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Hydroquinone (benzene-1,4-diol) is an aromatic organic compound and a type of phenol, having the formula C₆H₄(OH)₂.
Hydroquinone (benzene-1,4-diol) consists of a benzene ring with two hydroxyl groups in the para (1,4) position.
Hydroquinone (benzene-1,4-diol) appears as white to light tan crystals that darken on exposure to air and light.
Cas Number
123-31-9
Synonyms
Hydroquinol,p-Dihydroxybenzene,p-Benzenediol,1,4-Benzenediol,p-Hydroquinone,Quinol,1,4-Dihydroxybenzene
Introduction and Nomenclature
Benzene-1,4-diol, commonly known as hydroquinone, is an aromatic organic compound with the molecular formula C6H4(OH)2.
It is one of three isomeric dihydroxybenzenes, the others being catechol (1,2-dihydroxybenzene) and resorcinol (1,3-dihydroxybenzene).
Hydroquinone appears as a white, granular solid at room temperature and is slightly soluble in water but highly soluble in organic solvents.
Historically, hydroquinone was first isolated in the 19th century and has since become an important compound in various chemical industries.
Its name derives from its quinonoid structure and its relationship to quinones.
IUPAC nomenclature names it as benzene-1,4-diol, reflecting the position of the hydroxyl groups.
Chemical Structure and Properties
Hydroquinone has a symmetrical molecular structure characterized by a benzene ring substituted with two hydroxyl groups at the para (1,4) positions.
This configuration imparts significant resonance stabilization and allows it to readily undergo redox reactions.
Key chemical properties include:
Molecular formula: C6H6O2
Molecular weight: 110.11 g/mol
Melting point: 170-174 °C
Boiling point: 285 °C (decomposes)
Solubility in water: ~7 g/L at 25 °C
pKa values: 9.85 (first hydroxyl), 11.4 (second hydroxyl)
Hydroquinone is an efficient reducing agent, capable of converting to its oxidized form, benzoquinone, under mild conditions.
It also acts as an antioxidant by scavenging free radicals.
Synthesis and Industrial Production Methods
Hydroquinone is synthesized through various routes:
Hydroxylation of phenol: Catalytic hydroxylation of phenol using hydrogen peroxide yields hydroquinone and catechol as products.
Aniline oxidation: Oxidation of aniline with sulfuric acid and potassium dichromate historically yielded hydroquinone.
Hydrolysis of para-chlorophenol: An older industrial process where para-chlorophenol is hydrolyzed to form hydroquinone.
The phenol hydroxylation method is widely used due to its cost-effectiveness and selectivity. Catalysts such as titanium silicalite (TS-1) enhance the reaction efficiency and yield.
Applications
Hydroquinone has broad applications across diverse industries:
Cosmetics: Used in skin lightening creams to inhibit melanin production.
Photographic industry: Acts as a developing agent in black-and-white photography.
Polymer industry: Functions as a polymerization inhibitor for monomers such as styrene and methyl methacrylate.
Pharmaceuticals: Used in topical formulations for hyperpigmentation and melasma treatment.
Analytical chemistry: Serves as a redox indicator and reagent.
Its redox properties make it highly versatile for various chemical processes and formulations.
Mechanism of Action in Biological Systems
Hydroquinone exerts its effects by interacting with enzymatic systems and cellular antioxidants.
In skin-lightening applications, it inhibits the enzyme tyrosinase, which catalyzes melanin synthesis.
Its redox activity allows it to generate reactive oxygen species (ROS), leading to oxidative stress in cells.
Hydroquinone can undergo metabolic activation to quinones and semiquinone radicals, which can bind to DNA and proteins, potentially leading to cytotoxicity and genotoxicity.
These mechanisms are central to both its therapeutic and toxic effects.
Stability and Storage Conditions
Hydroquinone is sensitive to light, heat, and air. Exposure to oxygen can cause oxidation to quinones, leading to discoloration and degradation. To maintain stability:
Store in airtight containers under inert atmosphere
Protect from light using amber glass or opaque containers
Maintain at controlled room temperature
Stabilizers such as ascorbic acid or sulfites are often added to commercial formulations to enhance shelf life.
Analytical Methods for Detection and Quantification
Several techniques are used to detect and quantify hydroquinone:
High-performance liquid chromatography (HPLC): Most common, offering high sensitivity and specificity
Gas chromatography (GC): Requires derivatization due to thermal instability
UV-Visible spectroscopy: Utilizes absorbance at characteristic wavelengths
Mass spectrometry (MS): Used in conjunction with chromatography for structural analysis
Electrochemical methods: Exploit redox properties for detection
These methods are essential for quality control, environmental monitoring, and pharmacokinetic stud
SAFETY INFORMATION ABOUT BENZENE-1,4-DIOL (HYDROQUINONE)
First aid measures:
Description of first aid measures:
General advice:
Consult a physician.
Show this safety data sheet to the doctor in attendance.
Move out of dangerous area:
If inhaled:
If breathed in, move person into fresh air.
If not breathing, give artificial respiration.
Consult a physician.
In case of skin contact:
Take off contaminated clothing and shoes immediately.
Wash off with soap and plenty of water.
Consult a physician.
In case of eye contact:
Rinse thoroughly with plenty of water for at least 15 minutes and consult a physician.
Continue rinsing eyes during transport to hospital.
If swallowed:
Do NOT induce vomiting.
Never give anything by mouth to an unconscious person.
Rinse mouth with water.
Consult a physician.
Firefighting measures:
Extinguishing media:
Suitable extinguishing media:
Use water spray, alcohol-resistant foam, dry chemical or carbon dioxide.
Special hazards arising from the substance or mixture
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas
Advice for firefighters:
Wear self-contained breathing apparatus for firefighting if necessary.
Accidental release measures:
Personal precautions, protective equipment and emergency procedures
Use personal protective equipment.
Avoid breathing vapours, mist or gas.
Evacuate personnel to safe areas.
Environmental precautions:
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
Methods and materials for containment and cleaning up:
Soak up with inert absorbent material and dispose of as hazardous waste.
Keep in suitable, closed containers for disposal.
Handling and storage:
Precautions for safe handling:
Avoid inhalation of vapour or mist.
Conditions for safe storage, including any incompatibilities:
Keep container tightly closed in a dry and well-ventilated place.
Containers which are opened must be carefully resealed and kept upright to prevent leakage.
Storage class (TRGS 510): 8A: Combustible, corrosive hazardous materials
Exposure controls/personal protection:
Control parameters:
Components with workplace control parameters
Contains no substances with occupational exposure limit values.
Exposure controls:
Appropriate engineering controls:
Handle in accordance with good industrial hygiene and safety practice.
Wash hands before breaks and at the end of workday.
Personal protective equipment:
Eye/face protection:
Tightly fitting safety goggles.
Faceshield (8-inch minimum).
Use equipment for eye protection tested and approved under appropriate government standards such as NIOSH (US) or EN 166(EU).
Skin protection:
Handle with gloves.
Gloves must be inspected prior to use.
Use proper glove
removal technique (without touching glove's outer surface) to avoid skin contact with this product.
Dispose of contaminated gloves after use in accordance with applicable laws and good laboratory practices.
Wash and dry hands.
Full contact:
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
Splash contact
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
It should not be construed as offering an approval for any specific use scenario.
Body Protection:
Complete suit protecting against chemicals, The type of protective equipment must be selected according to the concentration and amount of the dangerous substance at the specific workplace.
Respiratory protection:
Where risk assessment shows air-purifying respirators are appropriate use a fullface respirator with multi-purpose combination (US) or type ABEK (EN 14387) respirator cartridges as a backup to engineering controls.
If the respirator is the sole means of protection, use a full-face supplied air respirator.
Use respirators and components tested and approved under appropriate government standards such as NIOSH (US) or CEN (EU).
Control of environmental exposure
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
Stability and reactivity:
Chemical stability:
Stable under recommended storage conditions.
Incompatible materials:
Strong oxidizing agents:
Hazardous decomposition products:
Hazardous decomposition products formed under fire conditions.
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas.
Disposal considerations:
Waste treatment methods:
Product:
Offer surplus and non-recyclable solutions to a licensed disposal company.
Contact a licensed professional waste disposal service to dispose of this material.
Contaminated packaging:
Dispose of as unused product
