Hydroquinone, also known as benzene-1,4-diol or quinol, is an
aromatic
In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
that is a type of
phenol, a derivative of
benzene, having the
chemical formula C
6H
4(OH)
2. It has two
hydroxyl groups
bonded to a
benzene ring in a
''para'' position. It is a white granular
solid
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
. Substituted derivatives of this parent compound are also referred to as hydroquinones. The name "hydroquinone" was coined by
Friedrich Wöhler in 1843.
Production
Hydroquinone is produced industrially in two main ways.
[Phillip M. Hudnall "Hydroquinone" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. 2005 Wiley-VCH, Weinheim. .]
* The most widely used route is similar to the
cumene process in reaction mechanism and 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 in acid to give
acetone and hydroquinone.
* A second route involves
hydroxylation of
phenol over a catalyst. The conversion uses
hydrogen peroxide and affords a mixture of hydroquinone and its ortho isomer
catechol (benzene-1,2-diol):
*:
C6H5OH + H2O2 -> C6H4(OH)2 + H2O
Other, less common methods include:
* A potentially significant synthesis of hydroquinone from
acetylene and
iron pentacarbonyl has been proposed
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
The Dakin oxidation (or Dakin reaction) is an organic redox reaction in which an '' ortho''- or '' para''-hydroxylated phenyl aldehyde ( 2-hydroxybenzaldehyde or 4-hydroxybenzaldehyde) or ketone reacts with hydrogen peroxide in base to form ...
.
* Hydroquinone was first obtained in 1820 by the French chemists
Pelletier and
Caventou via the
dry distillation of
quinic acid
Quinic acid is a cyclitol, a cyclic polyol, and a cyclohexanecarboxylic acid. It is a colorless solid that can be extracted from plant sources. Quinic acid is implicated in the perceived acidity of coffee.
Occurrence and preparation
The compound i ...
.
Reactions
The reactivity of hydroquinone's
hydroxyl groups resembles that of 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
1,4-Dihydroxyanthraquinone, also called quinizarin or Solvent Orange 86, is an organic compound derived from anthroquinone. Quinizarin is an orange or red-brown crystalline powder. It is formally derived from anthraquinone by replacement of two hy ...
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
Coenzyme Q, also known as ubiquinone and marketed as CoQ10, is a coenzyme family that is ubiquitous in animals and most bacteria (hence the name ubiquinone). In humans, the most common form is coenzyme Q10 or ubiquinone-10.
It is a 1,4-benzo ...
. 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 cocrystallized in a 1:1 ratio, a dark-green crystalline charge-transfer complex ( melting point 171 °C) called quinhydrone () is formed. This complex dissolves in hot water, where the two molecules dissociate in solution.
Amination
An important reaction is the conversion of hydroquinone to the mono- and diamine derivatives. Methylaminophenol, used in photography, is produced in this way:[
:C6H4(OH)2 + \overset -> HOC6H4NHCH3 + H2O
Diamines, useful in the rubber industry as antiozone agents, are similarly produced from aniline:
:C6H4(OH)2 + \overset -> 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
Metol (or Elon) is a trade name for the organic compound with the formula OC6H4NH2(CH3)SO4. It is the sulfate salt of ''N''-methylaminophenol. This colourless salt is a popular photographic developer used in black & white photography.Gerd Lö ...
, it reduces silver halides to elemental silver
Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
.
There are various other uses associated with its reducing power. As a polymerisation inhibitor, exploiting its antioxidant properties, hydroquinone prevents polymerization of acrylic acid, methyl methacrylate, cyanoacrylate, and other monomers that are susceptible to radical-initiated polymerization. By acting as a free radical scavenger, hydroquinone serves to prolong the shelflife of light-sensitive resins such as preceramic polymers The term preceramic polymer refers to one of various polymeric compounds, which through pyrolysis under appropriate conditions (generally in the absence of oxygen) are converted to ceramic compounds, having high thermal and chemical stability. Ce ...
.
Hydroquinone can lose a hydrogen cation from both hydroxyl groups to form a diphenolate ion. The disodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
diphenolate salt of hydroquinone is used as an alternating co monomer unit in the production of the polymer PEEK
Polyether ether ketone (PEEK) is a colourless organic thermoplastic polymer in the polyaryletherketone (PAEK) family, used in engineering applications. The polymer was first developed in November 1978, later being introduced to the market by ...
.
Skin depigmentation
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
Metol (or Elon) is a trade name for the organic compound with the formula OC6H4NH2(CH3)SO4. It is the sulfate salt of ''N''-methylaminophenol. This colourless salt is a popular photographic developer used in black & white photography.Gerd Lö ...
does. This is a prescription-only ingredient in some countries, including the member states of the European Union under Directives 76/768/EEC: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
Over-the-counter (OTC) drugs are medicines sold directly to a consumer without a requirement for a prescription from a healthcare professional, as opposed to prescription drugs, which may be supplied only to consumers possessing a valid prescr ...
preparations. The FDA officially banned hydroquinone in 2020 as part of a larger reform of the over-the-counter drug review process. The FDA stated that hydroquinone cannot be ruled out as a potential carcinogen. This conclusion was reached based on the extent of absorption
Absorption may refer to:
Chemistry and biology
*Absorption (biology), digestion
**Absorption (small intestine)
*Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials
*Absorption (skin), a route by which s ...
in humans and the incidence of neoplasm
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
s in rats in several studies where adult rats were found to have increased rates of tumours, including thyroid follicular cell hyperplasias, anisokaryosis (variation in nuclei sizes), 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, if taken orally, can cause exogenous ochronosis, 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. NTP evaluation showed some evidence of long-term carcinogenic and genotoxic effects
While hydroquinone remains widely prescribed for treatment of hyperpigmentation
Hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin.
Causes
Hyperpigmentation can be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris.James, William; Be ...
, questions raised about its safety profile by regulatory agencies in the EU, Japan, and USA encourage the search for other agents with comparable efficacy. Several such agents are already available or under research, including azelaic acid
Azelaic acid (AzA) is an organic compound with the formula HOOC(CH2)7 COOH. This saturated dicarboxylic acid exists as a white powder. It is found in wheat, rye, and barley. It is a precursor to diverse industrial products including polymers a ...
, kojic acid, retinoids, cysteamine, topical steroids, glycolic acid, and other substances. One of these, 4-butylresorcinol, has been proven 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.
Natural occurrences
Hydroquinones are one of the two primary reagents in the defensive glands of bombardier beetle
Bombardier beetles are ground beetles (Carabidae) in the tribes Brachinini, Paussini, Ozaenini, or Metriini—more than 500 species altogether—which are most notable for the defense mechanism that gives them their name: when disturbed, they e ...
s, 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.[Organic Chemistry, Solomon and Fryhle, 10th edition, Wiley Publishing, 2010.]
Hydroquinone is thought to be the active toxin in ''Agaricus hondensis
''Agaricus hondensis'', commonly known as the felt-ringed agaricus, is a species of fungus in the family Agaricaceae. The species was officially described in 1912 by mycologist William Alphonso Murrill, along with three other ''Agaricus'' specie ...
'' 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
Castoreum is a yellowish exudate from the castor sacs of mature beavers. Beavers use castoreum in combination with urine to scent mark their territory. Both beaver sexes have a pair of castor sacs and a pair of anal glands, located in two cavities ...
. This compound is gathered from the beaver's castor sacs.[The Beaver: Its Life and Impact. Dietland Muller-Schwarze, 2003, page 43]
book at google books
References
External links
*
{{Authority control
Photographic chemicals
IARC Group 3 carcinogens
Reducing agents