Hydroquinone
[CAS# 123-31-9]

Identification

• Classification: API >> Special medicine >> Dermatology medication
• Name: Hydroquinone
• Synonyms: Quinol; 1,4-Benzenediol; 1,4-Dihydroxybenzene
• Molecular Formula: C₆H₆O₂
• Molecular Weight: 110.11
• CAS Registry Number: 123-31-9
• EC Number: 204-617-8
• SMILES: C1=CC(=CC=C1O)O

Properties

• Density: 1.33 g/mL (20 °C)
• Melting point: 172-175 °C
• Index of Refraction: 1.63
• Boiling point: 285-287 °C
• Flash point: 165 °C
• Water solubility: 70 g/L (20 °C)

Safety Data

• Hazard Symbols: GHS05;GHS08;GHS07;GHS09 Danger
• Risk Statements: H351-H341-H302-H318-H317-H400
• Safety Statements: P203-P261-P264-P264+P265-P270-P272-P273-P280-P301+P317-P302+P352-P305+P354+P338-P317-P318-P321-P330-P333+P317-P362+P364-P391-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400
Carcinogenicity	Carc.	2	H351
Germ cell mutagenicity	Muta.	2	H341
Serious eye damage	Eye Dam.	1	H318
Acute toxicity	Acute Tox.	4	H302
Skin sensitization	Skin Sens.	1	H317
Chronic hazardous to the aquatic environment	Aquatic Chronic	1	H410
Acute toxicity	Acute Tox.	4	H312
Skin irritation	Skin Irrit.	2	H315
Skin sensitization	Skin Sens.	1B	H317
Acute toxicity	Acute Tox.	3	H301
Specific target organ toxicity - repeated exposure	STOT RE	1	H372
Eye irritation	Eye Irrit.	2	H319
Reproductive toxicity	Repr.	1B	H360
Germ cell mutagenicity	Muta.	1B	H340
Specific target organ toxicity - single exposure	STOT SE	1	H370
Respiratory sensitization	Resp. Sens.	1	H334

• Transport Information: UN 2662

Discovery and Applications

Hydroquinone, also known as 1,4-dihydroxybenzene, is an aromatic organic compound that has garnered significant attention for its diverse applications in various fields, including photography, cosmetics, and medicine. Discovered in the mid-19th century, hydroquinone is characterized by its two hydroxyl groups attached to a benzene ring. Its ability to act as a reducing agent makes it a valuable chemical in both industrial and laboratory settings.

The synthesis of hydroquinone can be achieved through several methods, including the reduction of quinone or the oxidation of catechol. The compound exists as a white crystalline solid at room temperature and is soluble in water, alcohol, and ether. Hydroquinone has been widely used in photography, particularly in the development of black-and-white films and prints. Its reducing properties allow it to convert exposed silver halides into metallic silver, thereby forming the image on the photographic medium. Despite the advent of digital photography, hydroquinone remains a crucial component in traditional photographic processes and is valued for its effectiveness and reliability.

In the cosmetics industry, hydroquinone is renowned for its skin-lightening properties. It is commonly used to treat hyperpigmentation, melasma, and age spots by inhibiting the enzyme tyrosinase, which is involved in the production of melanin. By reducing melanin production, hydroquinone can effectively lighten dark areas of the skin. Its popularity in skin care products, particularly in formulations aimed at evening out skin tone and reducing discoloration, has led to its widespread use in creams, lotions, and serums.

Despite its effectiveness, the use of hydroquinone in cosmetics has raised concerns regarding safety and potential side effects. Prolonged use can lead to a condition known as ochronosis, which results in a bluish-black discoloration of the skin. Additionally, there are concerns about the potential carcinogenicity of hydroquinone when applied to the skin over extended periods. As a result, several countries have implemented regulations regarding the concentration of hydroquinone in cosmetic products, with some banning its use altogether. In the United States, for instance, over-the-counter products containing hydroquinone are limited to a maximum concentration of 2%, while higher concentrations are available only by prescription.

Beyond its applications in photography and cosmetics, hydroquinone also serves as a crucial intermediate in the synthesis of various chemical compounds. It is utilized in the production of antioxidants, dyes, and resins, as well as in the manufacture of certain pharmaceuticals. Its reducing properties make it valuable in organic synthesis, where it is employed in redox reactions and as a reagent in various chemical transformations.

Research into the safety and efficacy of hydroquinone continues to evolve, with ongoing studies aimed at understanding its long-term effects and potential alternatives for skin lightening. Natural alternatives, such as licorice extract and kojic acid, are being explored as safer options for individuals seeking to address hyperpigmentation without the risks associated with hydroquinone.

Hydroquinone's discovery and subsequent applications in photography, cosmetics, and chemical synthesis highlight its versatility and importance in various fields. While it remains a key player in many industrial processes, the growing awareness of safety concerns has prompted a shift towards exploring alternative solutions that ensure efficacy while minimizing health risks. As research advances, the legacy of hydroquinone will undoubtedly influence future developments in both cosmetic science and organic chemistry.

References

1984. Metabolic fate and disposition of [14C]hydroquinone given orally to Sprague-Dawley rats. Toxicology.
DOI: 10.1016/0300-483x(84)90012-x

1994. Tobacco smoke tumor promoters, catechol and hydroquinone, induce oxidative regulation of protein kinase C and influence invasion and metastasis of lung carcinoma cells. Proceedings of the National Academy of Sciences of the United States of America.
DOI: 10.1073/pnas.91.25.12233

2005. Hydroquinone modulates reactivity of peroxynitrite and nitric oxide production. The Journal of Pharmacy and Pharmacology.
DOI: 10.1211/0022357055731