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. 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. A potentially industrially significant of
source of hydroquinone can be synthesized from acetylene and iron pentacarbonyl:
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. 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.
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. The
useful dye quinizarin is produced by diacylation of hydroquinone with phthalic anhydride
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: 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 76EEC:1976. 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. 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. 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. 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 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 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, 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. 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. Hydroquinone has been shown to be one of the
chemical constituents of the natural product propolis.
It is also one of the chemical compounds found in castoreum. This compound is gathered from
the beaver's castor sacs. See also
Pyrocatechol Resorcinol
Photographic developer Semiquinone
References ^ http:assets.openstudy.comattachments/4f0d5cb6e4b084a815fccd72-chmvijay-1326448458607-dipolemoment.pdf
^ 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.
^ a b Reppe, Walter; Kutepow, N; and Magin, A. "Cyclization of Acetylenic Compounds".
Angewandte Chemie International Edition in English 8: 727–733. doi:10.1002/anie.196907271.
Retrieved 26 December 2013. ^ Hubel, Karl and Braye, Henri. Process for
the preparation of substituted cyclic compounds and products resulting therefrom US3149138
A. Union Carbide Corp. ^ Pino, Piero; Braca, Giuseppe; and Sbrana,
Glauco. Preparation of hydroquinone US3355503 A. Lonza Ag.
^ Walter, Reppe and Magin, August. Production of hydroquinones US3394193 A. Basf Ag.
^ Piero, Pino; Giuseppe, Braca; Frediano, Settimo; and Glauco, Sbrana. Preparation of
hydroquinone US3459812 A. Lonza Ag. ^ Holmes, J. and Hagemeyer, H.. Process for
the production of hydroquinone US 3742071 A. Eastman Kodak Co.
^ Roscoe, Henry. A treatise on chemistry, Volume 3, Part 3. London: Macmillan & Co.
p. 165. ^ 76EEC:1976 Council Directive 76EEC of 27
July 1976 on the approximation of the laws of the Member States relating to cosmetic
products : http:eur-lex.europa.euLexUriServ.do?uri=CELEX:31976L0768:EN:HTML ^ Example of a product recall in Ireland
^ a b United States Food and Drug Administration. ﻿Skin Bleaching Drug Products for Over-the-Counter
Product Use; Proposed Rule﻿. 1978N-0065. http:www.fda.govDOCKETS78n-0065-npr0003.pdf.
^ Campaign For Safe Cosmetics - Hydroquinone ^ Olumide, YM; Akinkugbe, AO; Altraide, D;
Mohammed, T; Ahamefule, N; Ayanlowo, S; Onyekonwu, C; Essen, N. "Complications of chronic use
of skin lightening cosmetics". International Journal of Dermatology 47: 344–53. doi:10.1111/j.1365-4632.2008.02719.x.
PMID 18377596. ^
Organic Chemistry, Solomon and Fryhle, 10th edition, Wiley Publishing, 2010.
^ Joval, E; Kroeger, P; N. "Hydroquinone: the toxic compound of Agaricus hondensis".
Planta Medica 62: 185. doi:10.1055/s-2006-957852. PMID 17252436.
^ G.A. Burdock, Review of the biological properties and toxicity of bee propolis, Food and Chemical
Toxicology, Volume 36, Issue 4, 6 April 1998, Pages 347-363, ISSN 0278-6915, http:dx.doi.orgS0278-6915(97)00145-2.
^ The Beaver: Its Life and Impact. Dietland Muller-Schwarze, 2003, page 43
External links International Chemical Safety Card 0166
NIOSH Pocket Guide to Chemical Hazards IARC Monograph: "Hydroquinone"
IUPAC Nomenclature of Organic Chemistry