HYDROQUINONE

Hydroquinone is produced by the oxidation of aniline or phenol, by the reduction of quinone, or from a reaction of acetylene and carbon monoxide. 
Hydroquinone has two hydroxyl groups bonded to a benzene ring in a para position.
Hydroquinone is a white granular solid.

CAS Number: 123-31-9
Molecular Formula: C6H6O2
Molecular Weight: 110.11
EINECS Number: 204-617-8

Hydroquinone occurs naturally as a glucose ether, also known as arbutin, in the leaves of many plants and in fruits, as well as one of the agents used in the defense mechanism of the bombardier beetle, family Carabidae.
Hydroquinone, a colorless, hexagonal prism, has been reported to be a good antimitotic and tumor-inhibiting agent. 

Hydroquinone is a reducing agent used in a photographic developer, which polymerizes in the presence of oxidizing agents. 
In the manufacturing industry it may occur include bacteriostatic agent, drug, fur processing, motor fuel, paint, organic chemicals, plastics, stone coating, and styrene monomers.
Hydroquinone is a chemical compound that is commonly used in skincare products and cosmetics for its skin-lightening and depigmenting properties. 

Hydroquinone is considered a skin-lightening agent and is used to treat various skin conditions related to hyperpigmentation, such as melasma, age spots, freckles, and post-inflammatory hyperpigmentation (dark spots left after skin inflammation or injury).
Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. 

Hydroquinone has two hydroxyl groups bonded to a benzene ring in a para position. 
Substituted derivatives of this parent compound are also referred to as hydroquinones. 
The name "hydroquinone" was coined by Friedrich Wöhler in 1843.

Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2.
Hydroquinone is a white granular solid.

Hydroquinone works by inhibiting the activity of an enzyme called tyrosinase, which is involved in the production of melanin, the pigment responsible for the color of the skin, hair, and eyes. 
By blocking this enzyme, hydroquinone reduces the production of melanin, leading to a gradual lightening of the skin in areas where hyperpigmentation is present.
Hydroquinone is a skin-lightening agent. 

Hydroquinone bleaches the skin, which can be helpful when treating different forms of hyperpigmentation.
Hydroquinone is a topical skin-bleaching agent used in the cosmetic treatment of hyperpigmented skin conditions. 
The effect of skin lightening caused by hydroquinone is reversible when exposed to sunlight and therefore requires regular use until desired results are achieved. 

Various preparations are available including creams, emulsions, gels, lotions and solutions.
Hydroquinone produces reversible lightening of the skin by interfering with melanin production by the melanocytes. 
Specifically, inhibition of the enzymatic conversion of tyrosine to DOPA (dihydroxyphenylalanine) results in the desired chemical reduction of pigment. 

Ultimately, this causes a decrease in the number of melanocytes and decreased transfer of melanin leading to lighter skin.
Hydroquinone is a potent skin-lightening agent that is often used to diminish skin discolouration and promote an even skin tone. 
Hydroquinone functions by reducing the production and increasing the degradation of melanin pigments in the skin. 

This two-pronged action not only helps lighten the skin but also ensures a more uniform skin tone.
Hydroquinone is a depigmenting agent used to lighten areas of darkened skin such as freckles, age spots, chloasma, and melisma caused by pregnancy, birth control pills, hormone medicine, or injury to the skin. 
Hydroquinone decreases the formation of melanin in the skin. 

Melanin is the pigment in skin that gives it a brown color.
Hydroquinone is a chemical that a person can use to lighten their skin tone. 
Hydroquinone is available as a cream, gel, lotion, or emulsion. 

Hydroquinone is generally safe to use, but some people may experience side effects, such as dry skin.
Hydroquinone is frequently found in skin-lightening products like bleaching creams. 
Hydroquinone works by limiting your production of melanin, the hormone that darkens your skin. 

While some people use it to lighten their darker skin, hydroquinone creams are most commonly used to lighten small, dark patches like sunspots or hyperpigmentation.
The reactivity of the hydroxyl groups of hydroquinone is weakly acidic, similar to that of other phenols.
The resulting conjugated base undergoes easy O-alkylation to give mono- and diethers.

Similarly, Hydroquinone is highly sensitive to ring substitution by Friedel-Crafts reactions such as alkylation.
This reaction is exploited to obtain popular antioxidants such as 2-tert-butyl-4-methoxyphenol (BHA).
The useful dye quinizarin is produced by diacylation of Hydroquinone with phthalic anhydride.

Hydroquinone undergoes oxidation under mild conditions to give benzoquinone.
Some naturally occurring Hydroquinone derivatives exhibit such reactivity; an example of this is coenzyme Q.
Industrially, this reaction is used both with Hydroquinone itself and more often with its derivatives in which an OH is present.

Colorless Hydroquinone and benzoquinone, a bright yellow solid, are co-crystallized in a 1:1 ratio to give a dark green crystal.
A charge-transfer complex (melting point 171 °C) called quinhydron (C6H6O2·C6H4O2) is formed.
This complex dissolves in hot water, where the two molecules dissociate in solution.

Hydroquinone has a variety of uses, mainly related to its action as a water-soluble reducing agent.
Hydroquinone is a key ingredient in most black and white photo developers for film and paper.
Hydroquinone methol reduces silver halides to elemental silver.

There are also various other uses associated with reducing power.
Leveraging its antioxidant properties as a polymerization inhibitor, hydroquinone prevents the polymerization of acrylic acid.
Hydroquinone, cyanoacrylate and other monomers susceptible to radical-initiated polymerization.

Hydroquinone serves to extend the shelf life of photosensitive resins by acting as a free radical scavenger.
Hydroquinone can lose a hydrogen cation from either hydroxyl group to form a diphenolate ion.
The disodium diphenolate salt of hydroquinone is used as an alternative comonomer unit in polymer production.

Skin depigmentation Hydroquinone is used as a topical application to reduce skin discoloration in skin whitening.
Hydroquinone does not have the same predisposition to cause dermatitis as methol.
This is a prescription-only ingredient in some countries under the Directives, including member states of the European Union.

In 2006, the United States Food and Drug Administration revoked the previous approval of hydroquinone and recommended that it be banned.
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 on the basis of the extent of absorption in humans and the incidence of various species of neoplasms in rats.
Studies in which adult rats have found increased rates of tumors, including thyroid follicular cell hyperplasia, anisokaryosis (change in nucleus size), mononuclear cell leukemia, hepatocellular adenomas and renal tubule cell adenomas.

The Safe Cosmetics Campaign also drew attention to concerns.
Numerous studies have revealed that taking Hydroquinone by mouth can cause exogenous ochronosis, a disfiguring disease in the body.
which blue-black pigments are deposited in the skin; but skin preparations containing this ingredient applied topically.

The FDA classified hydroquinone as a safe product in 1982; however, it was generally considered safe and effective (GRASE).
Additional studies have been proposed within the scope of the National Toxicology Program (NTP) in order to determine whether there is a risk from the use of hydroquinone to humans.
NTP evaluation showed some evidence of long-term carcinogenic and genotoxic effects.

In some countries, hydroquinone is available both by prescription and over-the-counter (OTC). 
The concentration of hydroquinone in OTC products is typically lower than in prescription-strength products.
Hydroquinone is generally not recommended for long-term, continuous use. 

Hydroquinone is typically used for a limited duration, and once the desired results are achieved, users are advised to discontinue its use.
Hydroquinone can make the skin more sensitive to the sun.
Hydroquinone's essential to use sunscreen and sun protection measures when using hydroquinone-containing products to prevent further hyperpigmentation and sun damage.

While hydroquinone is generally considered safe when used as directed, it can cause side effects in some individuals. 
Common side effects may include skin dryness, redness, irritation, and a condition called "exogenous ochronosis," which can result in darkening of the skin in the treated areas. 
This is more commonly associated with long-term or misuse of hydroquinone.

Before using hydroquinone, especially in higher concentrations or for more severe skin conditions, it's advisable to consult with a dermatologist. 
They can recommend an appropriate treatment plan and monitor your progress.
In recent years, there has been growing interest in alternative skin-lightening agents, such as kojic acid, alpha arbutin, and licorice root extract, which are sometimes used as alternatives or in conjunction with hydroquinone for hyperpigmentation treatment.

Melting point: 172-175 °C(lit.)
Boiling point: 285 °C(lit.)
Density: 1.32
vapor density: 3.81 (vs air)
vapor pressure: 1 mm Hg ( 132 °C)
refractive index: 1.6320
Flash point: 165 °C
storage temp.: Store below +30°C.
solubility: H2O: 50 mg/mL, clear
pka: 10.35(at 20℃)
form: Needle-Like Crystals or Crystalline Powder
color: White to off-white
Odor: odorless
Water Solubility: 70 g/L (20 ºC)
Sensitive: Air & Light Sensitive
Merck: 14,4808
BRN: 605970
Henry's Law Constant (x 10-9 atm?m3/mol): <2.07 at 20 °C (approximate - calculated from: water solubility and vapor pressure)
Exposure limits    NIOSH REL: 15-min ceiling 2, IDLH 50; OSHA PEL: TWA 2; ACGIH TLV: TWA 2 (adopted).
Stability: Stable. Combustible, Incompatible with strong oxidizing agents.
InChIKey: QIGBRXMKCJKVMJ-UHFFFAOYSA-N
LogP: 0.59 at 20℃

Pregnant or nursing women are generally advised to avoid using hydroquinone-containing products due to concerns about potential risks to the developing fetus or infant. 
Hydroquinone's essential to consult with a healthcare provider or dermatologist for safe alternatives during pregnancy and breastfeeding.
In some cases, dermatologists may recommend combination therapies that include hydroquinone alongside other skin-lightening agents, such as retinoids or corticosteroids, to enhance the effectiveness of the treatment and reduce potential side effects.

Hydroquinone is typically used for a limited duration, often in cycles of a few months, to target specific areas of hyperpigmentation. 
After achieving the desired results, users may be advised to switch to maintenance therapy, which typically involves milder products to prevent recurrence of hyperpigmentation.
Hydroquinone typically works gradually, and improvement in skin tone and reduction of hyperpigmentation may take several weeks to months, depending on the severity of the condition.

Individuals with darker skin tones may be more prone to side effects like skin irritation or darkening (exogenous ochronosis) when using hydroquinone. 
Dermatologists often recommend lower concentrations or alternative treatments for individuals with darker skin.
Hydroquinone is applied topically to treat disorders characterized by excessive melanin in the epidermis, such as melasma. 

In the United States, nonprescription skin-lightening products contain hydroquinone at concentrations of 2% or less; higher concentrations are available by prescription.
Hydroquinone is marketed most aggressively to women of color for its whitening ability in skin creams. 
The chemical is allowed in personal care products in the United States in concentrations up to two percent. 

Although banned in the European Union, a UK news report found Hydroquinone containing hydroquinone were relatively easy to procure.
Hydroquinone are one of the two primary reagents in the defensive glands of bombardier beetles, 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.

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.
Hydroquinone is also one of the chemical compounds found in castoreum.

Hydroquinone is gathered from the beaver's castor sacs.
Hydroquinone is produced as an inhibitor, an antioxidant, and an intermediate in the synthesis of dyes, motor fuels, and oils; in photographic processing; and naturally in certain plant species, Hydroquinone is a phenol derivative with antioxidant properties that can cause toxicity in several organs, notably the kidney.

Hydroquinone is used as a topical treatment for skin hyperpigmentation and in various cosmetic products, it is metabolized mainly to glutathione conjugates and forms mutagenic DNA adducts in in-vitro systems.
Hydroquinone appears as light colored crystals or solutions.
Hydroquinone may irritate the skin, eyes and mucous membranes.

Hydroquinone is a benzenediol comprising benzene core carrying two hydroxy substituents para to each other.
Hydroquinone has a role as a cofactor, a carcinogenic agent, an Escherichia coli metabolite, a human xenobiotic metabolite, a skin lightening agent, an antioxidant and a mouse metabolite. Hydroquinone is a benzenediol and a member of hydroquinones.
Hydroquinone is used as a developing agent in black-and-white photography, lithography, and x-ray films. 

Hydroquinone is also used as an intermediate to produce antioxidants for rubber and food. 
Hydroquinone is added to a number of industrial monomers to inhibit polymerization during shipping, storage, and processing
Into a pressure reactor there was charged 100 ml of methanol and 1 g of diruthenium nonacarbonyl. 

Hydroquinones are one of the two primary reagents in the defensive glands of bombardier beetles, 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.

Production Methods Of Hydroquinone:
There are three current manufacturing processes for HQ: oxidative cleavage of diisopropylbenzene, oxidation of aniline, and hydroxylation of phenol.
Hydroquinone is air oxidized to the intermediate diisopropylbenzene bishydroperoxide. 
This hydroperoxide is purified by extraction and reacted further to form hydroquinone. 

The purified Hydroquinone is isolated by filtration and packaged. 
The process can be almost entirely closed, continuous, computer-controlled, and monitored.
Hydroquinone can also be prepared by oxidizing aniline to quinone in the presence of manganese dioxide and sulfuric acid. 

p-Benzoquinone is then reduced to Hydroquinone using iron oxide. 
The resulting hydroquinone is crystallized and dried. 
The process occurs in a closed system.

Hydroquinone is also manufactured by hydroxylation of phenol using hydrogen peroxide as a hydroxylation agent. 
The reaction is catalyzed by strong mineral acids or ferrous or cobalt salts.
Hydroquinone is produced industrially in two main ways.

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. 
Hydroquinone 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

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.

Hydroquinone was first obtained in 1820 by the French chemists Pelletier and Caventou via the dry distillation of quinic acid.
Hydroquinone is used to lighten the dark patches of skin (also called hyperpigmentation, melasma, "liver spots," "age spots," freckles) caused by pregnancy, birth control pills, hormone medicine, or injury to the skin.
This medicine works by blocking the process in the skin that leads to discoloration.

Hydroquinone is a chemical that a person can use to lighten their skin tone.
Hydroquinone is available as a cream, gel, lotion, or emulsion.
Hydroquinone is a chemical that bleaches the skin.

Hydroquinone can come as a cream, emulsion, gel, or lotion.
A person can apply these products directly to the skin.
Creams that contain hydroquinone are available with a prescription from a doctor.

People may use hydroquinone as a form of treatment for hyperpigmentation skin conditions, wherein some areas of skin grow darker than surrounding areas.
Hydroquinone is a depigmenting agent used to lighten areas of darkened skin such as freckles, age spots, chloasma, and melisma caused by pregnancy, birth control pills, hormone medicine, or injury to the skin.
Hydroquinone decreases the formation of melanin in the skin.

Melanin is the pigment in skin that gives it a brown color.
Hydroquinone is also known as 1,4-dihydroxy-benzene.
The detection of Hydroquinone along with resorcinol and phenol in air samples by synchronous fluorescence method has been proposed.

The electrochemical oxidation of Hydroquinone has been studied using cyclic and differential pulse voltammetry.
Hydroquinones enthalpies of sublimation, vaporization and fusion have been reported.
Hydroquinone may be used to synthesize bicyclic phosphonate derivative by reacting with phosphonic dichloride.

Hydroquinone is most commonly used in skin lighteners, products heavily marketed towards women of color.
Hydroquinone is linked to cancer and organ-system toxicity.
Hydroquinone is frequently found in skin-lightening products like bleaching creams.

Hydroquinone works by limiting your production of melanin, the hormone that darkens your skin.
While some people use it to lighten their darker skin, hydroquinone creams are most commonly used to lighten small, dark patches like sunspots or hyperpigmentation.
Creams with Hydroquinone as an ingredient are an excellent non-surgical aesthetic procedure to help you achieve the skin they have always wanted.

Unlike skin lightening surgery, hydroquinone creams are a cosmetic procedure that can be undertaken in the comfort of these home after the initial consultation with your dermatologist. 
Hydroquinone is used as a developing agent in photography and as an antioxidant in rubber and food.
Tinnitus (ringing in the ears), dizziness, headache, nausea, vomiting, dyspnea, erosion of the gastric mucosa, edema of internal organs, cyanosis, convulsions, delirium, and collapse may result from the ingestion of a large amount of hydroquinone in humans.

Hydroquinone is also a skin irritant in humans.
Chronic (long-term) occupational exposure to hydroquinone dust can result in eye irritation, corneal effects, and impaired vision. 
No information is available on the reproductive, developmental, or carcinogenic effects of hydroquinone in humans.

There was some evidence of carcinogenic activity in orally-exposed rodents.
Increased skin tumor incidence has been reported in mice treated dermally.
Hydroquinone has not classified hydroquinone for carcinogenicity.

Uses Of Hydroquinone:
Hydroquinone is a pigment-lightening agent used in bleaching creams. 
Hydroquinone combines with oxygen very rapidly and becomes brown when exposed to air. 
Although it occurs naturally, the synthetic version is the one commonly used in cosmetics. 

Hydroquinone application to the skin may cause allergic reaction and increase skin sun sensitivity. 
Hydroquinone is potentially carcinogenic and is associated with causing ochronosis, a discoloration of the skin. 
The u.S. FDA has banned hydroquinone from oTC cosmetic formulations, but allows 4 percent in prescription products. 

Hydroquinones use in cosmetics is prohibited in some european countries and in Australia.
Hydroquinone is a skin-lightening agent used topically for the treatment of hyperpigmentation.
Hydroquinone is used as a developing agent in black-and-white photography, lithography, and x-ray films.

Hydroquinone is also used as an intermediate to produce antioxidants for rubber and food.
Hydroquinone is added to a number of industrial monomers to inhibit polymerization during shipping, storage, and processing
Hydroquinone is primarily used to treat various forms of hyperpigmentation, including melasma, sunspots, age spots, and post-inflammatory hyperpigmentation.

Some individuals use hydroquinone for overall skin lightening to achieve a more even skin tone.
Hydroquinone is most commonly used to treat various forms of hyperpigmentation, which includes.
Hydroquinone is used skin condition characterized by dark patches on the face, often triggered by hormonal changes, pregnancy, or sun exposure.

Hydroquinone is used age Spots (Liver Spots): Dark spots that commonly appear on areas of the skin exposed to the sun, typically with age.
Hydroquinone is used small, brown spots on the skin, often related to sun exposure and genetics.
Hydroquinone is used dark spots or discoloration that remain after an inflammatory skin condition or injury, such as acne, eczema, or a wound.

Some individuals use hydroquinone for overall skin lightening to achieve a more even skin tone. 
Hydroquinone can be used to address concerns about uneven pigmentation or dark areas on the skin.
Hydroquinone is sometimes used in combination with other dermatological treatments, such as retinoids or corticosteroids, to enhance its effectiveness in treating hyperpigmentation.

Dermatologists may recommend hydroquinone to prepare the skin for certain procedures, such as chemical peels or laser therapy, as it can help improve the uniformity of skin tone.
Hydroquinone has a variety of uses principally associated with its action as a reducing agent that is soluble in water. 
Hydroquinone 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.

Hydroquinone is used 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.

Hydroquinone can lose a hydrogen cation from both hydroxyl groups 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.
Hydroquinone is most commonly used in skin lighteners, products heavily marketed towards women of color. 

Hydroquinone is linked to cancer and organ-system toxicity.
Hydroquinone is used in the following products: photo-chemicals, polymers, coating products, inks and toners and water treatment chemicals.
Hydroquinone has an industrial use resulting in manufacture of another substance (use of intermediates).

Hydroquinone is used in the following areas: printing and recorded media reproduction and formulation of mixtures and/or re-packaging.
Hydroquinone is used for the manufacture of: chemicals and plastic products.
Release to the environment of Hydroquinone can occur from industrial use: as processing aid, as an intermediate step in further manufacturing of another substance (use of intermediates) and for thermoplastic manufacture.

Hydroquinone is used in the following products: photo-chemicals.
Other release to the environment of Hydroquinone is likely to occur from: indoor use as reactive substance.
Hydroquinone has a variety of uses principally associated with its action as a reducing agent that is soluble in water.

Hydroquinone 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 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.
Hydroquinone can lose a hydrogen cation from both hydroxyl groups 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.

Hydroquinone is used in a cream or lotion formulation in a concentration of 1-5%.
Hydroquinone is often found in a combination formulation with other skin lightening agents such as topical retinoids (to increase efficiency) and low potency topical steroids (to reduce irritancy).
In New Zealand and many other countries, hydroquinone is only available on prescription, and may need to be compounded by the pharmacist.

Hydroquinone must be distinguished from monobenzyl ether of hydroquinone which can cause irreversible pigment loss.
Hydroquinone is a topical skin-bleaching agent used in the cosmetic treatment of hyperpigmented skin conditions.
The effect of skin lightening caused by hydroquinone is reversible when exposed to sunlight and therefore requires regular use until desired results are achieved.

Various preparations are available including creams, emulsions, gels, lotions and solutions.
Hydroquinone is available over the counter in a 2% cream and can be prescribed by your dermatologist in higher concentrations.
Hydroquinone produces reversible lightening of the skin by interfering with melanin production by the melanocytes.

Specifically, inhibition of the enzymatic conversion of tyrosine to DOPA (dihydroxyphenylalanine) results in the desired chemical reduction of pigment.
Ultimately, this causes a decrease in the number of melanocytes and decreased transfer of melanin leading to lighter skin.
Popularized by its usage as a photo-developer, hydroquinone can be used in any condition causing hyperpigmentation.

Common conditions include Hydroquinone, freckles, lentigines, age spots and acne scars.
Skin sensitivity to hydroquinone may be determined before treatment by applying a small amount of cream to the hyperpigmented area and noting any redness or itching.
As a general rule, always ensure the area is clean and dry then apply a thin film to the lesion and rub it into the skin well.

To maintain the desired affect, hydroquinone should be used concurrently with a strong sunscreen.
Hydroquinone is used many preparations are available as a combination product.

Health Hazard Of Hydroquinone:
Exposures to hydroquinone in large quantities by accidental oral ingestion produce toxicity and poisoning. 
The symptoms of poisoning include, but are not limited to, blurred speech, tinnitus, tremors, sense of suffocation, vomiting, muscular twitching, headache, convul- sions, dyspnea and cyanosis from methemoglobinemia, coma, and collapse from respira- tory failure. 
Occupational workers should be allowed to work with protective clothing and dust masks with full-face or goggles to protect the eyes, and under proper management.

Hydroquinone is very toxic; the probable oral lethal dose for humans is 50-500 mg/kg, or between 1 teaspoon and 1 ounce for a 150 lb. 
Hydroquinone is irritating but not corrosive. 
Fatal human doses have ranged from 5-12 grams, but 300-500 mg have been ingested daily for 3-5 months without ill effects. 

Fire Hazard Of Hydroquinone:
Dust cloud may explode if ignited in an enclosed area. 
Hydroquinone can react with oxidizing materials and is rapidly oxidized in the presence of alkaline materials.

Synonyms Of Hydroquinone:
hydroquinone
Benzene-1,4-diol
123-31-9
1,4-benzenediol
Quinol
1,4-Dihydroxybenzene
p-Benzenediol
p-Hydroquinone
p-Hydroxyphenol
4-Hydroxyphenol
p-Dihydroxybenzene
Benzoquinol
Eldoquin
hydroquinol
Eldopaque
Phiaquin
p-Dioxybenzene
Solaquin forte
Dihydroquinone
Hydroquinole
Idrochinone
Tecquinol
Benzohydroquinone
Hidroquinone
Arctuvin
Tequinol
Dihydroxybenzene
Eldopaque Forte
Eldoquin Forte
Derma-Blanch
Hydrochinon
Tenox HQ
Diak 5
Benzene, p-dihydroxy-
Hydrochinone
1,4-Dihydroxy-benzol
Artra
Usaf ek-356
1,4-Diidrobenzene
p-Dioxobenzene
1,4-Dihydroxybenzen
para-Dioxybenzene
para-Hydroquinone
NCI-C55834
Pyrogentistic acid
Black and White Bleaching Cream
1,4-Dihydroxy-benzeen
para-Dihydroxybenzene
beta-quinol
HE 5
Epiquin
Sunvanish
Idrochinone [Italian]
p-Dihydroquinone
alpha-hydroquinone
para-Hydroxyphenol
CHEBI:17594
NSC 9247
Hydrochinon [Czech, Polish]
CCRIS 714
1,4-Dihydroxybenzen [Czech]
1,4-Diidrobenzene [Italian]
quinnone
Eldopacque
HSDB 577
DTXSID7020716
p-Phenylenediol
1,4-Dihydroxy-benzeen [Dutch]
1,4-Dihydroxy-benzol [German]
p Benzendiol
p-Quinol
AI3-00072
CHEMBL537
UNII-XV74C1N1AE
NSC-9247
1,4-Benzoquinol
EINECS 204-617-8
XV74C1N1AE
UN2662
1,4-Hydroxybenzene
Hydroquinone [USP]
MFCD00002339
HQ
BQ(H)
DTXCID70716
Black & White Bleaching Cream
NSC9247
EC 204-617-8
Hydroquinone [UN2662] [Poison]
1,4-Dihydroxybenzene (ring-d4)
Hydroquinone (USP)
TRI-LUMA COMPONENT HYDROQUINONE
NCGC00015523-02
HYDROQUINONE COMPONENT OF TRI-LUMA
HYDROQUINONE (IARC)
HYDROQUINONE [IARC]
HYDROQUINONE (MART.)
HYDROQUINONE [MART.]
HYDROQUINONE (USP-RS)
HYDROQUINONE [USP-RS]
HYDROQUINONE (USP MONOGRAPH)
HYDROQUINONE [USP MONOGRAPH]
CAS-123-31-9
SMR000059154
SR-01000075920
BUTYLHYDROXYANISOLE IMPURITY A (EP IMPURITY)
BUTYLHYDROXYANISOLE IMPURITY A [EP IMPURITY]
4-DIHYDROXYBENZENE
hydrokinone
hydroquinon
Hidroquinona
Hydrokinon
Hydroquinoue
Accutin
Elopaque
hydroq uinone
hydroquinone gr
p-fenilenediol
p-hidroquinona
p-hidroxifenol
Reduced quinone
a-Hydroquinone
p-Dioxibenceno
4-hidroxifenol
p-Diphenol
p-Hydroxybenzene
b-Quinol
4-Benzenediol
Dihydroxybenzen e
14-Benzoquinol
p-Dihidroxibenceno
Hydroquinone, HQ
.beta.-Quinol
1,4 benzenediol
1,4-Bencenodiol
Hydroquinone,(S)
p-dihydroxy benzene
p -Dihydroxybenzene
PLQ
1,4-Benzendil
Artra (Salt/Mix)
HYDROP
.alpha.-Hydroquinone
1 4-p-Benzenediol
Hydroquinone (8CI)
phenol derivative, 4
1 4-Dihydroxybenzene
1,4-Dihidroxibenceno
4-hydroxyphenyl alcohol
Spectrum_001757
4e3h
HDQ (CHRIS Code)
PYROGENTISIC ACID
SpecPlus_000769
1,4-Dihydrobenzoquinone
ELDOQUIN (TN)
HYDROQUINONE BAKER
hydroquinone for synthesis
Spectrum2_001672
Spectrum3_000656
Spectrum4_000633
Spectrum5_001430
HYDROQUINONE [MI]
Lopac-H-9003
WLN: QR DQ
bmse000293
D03UOT
D08LQA
Epitope ID:116206
HYDROQUINONE [HSDB]
HYDROQUINONE [INCI]
HYDROQUINONE [VANDF]
1,4-Dihydroxybenzene Quinol
Lopac0_000577
SCHEMBL15516
BSPBio_002291
KBioGR_001246
KBioSS_002237
1,4-Dihydroxybenzene, XIII
MLS000069815
MLS001074911
BIDD:ER0340
DivK1c_006865
HYDROQUINONE [WHO-DD]
Hydroquinone, LR, >=99%
SPECTRUM1504237
Hydrochinon(CZECH, POLISH)
SPBio_001883
BDBM26190
Hydroquinone, puriss., 99.0%
KBio1_001809
KBio2_002237
KBio2_004805
KBio2_007373
KBio3_001511
Benzene-1,4-diol (Hydroquinone)
BENZENE, 1,4-DIHYDROXY-
HMS1922H15
HMS2093E08
HMS3261D16
HYDROQUINONE [ORANGE BOOK]
LS-23
Pharmakon1600-01504237
HY-B0951
Hydroquinone [UN2662] [Poison]
Tox21_110169
Tox21_202345
Tox21_300015
Tox21_500577
CCG-39082
NA2662
NSC758707
s4580
STK397446
UN3435
AKOS000119003
Tox21_110169_1
AM10548
DB09526
LP00577
NSC-758707
SDCCGSBI-0050559.P003
UN 2662
Hydroquinone, ReagentPlus(R), >=99%
Hydroquinone, USP, 99.0-100.5%
NCGC00015523-01
NCGC00015523-03
NCGC00015523-04
NCGC00015523-05
NCGC00015523-06
NCGC00015523-07
NCGC00015523-08
NCGC00015523-09
NCGC00015523-10
NCGC00015523-11
NCGC00015523-12
NCGC00015523-13
NCGC00015523-19
NCGC00090880-01
NCGC00090880-02
NCGC00090880-03
NCGC00090880-04
NCGC00090880-05
NCGC00254037-01
NCGC00259894-01
NCGC00261262-01
BP-21160
Hydroquinone, ReagentPlus(R), >=99.5%
SBI-0050559.P002
Hydroquinone, SAJ first grade, >=99.0%
EU-0100577
FT-0606877
H0186
Hydroquinone, SAJ special grade, >=99.0%
EN300-18053
Hydroquinone, meets USP testing specifications
C00530
D00073
H 9003
AB00053361_08
Quinol; 1,4-Benzenediol; 1,4-Dihydroxybenzene
Q419164
J-004910
J-521469
SR-01000075920-1
SR-01000075920-4
Q27102742
Z57127551
094CADDB-59BF-4EDF-B278-59791B203EA2
F1908-0167
Hydroquinone, certified reference material, TraceCERT(R)
Hydroquinone, United States Pharmacopeia (USP) Reference Standard
InChI=1/C6H6O2/c7-5-1-2-6(8)4-3-5/h1-4,7-8
Hydroquinone, Pharmaceutical Secondary Standard; Certified Reference Material