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			{{Short description|Chemical compound}}
			{{Use dmy dates|date=January 2024}}
	{{chembox		{{chembox
			| Watchedfields = changed
	| verifiedrevid = 407839026
			| verifiedrevid = 443862076
	| Name = Hydroquinone
	| ImageFile_Ref = {{chemboximage|correct|??}}		| Name = Hydroquinone
			| ImageFile_Ref = {{chemboximage|correct|??}}
	| ImageFile = Hydrochinon2.svg		| ImageFile = Hydrochinon2.svg
	| ImageSize = 150px		| ImageSize =
	| ImageName = Hydroquinone		| ImageName = Hydroquinone
	| ImageFile1 = Trans-hydroquinone-from-xtal-3D-balls.png		| ImageFile1 = Trans-hydroquinone-from-xtal-3D-balls.png
	| ImageSize1 = 150px		| ImageSize1 =
	| ImageFile2 = Hydroquinone crystal.jpg		| ImageFile2 = Hydroquinone crystal.jpg
			| PIN = Benzene-1,4-diol<ref name=iupac2013>{{cite book | title = Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book) | publisher = ] | date = 2014 | location = Cambridge | page = 691 | doi = 10.1039/9781849733069-FP001 | isbn = 978-0-85404-182-4| chapter = Front Matter }}</ref>
	| IUPACName = Benzene-1,4-diol
	| OtherNames = Hydroquinone<br />Idrochinone<br />Quinol/1-4 dihydroxy benzene/1-4 hydroxy benzene		| OtherNames = Hydroquinone<ref name=iupac2013 /><br />Idrochinone<br />Quinol<br />4-Hydroxyphenol<br />1,4-Dihydroxybenzene<br />''p''-Dihydroxybenzene<br />''p''-Benzenediol
	| Section1 = {{Chembox Identifiers		|Section1={{Chembox Identifiers
			| ChEBI_Ref = {{ebicite|correct|EBI}}
	| ChEBI = 17594
			| ChEBI = 17594
	| SMILES = c1cc(ccc1O)O		| SMILES = c1cc(ccc1O)O
	| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}		| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
	| ChemSpiderID = 764		| ChemSpiderID = 764
	| ChEMBL_Ref = {{ebicite|correct|EBI}}		| ChEMBL_Ref = {{ebicite|correct|EBI}}
	| ChEMBL = 537		| ChEMBL = 537
			| DrugBank = DB09526
	| UNII_Ref = {{fdacite|correct|FDA}}		| UNII_Ref = {{fdacite|correct|FDA}}
	| UNII = XV74C1N1AE		| UNII = XV74C1N1AE
	| KEGG_Ref = {{keggcite|correct|kegg}}		| KEGG_Ref = {{keggcite|correct|kegg}}
	| KEGG = D00073		| KEGG = D00073
			| Beilstein = 605970
			| Gmelin = 2742
			| PubChem = 785
			| EC_number = 204-617-8
			| UNNumber = 3077, 2662
	| InChI = 1/C6H6O2/c7-5-1-2-6(8)4-3-5/h1-4,7-8H		| InChI = 1/C6H6O2/c7-5-1-2-6(8)4-3-5/h1-4,7-8H
	| InChIKey = QIGBRXMKCJKVMJ-UHFFFAOYAX		| InChIKey = QIGBRXMKCJKVMJ-UHFFFAOYAX
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	| CASNo_Ref = {{cascite|correct|CAS}}		| CASNo_Ref = {{cascite|correct|CAS}}
	| CASNo = 123-31-9		| CASNo = 123-31-9
	| RTECS = MX3500000		| RTECS = MX3500000
	}}		}}
	| Section2 = {{Chembox Properties		|Section2={{Chembox Properties
			| C=6 | H=6 | O=2
	| Formula = C<sub>6</sub>H<sub>4</sub>(OH)<sub>2</sub>
	| MolarMass = 110.1 g/mol		| Appearance = White solid
	| Appearance = white solid		| Density = 1.3 g cm<sup>−3</sup>, solid
			| Solubility = 5.9 g/100 mL (15&nbsp;°C)
	| Density = 1.3 g/cm<sup>3</sup>, solid
			| MeltingPtC = 172
	| Solubility = 5.9 g/100 ml (15 °C)
	| MeltingPtC = 172		| BoilingPtC = 287
			| pKa = 9.9<ref>{{cite web |url= http://www.inchem.org/documents/sids/sids/123319.pdf |title= Hydroquinone |publisher= UNEP Publications |work= ] |access-date= 17 September 2018 |archive-date= 20 October 2016 |archive-url= https://web.archive.org/web/20161020081945/http://www.inchem.org/documents/sids/sids/123319.pdf |url-status= dead }}</ref>
	| BoilingPtC = 287
			| VaporPressure = {{val|e=-5|u=mmHg}} (20&nbsp;°C)<ref name=PGCH/>
	| pKa =
			| MagSus = {{val|-64.63e-6|u=cm<sup>3</sup>/mol}}
	}}		}}
	| Section3 = {{Chembox Structure		|Section3={{Chembox Structure
			| Dipole = {{val|1.4|0.1|u=]}}<ref>{{cite journal|doi=10.1021/ja01218a051|title=The Dipole Moments of Catechol, Resorcinol and Hydroquinone|year=1945|last1=Lander|first1=John J.|last2=Svirbely|first2=John J. Lander, W. J.|journal=Journal of the American Chemical Society|volume=67|issue=2|pages=322–324}}</ref>
	| Dipole = zero
	}}		}}
	| Section7 = {{Chembox Hazards		| Section6 = {{Chembox Pharmacology
			| Pharmacology_ref =
	| ExternalMSDS =
			| ATCCode_prefix = D11
	| EUClass = Harmful ('''Xn''')<br />]<br />]<br />Dangerous for<br />the environment ('''N''')
	| NFPA-H = 2		| ATCCode_suffix = AX11
	| NFPA-F = 1		| ATC_Supplemental =
	| NFPA-R =		| ATCvet =
			| Licence_EU =
	| RPhrases = {{R22}} {{R40}} {{R41}} {{R43}} {{R50}} {{R68}}
			| INN =
	| SPhrases = {{S2}} {{S26}} {{S36/37/39}} {{S61}}
	| FlashPt = 165 °C		| INN_EMA =
			| Licence_US =
			| Legal_status =
			| Legal_AU =
			| Legal_AU_comment =
			| Legal_CA =
			| Legal_CA_comment =
			| Legal_NZ =
			| Legal_NZ_comment =
			| Legal_UK =
			| Legal_UK_comment =
			| Legal_US =
			| Legal_US_comment =
			| Legal_EU =
			| Legal_EU_comment =
			| Legal_UN =
			| Legal_UN_comment =
			| Pregnancy_category =
			| Pregnancy_AU =
			| Pregnancy_AU_comment =
			| Dependence_liability =
			| AdminRoutes =
			| Bioavail =
			| ProteinBound =
			| Metabolism =
			| Metabolites =
			| OnsetOfAction =
			| HalfLife =
			| DurationOfAction =
			| Excretion =
	}}		}}
	| Section8 = {{Chembox Related		|Section7={{Chembox Hazards
			| ExternalSDS =
	| Function = ]s
			| NFPA-H = 2
	| OtherFunctn = ]<br />]
			| NFPA-F = 1
	| OtherCpds = ]
			| NFPA-R =
			| NFPA_ref =<ref>{{cite web |url=http://assets.openstudy.com/updates/attachments/4f0d5cb6e4b084a815fccd72-chmvijay-1326448458607-dipolemoment.pdf |title=Archived copy |access-date=25 January 2014 |url-status=live |archive-url=https://web.archive.org/web/20140202184040/http://assets.openstudy.com/updates/attachments/4f0d5cb6e4b084a815fccd72-chmvijay-1326448458607-dipolemoment.pdf |archive-date=2 February 2014 }}</ref>
			| GHSPictograms = {{GHS05}}{{GHS07}}{{GHS08}}{{GHS09}}
			| GHSSignalWord = Danger
			| HPhrases = {{H-phrases|302|317|318|341|351|400}}
			| PPhrases = {{P-phrases|201|202|261|264|270|272|273|280|281|301+312|302+352|305+351+338|308+313|310|321|330|333+313|363|391|405|501}}
			| FlashPtC = 165
			| PEL = TWA 2 mg/m<sup>3</sup><ref name=PGCH>{{PGCH|0338}}</ref>
			| IDLH = 50 mg/m<sup>3</sup><ref name=PGCH/>
			| REL = C 2 mg/m<sup>3</sup> <ref name=PGCH/>
			| LD50 = 490 mg/kg (mammal, oral)<br/>245 mg/kg (mouse, oral)<br/>200 mg/kg (rabbit, oral)<br/>320 mg/kg (rat, oral)<br/>550 mg/kg (guinea pig, oral)<br/>200 mg/kg (dog, oral)<br/>70 mg/kg (cat, oral)<ref>{{IDLH|123319|Hydroquinone}}</ref>
			}}
			|Section8={{Chembox Related
			| OtherFunction_label = ]s
			| OtherFunction = ]<br />]
			| OtherCompounds = ]
	}}		}}
	}}		}}
	'''Hydroquinone''', also '''benzene-1,4-diol''' or '''quinol''', is an ] ] that is a type of ], having the ] C<sub>6</sub>H<sub>4</sub>(OH)<sub>2</sub>. Its ], shown in the table at right, has two ]s ] to a ] in a ] position. It is a white granular ]. Substituted derivatives of this parent compound are also referred to as hydroquinones.		'''Hydroquinone''', also known as '''benzene-1,4-diol''' or '''quinol''', is an ] ] that is a type of ], a derivative of ], having the ] C<sub>6</sub>H<sub>4</sub>(OH)<sub>2</sub>. It has two ]s ] to a ] in a ] position. It is a white granular ]. Substituted derivatives of this parent compound are also referred to as hydroquinones. The name "hydroquinone" was coined by ] in 1843.<ref>F. Wöhler (1844) "Untersuchungen über das Chinon" (Investigations of quinone), ''Annalen der Chemie und Pharmacie'', '''51''' : 145-163. ''"Das so erhaltene Destillat … enthält … einen neuen, krystallisierenden Körper, den ich unter dem Namen ''farbloses Hydrochinon'' weiter unten näher beschreiben werde."'' (The distillate so obtained … contains … a new, crystallizable substance, that I will describe, under the name of ''colorless hydroquinone'', further below in more detail.) </ref>
			In 2022, it was the 268th most commonly prescribed medication in the United States, with more than 900,000 prescriptions.<ref>{{cite web | title=The Top 300 of 2022 | url=https://clincalc.com/DrugStats/Top300Drugs.aspx | website=ClinCalc | access-date=30 August 2024 | archive-date=30 August 2024 | archive-url=https://web.archive.org/web/20240830202410/https://clincalc.com/DrugStats/Top300Drugs.aspx | url-status=live }}</ref><ref>{{cite web | title = Hydroquinone Drug Usage Statistics, United States, 2013 - 2022 | website = ClinCalc | url = https://clincalc.com/DrugStats/Drugs/Hydroquinone | access-date = 30 August 2024 }}</ref>
	==Production==		==Production==
	Hydroquinone is produced industrially by three routes, two of which are dominant.<ref name=Ullmann>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}}.</ref> Similar to the ] in reaction mechanism, the most widely used route involves the dialkylation of benzene with ] to give ]. This compound reacts with air to afford the bis(hydroperoxide), which is structurally similar to ] and rearranges to give acetone and hydroquininone in acid. A second route involves ] of ]. The conversion uses ] and affords a mixture of hydroquinone and ]:		Hydroquinone is produced industrially in two main ways.<ref name=Ullmann>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}}.</ref>
			* The most widely used route is similar to the ] in reaction mechanism and involves the dialkylation of ] with ] to give 1,4-diisopropylbenzene. This compound reacts with air to afford the bis(hydroperoxide), which is structurally similar to ] and rearranges in acid to give ] and hydroquinone.<ref>Gerhard Franz, Roger A. Sheldon "Oxidation" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim, 2000 {{doi|10.1002/14356007.a18_261}}</ref>
			* A second route involves ] of ] over a catalyst. The conversion uses ] and affords a mixture of hydroquinone and its ortho isomer ] (benzene-1,2-diol):
			*:<chem>C6H5OH + H2O2 -> C6H4(OH)2 + H2O</chem>
	:C<sub>6</sub>H<sub>5</sub>OH + H<sub>2</sub>O<sub>2</sub> → C<sub>6</sub>H<sub>4</sub>(OH)<sub>2</sub> + H<sub>2</sub>O
			Other, less common methods include:
			* A potentially significant synthesis of hydroquinone from ] and ] has been proposed<ref name=Cyclization>{{cite journal|author1=Reppe, Walter |author2=Kutepow, N |author3=Magin, A |title=Cyclization of Acetylenic Compounds|journal=Angewandte Chemie International Edition in English|year=1969|volume=8|issue=10|pages=727–733|doi=10.1002/anie.196907271}}</ref><ref>{{cite book|author1=Hubel, Karl |author2=Braye, Henri|title=Process for the preparation of substituted cyclic compounds and products resulting therefrom US3149138 A|year=1960|publisher=Union Carbide Corp.|url=https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US3149138.pdf}}</ref><ref>{{cite book|author1=Pino, Piero |author2=Braca, Giuseppe |author3=Sbrana, Glauco |title=Preparation of hydroquinone US3355503 A|year=1964|publisher=Lonza Ag|url=https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US3355503.pdf}}</ref><ref>{{cite book|author1=Walter, Reppe |author2=Magin, August|title=Production of hydroquinones US3394193 A|year=1966|publisher=Basf Ag|url=https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US3394193.pdf}}</ref><ref>{{cite book|author1=Piero, Pino |author2=Giuseppe, Braca |author3=Frediano, Settimo |author4=Glauco, Sbrana |title=Preparation of hydroquinone US3459812 A|year=1967|publisher=Lonza Ag|url=https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US3459812.pdf}}</ref><ref>{{cite book|author1=Holmes, J. |author2=Hagemeyer, H.|title=Process for the production of hydroquinone US 3742071 A|year=1971|publisher=Eastman Kodak Co.|url=https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US3742071.pdf}}</ref> Iron pentacarbonyl serves as a ], rather than as a ], in the presence of free ] gas. ] or ] 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.<ref name=Cyclization />
	The third method, practiced only in China, is the oxidation of ] by ] followed by reduction of the resulting 1,4-benzoquinone. The process is conducted batchwise and generates a substantial waste stream.
			* Hydroquinone and its ] can also be prepared by oxidation of various phenols, such as ] and ].<ref name=":1">{{Cite journal |vauthors=Shi Y, Xia Y, Xu G, Wen L, Gao G, Zong B |date=28 October 2021 |title=Hydrogen peroxide and applications in green hydrocarbon nitridation and oxidation |doi=10.1016/j.cjche.2021.09.030 |journal=Chinese Journal of Chemical Engineering |type=Review article |volume=41 |pages=145–161|s2cid=240248911 }}</ref> Examples include ] and ].
			* Hydroquinone was first obtained in 1820 by the French chemists ] and ] via the ] of ].<ref>See:
			* Pelletier and Caventou (1820) "Recherches chimiques sur les quinquinas" (Chemical investigations of quinquinas ] trees]), ''Annales de Chimie et de Physique'', 2nd series, '''15''' : 289–318, 337-364. , the preparation and properties of ''l'acide pyro-kinique'' (pyroquinic acid or hydroquinone) are discussed.
			* {{cite book|last=Roscoe|first=Henry|title=A Treatise on Chemistry, Volume 3, Part 3|year=1891|publisher=Macmillan & Co.|location=London|pages=165|url=https://books.google.com/books?id=HEY9AAAAYAAJ&pg=PA165}}</ref>
			* ] of chlorinated ], described as being used in China.<ref name=":1" />
			Note that methods such as hydrolysis of chlorinated phenol and oxidation of phenols are much more polluting methods than some others.<ref name=":1" />
	==Reactions==		==Reactions==
	In term of the reactivity of its O-H groups, hydroquinone resembles other ]s, being weakly acidic. The resulting ] undergoes easy O-alkylation to give mono- and diethers. Similarly, hydroquinone is highly susceptible to ring substitution by ]s such as alkylation. This reaction is exploited en route to popular antioxidants such as 2-tert-butyl-		The reactivity of hydroquinone's ] groups resembles that of other ]s, being weakly acidic. The resulting ] undergoes easy ''O''-alkylation to give ]. Similarly, hydroquinone is highly susceptible to ring substitution by ]s such as alkylation. This reaction is exploited en route to popular antioxidants such as 2-''tert''-butyl-4-methoxyphenol (]). The useful dye ] is produced by diacylation of hydroquinone with ].<ref name=Ullmann/>
	4-methoxyphenol (]). The useful dye ] is produced by diacylation of hydroquinone with ]<ref name=Ullmann/>
	===Redox===		===Redox===
	Hydroquinone undergoes ] under mild conditions to give ]. This process can be reversed. Some naturally occurring hydroquinone derivatives exhibit this sort of reactivity, one example being ]. Industrially this reaction is exploited both with hydroquinone itself but more often with its derivatives where one OH has been replaced by an amine.		Hydroquinone undergoes ] under mild conditions to give ]. This process can be reversed. Some naturally occurring hydroquinone derivatives exhibit this sort of reactivity, one example being ]. Industrially this reaction is exploited both with hydroquinone itself but more often with its derivatives where one OH has been replaced by an amine.
			When colorless hydroquinone and benzoquinone, a bright yellow solid, are ]lized in a 1:1 ratio, a dark-green crystalline ] (] 171&nbsp;°C) called ] ({{chem2|C6H6O2*C6H4O2}}) is formed. This complex dissolves in hot water, where the two molecules dissociate in solution.<ref>{{Cite book|title=Introduction to organic chemistry|author=Streitwieser, Andrew|date=1992|publisher=Prentice Hall|others=Heathcock, Clayton H., 1936-, Kosower, Edward M.|isbn=978-0139738500|edition= 4th|location=Upper Saddle River, N.J.|oclc=52836313}}</ref>
	===Amination===		===Amination===
	An important reaction is the conversion of hydroquinone to the mono- and diamino derivatives. Methylaminophenol, used in photography, is produced in this way:<ref name=Ullmann/>		An important reaction is the conversion of hydroquinone to the mono- and diamine derivatives. ], used in photography, is produced in this way:<ref name=Ullmann/>
			:<chem>C6H4(OH)2 + \overset{methylamine}{CH3NH2} -> HOC6H4NHCH3 + H2O</chem>
	:C<sub>6</sub>H<sub>4</sub>(OH)<sub>2</sub> + CH<sub>3</sub>NH<sub>2</sub> → C<sub>6</sub>H<sub>4</sub>(OH)(N(H)CH<sub>3</sub>) + H<sub>2</sub>O
	Similarly diamines, useful in the rubber industry as antiozone agents, are produced similarly from ]:		Diamines, useful in the rubber industry as antiozone agents, are similarly produced from ]:
			:<chem>C6H4(OH)2 + \overset{aniline}{2 C6H5NH2} -> C6H4(N(H)C6H5)2 + 2 H2O</chem>
	:C<sub>6</sub>H<sub>4</sub>(OH)<sub>2</sub> + 2 C<sub>6</sub>H<sub>5</sub>NH<sub>2</sub> → C<sub>6</sub>H<sub>4</sub>(N(H)CH<sub>6</sub>H<sub>5</sub>)<sub>2</sub> + 2 H<sub>2</sub>O
	==Uses==		==Uses==
	Hydroquinone has a variety of uses principally associated with its action as a reducing agent which is ] in water. It is a major component in most ]s for film and paper where, with the compound ], it reduces silver ] to elemental ].		Hydroquinone has a variety of uses principally associated with its action as a reducing agent that is ] in water. It is a major component in most black and white ]s for film and paper where, with the compound ], it reduces silver ] to elemental ].
	There are a variety of other uses associated with its ]. As a polymerization inhibitor, hydroquinone prevents polymerization of ], ], and other monomers that are susceptible to radical-initiated ]. This application exploits the ] properties of ].		There are various other uses associated with its ]. As a ], exploiting its ] properties, hydroquinone prevents polymerization of ], ], ], and other monomers that are susceptible to radical-initiated ]. By acting as a free radical scavenger, hydroquinone serves to prolong the shelflife of light-sensitive resins such as ].<ref> ''Additive manufacturing 2019'' vol. 27. pp 80-90</ref>
			Hydroquinone can lose a hydrogen cation from both hydroxyl groups to form a diphenolate ion. The di] diphenolate ] of hydroquinone is used as an alternating co] unit in the production of the ] ].
	Hydroquinone can undergo mild ] to convert to the compound ''parabenzoquinone'', C<sub>6</sub>H<sub>4</sub>O<sub>2</sub>, often called ''p''-quinone or simply ]. ] 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 ], and can undergo similar ] interconversions.
			=== Skin depigmentation ===
	Hydroquinone can lose an H<sup>+</sup> from both to form a diphenolate ion. The di] diphenolate ] of hydroquinone is used as an alternating co] unit in the production of the ] ].
			Hydroquinone is used as a topical application in ] to reduce the ] of skin. It does not have the same predisposition to cause ] as ] does. This is a prescription-only ingredient in some countries, including the member states of the European Union under ] 76/768/EEC:1976.<ref>{{CELEX|31976L0768|text=Council Directive 76/768/EEC of 27 July 1976 on the approximation of the laws of the Member States relating to cosmetic products}}</ref><ref>{{cite web|url=https://www.beskinformed.com/hydroquinone/clear-n-smooth-skin-toning-cream-recalled/|title=Clear N Smooth Skin Toning Cream recalled|date=4 October 2011|access-date=4 April 2018}}</ref>
			In 2006, the United States ] revoked its previous approval of hydroquinone and proposed a ban on all ] preparations.<ref name="FDA 2006">{{cite report | author = United States Food and Drug Administration | year = 2006 | title = Skin Bleaching Drug Products for Over-the-Counter Product Use; Proposed Rule | url = https://www.fda.gov/OHRMS/DOCKETS/98fr/78n-0065-npr0003.pdf | docket = 1978N-0065 | url-status = live | archive-url = https://web.archive.org/web/20110516100158/https://www.fda.gov/OHRMS/DOCKETS/98fr/78n-0065-npr0003.pdf | archive-date = 16 May 2011 }}</ref> The FDA officially banned hydroquinone in 2020 as part of a larger reform of the over-the-counter drug review process.<ref>{{Cite web|title=Congress Enacts OTC Monograph Reform|url=https://www.jdsupra.com/legalnews/congress-enacts-otc-monograph-reform-96907/|access-date=19 December 2021|website=JD Supra|language=en}}</ref> The FDA stated that hydroquinone cannot be ruled out as a potential ].<ref name=":0">{{Cite web|url=https://www.fda.gov/AboutFDA/CentersOffices/OfficeofMedicalProductsandTobacco/CDER/ucm203112.htm|title=About the Center for Drug Evaluation and Research - Hydroquinone Studies Under The National Toxicology Program (NTP)|last=Research|first=Center for Drug Evaluation and|website=www.fda.gov|language=en|access-date=12 February 2017|url-status=live|archive-url=https://web.archive.org/web/20170122222134/https://www.fda.gov/aboutfda/centersoffices/officeofmedicalproductsandtobacco/cder/ucm203112.htm|archive-date=22 January 2017}}</ref> This conclusion was reached based on the extent of ] in humans and the incidence of ]s in rats in several studies where adult rats were found to have increased rates of tumours, including ]s, anisokaryosis (variation in nuclei sizes), mononuclear cell leukemia, ]s and ]s. The Campaign for Safe Cosmetics has also highlighted concerns.<ref> {{webarchive|url=https://web.archive.org/web/20101127071016/http://safecosmetics.org/article.php?id=289 |date=27 November 2010 }}</ref>
	===Skin depigmentation===
	In human medicine, hydroquinone is used as a topical application in ] to reduce the color of skin as it does not have the same predisposition to cause ] as ] does. This use is banned in some countries, including the member states of the European Union under Directive 76/768/EEC:1976.<ref>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</ref><ref></ref>
			Numerous studies have revealed that hydroquinone, if taken orally, can cause exogenous ], a disfiguring disease in which blue-black pigments are deposited onto the skin; however, skin preparations containing the ingredient are administered topically. The FDA had classified hydroquinone in 1982 as a safe product - generally recognized as safe and effective (GRASE), however additional studies under the National Toxicology Program (NTP) were suggested in order to determine whether there is a risk to humans from the use of hydroquinone.<ref name="FDA 2006"/><ref>{{cite journal |author1=Olumide, YM |author2=Akinkugbe, AO |author3=Altraide, D |author4=Mohammed, T |author5=Ahamefule, N |author6=Ayanlowo, S |author7=Onyekonwu, C |author8=Essen, N | title=Complications of chronic use of skin lightening cosmetics| date= April 2008 | volume = 47 | issue = 4 | pmid=18377596 |pages = 344–53 | journal = International Journal of Dermatology | doi=10.1111/j.1365-4632.2008.02719.x|s2cid=8159382 }}</ref><ref name=":0" /> NTP evaluation showed some evidence of long-term carcinogenic and genotoxic effects<ref>{{Cite web |title=Hydroquinone 10022-H |url=https://ntp.niehs.nih.gov/static/whatwestudy/testpgm/status/ts-10022-h.html |url-status=live |archive-url=https://web.archive.org/web/20171001031226/https://ntp.niehs.nih.gov/testing/status/agents/ts-10022-h.html |archive-date=1 October 2017 |access-date=18 August 2023 |website=ntp.niehs.nih.gov |language=en-US}}</ref>
	In 2006, the United States ] revoked its previous approval of hydroquinone and proposed a ban on all ] preparations.<ref name="FDA 2006">{{cite report| author = United States Food and Drug Administration | year = 2006 | title=Skin Bleaching Drug Products for Over-the-Counter Product Use; Proposed Rule | url=http://www.fda.gov/OHRMS/DOCKETS/98fr/78n-0065-npr0003.pdf | docket=1978N-0065}}</ref> The FDA stated that hydroquinone cannot be ruled out as a potential ]. This conclusion was reached based on the extent of ] in humans and the incidence of ]s in rats in several studies where adult rats were found to have increased rates of tumours, including ]s, anisokaryosis, mononuclear cell leukemia, ]s and ]s. The Campaign for Safe Cosmetics has also highlighted concerns.<ref>]</ref>
			While hydroquinone remains widely prescribed for treatment of ], questions raised about its safety profile by regulatory agencies in the EU, Japan, and USA encourage the search for other agents with comparable efficacy.<ref>{{Cite journal|last=Draelos|first=Zoe Diana|date=1 September 2007|title=Skin lightening preparations and the hydroquinone controversy|journal=Dermatologic Therapy|volume=20|issue=5|pages=308–313|doi=10.1111/j.1529-8019.2007.00144.x|issn=1529-8019|pmid=18045355|s2cid=24913995|doi-access=free}}</ref> Several such agents are already available or under research,<ref>{{Cite journal|last=Bandyopadhyay|first=Debabrata|date=1 January 2009|journal=Indian Journal of Dermatology|volume=54|issue=4|pages=303–309|doi=10.4103/0019-5154.57602|issn=0019-5154|pmc=2807702|pmid=20101327|title=Topical treatment of melasma |doi-access=free }}</ref> including ],<ref>{{Cite journal|last1=Mazurek|first1=Klaudia|last2=Pierzchała|first2=Ewa|date=1 September 2016|title=Comparison of efficacy of products containing azelaic acid in melasma treatment|journal=Journal of Cosmetic Dermatology|volume=15|issue=3|pages=269–282|doi=10.1111/jocd.12217|issn=1473-2165|pmid=27028014|s2cid=25303091}}</ref> ], retinoids, cysteamine,<ref>{{Cite journal|last1=Mansouri|first1=P.|last2=Farshi|first2=S.|last3=Hashemi|first3=Z.|last4=Kasraee|first4=B.|date=1 July 2015|title=Evaluation of the efficacy of cysteamine 5% cream in the treatment of epidermal melasma: a randomized double-blind placebo-controlled trial|journal=The British Journal of Dermatology|volume=173|issue=1|pages=209–217|doi=10.1111/bjd.13424|issn=1365-2133|pmid=25251767|s2cid=21618233}}</ref> topical steroids, ], and other substances. One of these, ], has been proved to be more effective at treating melanin-related skin disorders by a wide margin, as well as safe enough to be made available over the counter.<ref>{{cite book|title=Phenols—Advances in Research and Application: 2013 Edition|date=2013|page=76|chapter=Hydroquinones|publisher=Scholastic}}</ref>
	Numerous studies have revealed that hydroquinone can cause exogenous ], a disfiguring disease in which blue-black pigments are deposited onto the skin.<ref name="FDA 2006"/><ref>{{cite journal | author=Olumide, YM; Akinkugbe, AO; Altraide, D; Mohammed, T; Ahamefule, N; Ayanlowo, S; Onyekonwu, C; Essen, N | title=Complications of chronic use of skin lightening cosmetics
	| date= April 2008 | volume = 47 | issue = 4 | pmid=18377596 |pages = 344–53 | journal = International Journal of Dermatology | doi=10.1111/j.1365-4632.2008.02719.x}}</ref>
			In the ] substituted hydroquinones, typically anthrahydroquinone are used to produce ] which forms spontaneously on reaction with oxygen. The type of substituted hydroquinone is selected depending on reactivity and recyclability.
	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 ] rating reduces the risk of further damage. Hydroquinone is sometimes combined with ] 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==		==Natural occurrences==
	Hydroquinones are one of the two primary reagents in the defensive glands of ]s, along with ] (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 ]s and ]s. When the contents of the reservoir are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide and ] the ] of the hydroquinones into ]. 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 ].<ref name="Solomons"> Organic Chemistry, Solomon and Fryhle, 10th edition, Wiley Publishing, 2010. </ref>		Hydroquinones are one of the two primary reagents in the defensive glands of ]s, along with ] (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 ]s and ]s. When the contents of the reservoir are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide and ] the ] of the hydroquinones into ]. 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 ].<ref name="Solomons">Organic Chemistry, Solomon and Fryhle, 10th edition, Wiley Publishing, 2010.{{page needed|date=October 2016}}</ref>
			Hydroquinone is thought to be the active toxin in '']'' mushrooms.<ref>{{cite journal | doi=10.1055/s-2006-957852 |author1=Joval, E |author2=Kroeger, P |author3=N | title=Hydroquinone: the toxic compound of Agaricus hondensis| date= April 1996 | volume = 62 | issue = 2 |pages = 185 | journal = Planta Medica | pmid=17252436|s2cid=260249338 }}</ref>
	] hydroquinone derivatives are the principal irritants exuded by the ], which can cause severe ] in humans.
			Hydroquinone has been shown to be one of the chemical constituents of the natural product ].<ref name="Propolis">{{cite journal | last1 = Burdock | first1 = G.A. | year = 1998 | title = Review of the biological properties and toxicity of bee propolis (propolis) | journal = Food and Chemical Toxicology | volume = 36 | issue = 4| pages = 347–363 | doi = 10.1016/S0278-6915(97)00145-2 | pmid = 9651052 }}</ref>
	Hydroquinone is thought to be the active toxin in '']'' mushrooms.<ref>{{cite journal | doi=10.1055/s-2006-957852 | author=Joval, E; Kroeger, P; N | title=Hydroquinone: the toxic compound of Agaricus hondensis.
	| date= April 1996 | volume = 62 | issue = 2 |pages = 185 | journal = Planta Medica | pmid=17252436}}</ref>
			It is also one of the chemical compounds found in ]. This compound is gathered from the ]'s castor sacs.<ref>The Beaver: Its Life and Impact. Dietland Muller-Schwarze, 2003, page 43 ()</ref>
	==See also==
	* ]
	* ]
	* ]
	* ]
	==References==		==References==
			{{Reflist}}
	<references/>
	==External links==		== External links ==
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	*{{ecb}}
	* (online version of the "''Blue Book''")
	{{Other dermatological preparations}}		{{Other dermatological preparations}}
			{{Authority control}}
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