# Hydroquinone

Hydroquinone
Identifiers
CAS number 123-31-9
ChemSpider 764
UNII XV74C1N1AE
KEGG D00073
ChEBI CHEBI:17594
ChEMBL CHEMBL537
RTECS number MX3500000
ATC code D11AX11
Jmol-3D images Image 1
Properties
Molecular formula C6H6O2
Molar mass 110.11 g mol−1
Appearance white solid
Density 1.3 g cm−3, solid
Melting point 172 °C (342 °F; 445 K)
Boiling point 287 °C (549 °F; 560 K)
Solubility in water 5.9 g/100 mL (15 °C)
Structure
Dipole moment 1.4 D
Hazards
EU classification Harmful (Xn)
Carc. Cat. 3
Muta. Cat. 3
Dangerous for
the environment (N)
R-phrases R22 R40 R41 R43 R50 R68
S-phrases (S2) S26 S36/37/39 S61
NFPA 704
Flash point 165 °C (329 °F; 438 K)
Related compounds
Related benzenediols Pyrocatechol
Resorcinol
Related compounds 1,4-benzoquinone
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Hydroquinone, also benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, having the chemical formula C6H4(OH)2. Its chemical structure, shown in the table at right, features two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid. Substituted derivatives of this parent compound are also referred to as hydroquinones.

## Production

Hydroquinone is produced industrially by three routes, two of which are dominant.[2] Similar to the cumene process in reaction mechanism, the most widely used route involves the dialkylation of benzene with propene to give 1,4-diisopropylbenzene. This compound reacts with air to afford the bis(hydroperoxide), which is structurally similar to cumene hydroperoxide and rearranges to give acetone and hydroquinone in acid. A second route involves hydroxylation of phenol. The conversion uses hydrogen peroxide and affords a mixture of hydroquinone and catechol:

C6H5OH + H2O2 → C6H4(OH)2 + H2O

The third method, practiced only in China, is the oxidation of aniline by manganese dioxide followed by reduction of the resulting 1,4-benzoquinone. The process is conducted batchwise and generates a substantial waste stream.[citation needed]

A potentially industrially significant of source of hydroquinone can be synthesized from acetylene and iron pentacarbonyl:[3][4][5][6][7][8]

$\mathrm{ Fe(CO)_5 + 4C_2H_2 + 2H_2O \xrightarrow[basic\ conditions]{ \begin{array}{c} 50-80\ ^{\circ} \mathrm{C} \\ 20-25\ \mathrm{atm} \end{array} } 2C_6H_4(OH)_2 + FeCO_3 }$

Iron pentacarbonyl serves as a catalyst, rather than as a reagent, in the presence of free carbon monoxide gas. Rhodium or ruthenium can substitute for iron as the catalyst with favorable chemical yields but are not typically used due to their cost of recovery from the reaction mixture.[3]

Hydroquinone and its derivatives can also be prepared by oxidation of various phenols. Examples include Elbs persulfate oxidation and Dakin oxidation:

Historically hydroquinone was obtained from the dry distillation of quinic acid.[9]

## Reactions

In term of the reactivity of its O-H groups, hydroquinone resembles other phenols, being weakly acidic. The resulting conjugate base undergoes easy O-alkylation to give mono- and diethers. Similarly, hydroquinone is highly susceptible to ring substitution by Friedel-Crafts reactions such as alkylation. This reaction is exploited en route to popular antioxidants such as 2-tert-butyl-4-methoxyphenol ("BHA"). The useful dye quinizarin is produced by diacylation of hydroquinone with phthalic anhydride[2]

### Redox

Hydroquinone undergoes oxidation under mild conditions to give benzoquinone. This process can be reversed. Some naturally occurring hydroquinone derivatives exhibit this sort of reactivity, one example being coenzyme Q. Industrially this reaction is exploited both with hydroquinone itself but more often with its derivatives where one OH has been replaced by an amine.

### Amination

An important reaction is the conversion of hydroquinone to the mono- and diamino derivatives. Methylaminophenol, used in photography, is produced in this way:[2]

C6H4(OH)2 + CH3NH2 → C6H4(OH)(N(H)CH3) + H2O

Similarly diamines, useful in the rubber industry as antiozone agents, are produced similarly from aniline:

C6H4(OH)2 + 2 C6H5NH2 → C6H4(N(H)C6H5)2 + 2 H2O

## Uses

Hydroquinone has a variety of uses principally associated with its action as a reducing agent that is soluble in water. It is a major component in most black and white photographic developers for film and paper where, with the compound Metol, it reduces silver halides to elemental silver.

There are various other uses associated with its reducing power. As a polymerization inhibitor, hydroquinone prevents polymerization of acrylic acid, methyl methacrylate, cyanoacrylate, and other monomers that are susceptible to radical-initiated polymerization. This application exploits the antioxidant properties of hydroquinone.

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 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.

### Skin depigmentation

In human medicine, hydroquinone is used as a topical application in skin whitening to reduce the color of skin. It does not have the same predisposition to cause dermatitis as metol does. This use is banned in some countries, including the member states of the European Union under Directive 76/768/EEC:1976.[10][11]

In 2006, the United States Food and Drug Administration revoked its previous approval of hydroquinone and proposed a ban on all over-the-counter preparations.[12] The FDA stated that hydroquinone cannot be ruled out as a potential carcinogen. This conclusion was reached based on the extent of absorption in humans and the incidence of neoplasms in rats in several studies where adult rats were found to have increased rates of tumours, including thyroid follicular cell hyperplasias, anisokaryosis, mononuclear cell leukemia, hepatocellular adenomas and renal tubule cell adenomas. The Campaign for Safe Cosmetics has also highlighted concerns.[13]

Numerous studies have revealed that hydroquinone can cause exogenous ochronosis, a disfiguring disease in which blue-black pigments are deposited onto the skin, if taken orally; however, skin preparations containing the ingredient are administered topically. The FDA has classified hydroquinone currently as a safe product, as currently used.[12][14]

While using hydroquinone as lightening agent can be effective with proper use, it can also cause skin sensitivity. Using a daily sunscreen with a high PPD (persistent pigment darkening) rating reduces the risk of further damage. Hydroquinone is sometimes combined with alpha hydroxy acids that exfoliate the skin to quicken the lightening process. In the United States, topical treatments usually contain up to 2% in hydroquinone. Otherwise, higher concentrations (up to 4%) should be prescribed and used with caution.

## Natural occurrences

Hydroquinones are one of the two primary reagents in the defensive glands of bombardier beetles, along with hydrogen peroxide (and perhaps other compounds, depending on the species), which collect in a reservoir. The reservoir opens through a muscle-controlled valve onto a thick-walled reaction chamber. This chamber is lined with cells that secrete catalases and peroxidases. When the contents of the reservoir are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide and catalyze the oxidation of the hydroquinones into p-quinones. These reactions release free oxygen and generate enough heat to bring the mixture to the boiling point and vaporize about a fifth of it, producing a hot spray from the beetle's abdomen.[15]

Farnesyl hydroquinone derivatives are the principal irritants exuded by the poodle-dog bush, which can cause severe contact dermatitis in humans.

Hydroquinone is thought to be the active toxin in Agaricus hondensis mushrooms.[16]

Hydroquinone has been shown to be one of the chemical constituents of the natural product propolis.[17]

It is also one of the chemical compounds found in castoreum. This compound is gathered from the beaver's castor sacs.[18]

## References

2. ^ a b c Phillip M. Hudnall "Hydroquinone" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. 2005 Wiley-VCH, Weinheim. doi:10.1002/14356007.a13_499.
3. ^ a b Reppe, Walter; Kutepow, N; and Magin, A (1969). "Cyclization of Acetylenic Compounds". Angewandte Chemie International Edition in English 8 (10): 727–733. doi:10.1002/anie.196907271. Retrieved 26 December 2013.
4. ^ Hubel, Karl and Braye, Henri (1960). Process for the preparation of substituted cyclic compounds and products resulting therefrom US3149138 A. Union Carbide Corp.
5. ^ Pino, Piero; Braca, Giuseppe; and Sbrana, Glauco (1964). Preparation of hydroquinone US3355503 A. Lonza Ag.
6. ^ Walter, Reppe and Magin, August (1966). Production of hydroquinones US3394193 A. Basf Ag.
7. ^ Piero, Pino; Giuseppe, Braca; Frediano, Settimo; and Glauco, Sbrana (1967). Preparation of hydroquinone US3459812 A. Lonza Ag.
8. ^ Holmes, J. and Hagemeyer, H. (1971). Process for the production of hydroquinone US 3742071 A. Eastman Kodak Co.
9. ^ Roscoe, Henry (1891). A treatise on chemistry, Volume 3, Part 3. London: Macmillan & Co. p. 165.
10. ^ 76/768/EEC:1976 Council Directive 76/768/EEC of 27 July 1976 on the approximation of the laws of the Member States relating to cosmetic products : http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31976L0768:EN:HTML
11. ^ Example of a product recall in Ireland
12. ^ a b United States Food and Drug Administration (2006). ﻿Skin Bleaching Drug Products for Over-the-Counter Product Use; Proposed Rule﻿ (Report). 1978N-0065.
13. ^ Campaign For Safe Cosmetics - Hydroquinone
14. ^ Olumide, YM; Akinkugbe, AO; Altraide, D; Mohammed, T; Ahamefule, N; Ayanlowo, S; Onyekonwu, C; Essen, N (April 2008). "Complications of chronic use of skin lightening cosmetics". International Journal of Dermatology 47 (4): 344–53. doi:10.1111/j.1365-4632.2008.02719.x. PMID 18377596.
15. ^ Organic Chemistry, Solomon and Fryhle, 10th edition, Wiley Publishing, 2010.
16. ^ Joval, E; Kroeger, P; N (April 1996). "Hydroquinone: the toxic compound of Agaricus hondensis". Planta Medica 62 (2): 185. doi:10.1055/s-2006-957852. PMID 17252436.
17. ^ G.A. Burdock, Review of the biological properties and toxicity of bee propolis (propolis), Food and Chemical Toxicology, Volume 36, Issue 4, 6 April 1998, Pages 347-363, ISSN 0278-6915, http://dx.doi.org/10.1016/S0278-6915(97)00145-2. (http://www.sciencedirect.com/science/article/pii/S0278691597001452)
18. ^ The Beaver: Its Life and Impact. Dietland Muller-Schwarze, 2003, page 43 (book at google books)