independently rediscovered catechol; in reporting their discovery, Philosophical Magazine coined the name pyrocatechin. [5] By 1852, Erdmann realized that catechol was benzene with two oxygen atoms added; in 1867, August Kekulé found that catechol was a diol of benzene, so by 1868, catechol was listed as an ortho Hydroquinone [6]. In 1879, the Journal of the Chemical Society suggested that catechol be called "catechol", and it was listed as "catechol" the following year [7].

Catechol has been shown to occur naturally in free form in kino and beech wood tar. Its sulfonic acid has been detected in horse and human urine. [8]

Catechol is produced industrially by the hydroxylation of phenol with hydrogen peroxide.

It can be produced by the reaction of salicylaldehyde with base and hydrogen peroxide (Dakin oxidation) [10] and by the hydrolysis of 2-substituted phenols, especially 2-chlorophenol, with hot aqueous solutions containing alkali metal hydroxides. Its methyl ether derivative, guaiacol, is converted to catechol by hydrolysis of the CH3-O bond facilitated by hydroiodic acid (HI).

An alkaline solution of catechol reacts with iron(III) to form red [Fe(C6H4O2)3]3−. Ferric chloride is green in aqueous solution, while alkaline solution will quickly turn green and finally black when exposed to air. [12] Iron-containing dioxygenases catalyze the cleavage of catechols.