Hydroquinone
Encyclopedia
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, has 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.
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 hydroquininone in acid. A second route involves hydroxylation
of phenol
. The conversion uses hydrogen peroxide
and affords a mixture of hydroquinone and catechol:
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.
s, 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 reaction
s 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
. 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this reaction is exploited both with hydroquinone itself but more often with its derivatives where one OH has been replaced by an amine.
Similarly diamines, useful in the rubber industry as antiozone agents, are produced similarly from aniline
:
s for film and paper where, with the compound Metol
, it reduces silver halides to elemental silver
.
There are a variety of other uses associated with its reducing power
. As a polymerization inhibitor, hydroquinone prevents polymerization of acrylic acid
, methyl methacrylate
, 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
.
to reduce the color of skin as 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.
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 adenoma
s and renal tubule cell ademona
s. 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 (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.
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 catalase
s and peroxidase
s. 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.
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
that is a type of phenol
Phenols
In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl group bonded directly to an aromatic hydrocarbon group...
, having the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
C6H4(OH)2. Its chemical structure
Chemical structure
A chemical structure includes molecular geometry, electronic structure and crystal structure of molecules. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. Molecular geometry can range from the very simple, such as...
, shown in the table at right, has two hydroxyl groups bonded
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
to a benzene ring in a para position. It is a white granular solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
. 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 processCumene process
The Cumene process is an industrial process for developing phenol and acetone from benzene and propylene. The term stems from cumene , the intermediate material during the process. It was invented by Heinrich Hock in 1944 and independently by R. Ūdris and P...
in reaction mechanism, the most widely used route involves the dialkylation of benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
with propene to give 1,4-diisopropylbenzene. This compound reacts with air to afford the bis(hydroperoxide), which is structurally similar to cumene hydroperoxide
Cumene hydroperoxide
Cumene hydroperoxide is an intermediate in the cumene process for developing phenol and acetone from benzene and propylene. It is typically used as an oxidising agent. Products of decomposition of cumene hydroperoxide are methylstyrene, acetophenone and cumyl alcohol. Its formula is...
and rearranges to give acetone
Acetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...
and hydroquininone in acid. A second route involves hydroxylation
Hydroxylation
Hydroxylation is a chemical process that introduces a hydroxyl group into an organic compound. In biochemistry, hydroxylation reactions are often facilitated by enzymes called hydroxylases. Hydroxylation is the first step in the oxidative degradation of organic compounds in air...
of phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
. The conversion uses hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
and affords a mixture of hydroquinone and catechol:
- C6H5OH + H2O2 → C6H4(OH)2 + H2O
The third method, practiced only in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, is the oxidation of aniline
Aniline
Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...
by manganese dioxide followed by reduction
Reduction
Reduction, reduced, or reduce may refer to:- Chemistry :* Reduction, part of a reduction-oxidation reaction where oxygen is being removed from a compound.** Reduced gas, a gas with a low oxidation number...
of the resulting 1,4-benzoquinone
1,4-Benzoquinone
1,4-Benzoquinone, commonly known as para-quinone, is a chemical compound with the formula C6H4O2. In a pure state, it forms bright-yellow crystals with a characteristic irritating odor, resembling that of chlorine, bleach, and hot plastic. Impure samples are often dark-colored due to the presence...
. The process is conducted batchwise and generates a substantial waste stream.
Reactions
In term of the reactivity of its O-H groups, hydroquinone resembles other phenolPhenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
s, 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 reaction
Friedel-Crafts reaction
The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. This reaction type is a form of electrophilic aromatic substitution...
s such as alkylation. This reaction is exploited en route to popular antioxidants such as 2-tert-butyl-
4-methoxyphenol ("BHA"
Butylated hydroxyanisole
Butylated hydroxyanisole is an antioxidant consisting of a mixture of two isomeric organic compounds, 2-tert-butyl-4-hydroxyanisole and 3-tert-butyl-4-hydroxyanisole. It is prepared from 4-methoxyphenol and isobutylene. It is a waxy solid used as a food additive with the E number E320...
). The useful dye quinizarin
1,4-Dihydroxyanthraquinone
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 hydrogen atoms by hydroxyl groups...
is produced by diacylation of hydroquinone with phthalic anhydride
Phthalic anhydride
Phthalic anhydride is the organic compound with the formula C6H42O. It is the anhydride of phthalic acid. This colourless solid is an important industrial chemical, especially for the large-scale production of plasticizers for plastics. In 2002, approximately 4.6 billion kilograms were...
Redox
Hydroquinone undergoes oxidation under mild conditions to give benzoquinone1,4-Benzoquinone
1,4-Benzoquinone, commonly known as para-quinone, is a chemical compound with the formula C6H4O2. In a pure state, it forms bright-yellow crystals with a characteristic irritating odor, resembling that of chlorine, bleach, and hot plastic. Impure samples are often dark-colored due to the presence...
. This process can be reversed. Some naturally occurring hydroquinone derivatives exhibit this sort of reactivity, one example being coenzyme Q
Coenzyme Q
Coenzyme Q10, also known as ubiquinone, ubidecarenone, coenzyme Q, and abbreviated at times to CoQ10 , CoQ, Q10, or Q, is a 1,4-benzoquinone, where Q refers to the quinone chemical group, and 10 refers to the number of isoprenyl chemical subunits in its tail.This oil-soluble, vitamin-like substance...
. 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
Aniline
Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...
:
- C6H4(OH)2 + 2 C6H5NH2 → C6H4(N(H)CH6H5)2 + 2 H2O
Uses
Hydroquinone has a variety of uses principally associated with its action as a reducing agent which is soluble in water. It is a major component in most photographic developerPhotographic developer
In the processing of photographic films, plates or papers, the photographic developer is a chemical that makes the latent image on the film or print visible. It does this by reducing the silver halides that have been exposed to light to elemental silver in the gelatine matrix...
s for film and paper where, with the compound Metol
Metol
Metol is the chemical compound with the name monomethyl-p-aminophenol hemisulfate. It is a developing agent used in black & white photographic developers...
, it reduces silver halides to elemental silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
.
There are a variety of other uses associated with its reducing power
Reducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...
. As a polymerization inhibitor, hydroquinone prevents polymerization of acrylic acid
Acrylic acid
Acrylic acid is an organic compound with the formula CH2=CHCO2H. It is the simplest unsaturated carboxylic acid, consisting of a vinyl group connected directly to a carboxylic acid terminus. This colorless liquid has a characteristic acrid or tart smell. It is miscible with water, alcohols,...
, methyl methacrylate
Methyl methacrylate
Methyl methacrylate is an organic compound with the formula CH2=CCOOCH3. This colourless liquid, the methyl ester of methacrylic acid is a monomer produced on a large scale for the production of poly .-Production:...
, and other monomers that are susceptible to radical-initiated polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
. This application exploits the antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...
properties of hydroquinone
Hydroquinone
Hydroquinone, also benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, having the chemical formula C6H42. Its chemical structure, shown in the table at right, has two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid...
.
Hydroquinone can undergo mild oxidation to convert to the compound parabenzoquinone
1,4-Benzoquinone
1,4-Benzoquinone, commonly known as para-quinone, is a chemical compound with the formula C6H4O2. In a pure state, it forms bright-yellow crystals with a characteristic irritating odor, resembling that of chlorine, bleach, and hot plastic. Impure samples are often dark-colored due to the presence...
, C6H4O2, often called p-quinone or simply quinone
Quinone
A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...
. Reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
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
Coenzyme Q
Coenzyme Q10, also known as ubiquinone, ubidecarenone, coenzyme Q, and abbreviated at times to CoQ10 , CoQ, Q10, or Q, is a 1,4-benzoquinone, where Q refers to the quinone chemical group, and 10 refers to the number of isoprenyl chemical subunits in its tail.This oil-soluble, vitamin-like substance...
, and can undergo similar redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
interconversions.
Hydroquinone can lose an H+ from both to form a diphenolate ion. The disodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
diphenolate salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
of hydroquinone is used as an alternating comonomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
unit in the production of the polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
PEEK
PEEK
Polyether ether ketone is a colourless organic polymer thermoplastic used in engineering applications.-Synthesis:PEEK polymers are obtained by step-growth polymerization by the dialkylation of bisphenolate salts. Typical is the reaction of 4,4'-difluorobenzophenone with the disodium salt of...
.
Skin depigmentation
In human medicine, hydroquinone is used as a topical application in skin whiteningSkin whitening
Skin whitening, skin lightening and skin bleaching refers to the practice of using chemical substances in an attempt to lighten skin tone or provide an even skin complexion by lessening the concentration of melanin...
to reduce the color of skin as it does not have the same predisposition to cause dermatitis
Dermatitis
-Etymology:Dermatitis derives from Greek derma "skin" + -itis "inflammation" and genetic disorder.-Terminology:There are several different types of dermatitis. The different kinds usually have in common an allergic reaction to specific allergens. The term may describe eczema, which is also called...
as metol
Metol
Metol is the chemical compound with the name monomethyl-p-aminophenol hemisulfate. It is a developing agent used in black & white photographic developers...
does. This use is banned in some countries, including the member states of the European Union under Directive 76/768/EEC:1976.
In 2006, the United States Food and Drug Administration
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...
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
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...
. This conclusion was reached based on the extent of absorption
Absorption (Pharmacokinetics)
In pharmacology , absorption is the movement of a drug into the bloodstream.Absorption involves several phases...
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 adenoma
Hepatocellular adenoma
Hepatocellular adenoma, also hepatic adenoma, or rarely hepadenoma, is an uncommon benign liver tumor which is associated with the use of hormonal contraception with a high estrogen content...
s and renal tubule cell ademona
Kidney cancer
Kidney cancer is a type of cancer that starts in the cells in the kidney.The two most common types of kidney cancer are renal cell carcinoma and urothelial cell carcinoma of the renal pelvis...
s. The Campaign for Safe Cosmetics has also highlighted concerns.
Numerous studies have revealed that hydroquinone can cause exogenous ochronosis
Ochronosis
Ochronosis is the syndrome caused by the accumulation of homogentisic acid in connective tissues. The phenomenon was first described by Rudolf Virchow in 1865. The condition was named after the yellowish discoloration of the tissue seen on microscopic examination...
, 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 (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 beetleBombardier beetle
Bombardier beetles are ground beetles 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, the beetle ejects a noxious chemical spray in a rapid burst of pulses from...
s, along with hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
(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 catalase
Catalase
Catalase is a common enzyme found in nearly all living organisms that are exposed to oxygen, where it catalyzes the decomposition of hydrogen peroxide to water and oxygen...
s and peroxidase
Peroxidase
Peroxidases are a large family of enzymes that typically catalyze a reaction of the form:For many of these enzymes the optimal substrate is hydrogen peroxide, but others are more active with organic hydroperoxides such as lipid peroxides...
s. When the contents of the reservoir are forced into the reaction chamber, the catalases and peroxidases rapidly break down the hydrogen peroxide and catalyze
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
the oxidation of the hydroquinones into p-quinones
Quinone
A quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...
. 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
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
.
Farnesyl
Farnesol
Farnesol is a natural organic compound which is an acyclic sesquiterpene alcohol found as a colorless liquid. It is insoluble in water, but miscible with oils...
hydroquinone derivatives are the principal irritants exuded by the poodle-dog bush, which can cause severe contact dermatitis
Contact dermatitis
Contact dermatitis is a term for a skin reaction resulting from exposure to allergens or irritants . Phototoxic dermatitis occurs when the allergen or irritant is activated by sunlight....
in humans.
Hydroquinone is thought to be the active toxin in Agaricus hondensis
Agaricus hondensis
Agaricus hondensis is a mushroom of thegenus Agaricus.-External links:* -See also:List of Agaricus species...
mushrooms.
External links
- International Chemical Safety Card 0166
- NIOSH Pocket Guide to Chemical Hazards
- IARC Monograph: "Hydroquinone"
- IUPAC Nomenclature of Organic Chemistry (online version of the "Blue Book")