 | |
 | |
 | |
| Names | |
|---|---|
| Preferred IUPAC name Benzene-1,4-diol[1] | |
| Other names | |
| Identifiers | |
| |
3D model (JSmol) | |
| 605970 | |
| ChEBI | |
| ChEMBL | |
| ChemSpider |
|
| DrugBank | |
| ECHA InfoCard | 100.004.199 |
| EC Number |
|
| 2742 | |
| KEGG |
|
| RTECS number |
|
| UNII | |
| UN number | 3077, 2662 |
| |
| |
| Properties | |
| C6H6O2 | |
| Molar mass | 110.112 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) |
| 5.9 g/100 mL (15 °C) | |
| Vapor pressure | 10−5 mmHg (20 °C)[2] |
| Acidity (pKa) | 9.9[3] |
| −64.63×10−6 cm3/mol | |
| Structure | |
| 1.4±0.1 D[4] | |
| Pharmacology | |
| D11AX11 (WHO) | |
| Hazards | |
| GHS labelling: | |
    | |
| Danger | |
| H302,H317,H318,H341,H351,H400 | |
| P201,P202,P261,P264,P270,P272,P273,P280,P281,P301+P312,P302+P352,P305+P351+P338,P308+P313,P310,P321,P330,P333+P313,P363,P391,P405,P501 | |
| NFPA 704 (fire diamond) | |
| Flash point | 165 °C (329 °F; 438 K) |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 490 mg/kg (mammal, oral) 245 mg/kg (mouse, oral) 200 mg/kg (rabbit, oral) 320 mg/kg (rat, oral) 550 mg/kg (guinea pig, oral) 200 mg/kg (dog, oral) 70 mg/kg (cat, oral)[5] |
| NIOSH (US health exposure limits): | |
PEL (Permissible) | TWA 2 mg/m3[2] |
REL (Recommended) | C 2 mg/m3 [15-minute][2] |
IDLH (Immediate danger) | 50 mg/m3[2] |
| Related compounds | |
Relatedbenzenediols | Pyrocatechol Resorcinol |
Related compounds | 1,4-benzoquinone |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Hydroquinone, also known asbenzene-1,4-diol orquinol, is anaromaticorganic compound that is a type ofphenol, a derivative ofbenzene, having thechemical formula C6H4(OH)2. It has twohydroxyl groupsbonded to abenzene ring in apara position. It is a white granularsolid. Substituted derivatives of this parent compound are also referred to as hydroquinones. The name "hydroquinone" was coined byFriedrich Wöhler in 1843.[7]
In 2022, it was the 268th most commonly prescribed medication in the United States, with more than 900,000 prescriptions.[8][9]
Hydroquinone is produced industrially in two main ways.[10]
Other, less common methods include:
The latter three methods are generally lessatom-economical than oxidation with hydrogen peroxide, and their commercial practice in China produced seriouspollution in 2022.[20]
The reactivity of hydroquinone'shydroxyl groups resembles that of otherphenols, being weakly acidic. The resultingconjugate base easily undergoesO-alkylation to givemono- and diethers. Similarly, hydroquinone is highly susceptible to ring substitution viaFriedel–Crafts alkylation. This reaction is often used for the production of several popular antioxidants, namely 2-tert-butyl-4-methoxyphenol (BHA). The useful dyequinizarin is produced bydiacylation of hydroquinone withphthalic anhydride.[10]
Hydroquinone can be reversiblyoxidised under mild conditions to givebenzoquinone. Naturally occurring hydroquinone derivatives, such ascoenzyme Q, exhibit similar reactivity, wherein one hydroxyl group is exchanged for an amino group. Given the conditional reversibility and relative ubiquity of reagents, oxidation reactions of hydroquinones and hydroquinone derivatives are of significant commercial use, often used at an industrial scale.
When colorless hydroquinone and benzoquinone - bright yellow in solid form - arecocrystallized at a 1:1 ratio, a dark-green crystallinecharge-transfer complex (melting point 171 °C), known asquinhydrone (C6H6O2·C6H4O2), is formed. This complex dissolves in hot water, dissociating both quinone molecules in solution.[21]
An important reaction involves the conversion of hydroquinone to its mono- and di-amine derivatives. One such derivative,methylaminophenol, used in photography, is produced according to the stochiometry:[10]
Diamines - used in the rubber industry as antiozone agents - aminated fromaniline, are formed via a similar pathway:
Hydroquinone has a variety of uses principally associated with its action as a reducing agent that issoluble in water. It is a major component in most black and whitephotographic developers for film and paper where, with the compoundmetol, it reduces silverhalides to elementalsilver.
There are various other uses associated with itsreducing power. As apolymerisation inhibitor, exploiting itsantioxidant properties, hydroquinone prevents polymerization ofacrylic acid,methyl methacrylate,cyanoacrylate, and other monomers that are susceptible to radical-initiatedpolymerization. By acting as a free radical scavenger, hydroquinone serves to prolong the shelflife of light-sensitive resins such aspreceramic polymers.[22]
Hydroquinone can lose a hydrogen cation from both hydroxyl groups to form a diphenolate ion. The disodium diphenolatesalt of hydroquinone is used as an alternating comonomer unit in the production of thepolymerPEEK.
Hydroquinone is used as a topical application inskin whitening to reduce thecolor of skin. It does not have the same predisposition to causedermatitis asmetol does. This is a prescription-only ingredient in some countries, including the member states of the European Union underDirectives 76/768/EEC:1976.[23][24]
In 2006, the United StatesFood and Drug Administration revoked its previous approval of hydroquinone and proposed a ban on allover-the-counter preparations.[25] The FDA officially banned hydroquinone in 2020 as part of a larger reform of the over-the-counter drug review process.[26] The FDA stated that hydroquinone cannot be ruled out as a potentialcarcinogen.[27] This conclusion was reached based on the extent ofabsorption in humans and the incidence ofneoplasms in rats in several studies where adult rats were found to have increased rates of tumours, includingthyroid follicular cell hyperplasias, anisokaryosis (variation in nuclei sizes), mononuclear cell leukemia,hepatocellular adenomas andrenal tubule cell adenomas. The Campaign for Safe Cosmetics has also highlighted concerns.[28]
Numerous studies have revealed that hydroquinone, if taken orally, can cause exogenousochronosis, 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.[25][27][29] NTP evaluation showed some evidence of long-term carcinogenic and genotoxic effects.[30]
While hydroquinone remains widely prescribed for treatment ofhyperpigmentation, questions raised about its safety profile by regulatory agencies in the EU, Japan, and USA encourage the search for other agents with comparable efficacy.[31] Several such agents are already available or under research,[32] includingazelaic acid,[33]kojic acid, retinoids, cysteamine,[34] topical steroids,glycolic acid, and other substances. One of these,4-butylresorcinol, 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.[35]
In theanthraquinone process substituted hydroquinones, typically anthrahydroquinone are used to producehydrogen peroxide which forms spontaneously on reaction with oxygen. The type of substituted hydroquinone is selected depending on reactivity and recyclability.
Hydroquinones are one of the two primaryreagents in the defensive glands ofbombardier beetles, along withhydrogen 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 secretecatalases andperoxidases. When the contents of the reservoir are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide andcatalyze theoxidation of the hydroquinones intop-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'sabdomen.[36]
Hydroquinone is thought to be the active toxin inAgaricus hondensis mushrooms.[37]
Hydroquinone has been shown to be one of the chemical constituents of the natural productpropolis.[38]
It is also one of the chemical compounds found incastoreum. This compound is gathered from thebeaver's castor sacs.[39]
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