Benzenediol

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Benzenediols or dihydroxybenzenes are aromatic chemical compounds and polyphenols in which two hydroxyl groups are substituted onto a benzene ring. Because they have at least one hydroxyl group covalently bonded directly to a carbon atom in a benzene ring, they are in a class of organic compounds called phenols. There are three isomers of bezenediol, each of which has its particular name as shown in the mini-table below. Various other ways of naming these three chemical compounds are also shown:

ortho isomer	meta isomer	para isomer
Pyrocatechol 1,2-benzenediol o-benzenediol 1,2-dihydroxybenzene o-dihydroxybenzene catechol	Resorcinol 1,3-benzenediol m-benzenediol 1,3-dihydroxybenzene m-dihydroxybenzene resorcin	Hydroquinone 1,4-benzenediol p-benzenediol 1,4-dihydroxybenzene p-dihydroxybenzene

All three of these compounds are colorless to white granular solids at room temperature and pressure, but upon exposure to oxygen they may darken. All three isomers have the chemical formula C₆H₆O₂. The chemical structures are shown here:

Similar to other phenols, the hydroxyl groups on the aromatic ring of a benzenediol are weakly acidic. Each benzenediol can lose an H⁺ from one of the hydroxyls to form a type of phenolate ion. Hydroquinone can lose an H⁺ from both to form a diphenolate ion. The disodium diphenolate salt of hydroquinone is used as an alternating comonomer unit in the production of the polymer PEEK.

Occurrence and uses

Urushiols are naturally existing organic compounds which have the catechol skeleton structure and diphenol functionality but with alkyl groups substituted onto the aromatic ring. Urushiols are the skin-irritating poisons found in plants like poison ivy, etc. Catecholamines are biochemically significant hormones/neurotransmitters which are phenethylamines where the phenyl group has a catechol skeleton structure. Two parts of a molecule of catechin, another natural compound which is used for tea, have the catechol skeleton structure in it.

Both hydroquinone and catechol have been used as film developing chemicals. The use of hydroquinone for film developing is common.

Hydroquinone can undergo mild oxidation to convert to the compound parabenzoquinone, C₆H₄O₂, often called p-quinone or simply quinone. Reduction of quinone reverses this reaction back to hydroquinone. Some biochemical compounds in nature have this sort of hydroquinone or quinone section in their structures, such as Coenzyme Q, and can undergo similar redox interconversions. Hydroquinone has a variety of uses principally associated with its action as a reducing agent which is soluble in water. It is a major component in most photographic developers where, with the compound Metol, it reduces silver halides to elemental silver.

In human medicine, hydroquinone also used as a topical application to reduce the color of skin. It does not have the same predisposition to cause dermatitis as Metol does.

See also

• Urushiol