# Hydroquinone

Hydroquinone is an aromatic organic compound with the chemical formula $${C_6H_4(OH)_2}$$, also known as benzene-1,4-diol or quinol, which is a kind of phenol and also a derivative of benzene. It contains two hydroxyl groups bonded in a para position to the benzene ring. It is a granular white solid. This parent compound’s substituted derivatives are also known as hydroquinones. Friedrich Wöhler coined the term “hydroquinone” in 1843.

## What is Hydroquinone?

Hydroquinone is an aromatic organic compound. Its chemical formula is $${C_6H_4(OH)_2}$$, it is also known as benzene-1,4-diol or quinol. Two hydroxyl groups bound to a benzene ring in the para position to form hydroquinone. It is known as a melanin synthesis inhibitor and is a phenol derivative that has antioxidant properties. It has the appearance of granular solid white.

Hydroquinone has several applications, which are primarily associated with its function as a reducing agent that is soluble in water. It is a major component of most black and white photographers for film and paper where, with the compound metol, it transforms silver halides into elemental silver.

### Properties of Hydroquinone

• Formula of hydroquinone is $${C_6H_6O_2}$$.
• The Molecular Weight is 110.11 g/mol.
• The Boiling Point of hydroquinone is 287°C
• Also, the Melting Point of hydroquinone is 172°C.
• The Density of hydroquinone is $${1.3 g cm^{−3}}$$.
• It is soluble in water.
• It has a white-solid appearance.

### Production of Hydroquinone

Industrial production of hydroquinone usually happens in two ways. The most commonly used technique is identical to the cumene process in the reaction mechanism and, it includes the dialkylation of benzene with propene to produce 1,4-diisopropyl benzene. The compound reacts with air to form bishydroperoxide, which has a similar structure compared to cumene hydroperoxide and, it rearranges in acid to form acetone and hydroquinone.

The second method includes the hydroxylation of phenol over a catalyst. The conversion process uses hydrogen peroxide and provides a combination of hydroquinone and catechol (benzene-1,2-diol):

$${C_6H_5OH + H_2O_2 \longrightarrow C_6H_4(OH)_2 + H_2O}$$

Some other methods for producing hydroquinone are:

Oxidation of various phenols can also produce hydroquinone and its derivatives. Examples of such method are Elbs persulfate oxidation and Dakin oxidation.

French chemists Pelletier and Caventou first obtained hydroquinone in 1820 through the dry distillation process of quinic acid.

### Uses of Hydroquinone

The uses of hydroquinone are:

• As a reducing agent.
• For the preventive measures of methyl methacrylate.
• In skin whitening.
• Helpful as a biomarker for benzene exposure.
• By photographic developers
• In the treatment of acne scars
• In various cosmetic products

### Applications of Hydroquinone

• Hydroquinone has several applications, which are primarily associated with its function as a reducing agent that is soluble in water. It is a major component of most black and white photographers for film and paper where, with the compound metol, it transforms silver halides into elemental silver.
• There are several other applications for its reducing power. As a polymerization barrier, hydroquinone inhibits the polymerization of acrylic acid, methyl methacrylate, cyanoacrylate, and other monomers vulnerable to radical-initiated polymerization by using its antioxidant properties.
• By serving as a free-radical scavenger, hydroquinone helps in improving the shelflife of light-sensitive resins such as preceramic polymers. Hydroquinone can form a diphenolate ion by losing a hydrogen cation from both hydroxyl groups.

## FAQs on Hydroquinone

Q.1. How is hydroquinone used in hyperpigmentation treatment?

Answer. Hydroquinone is useful in lightening the dark areas of skin, also known as hyperpigmentation, melasma, “liver marks,” “ageing spots,” and freckles that occur due to pregnancy, birth control drugs, hormone therapy, or damage to the skin. This drug works by suppressing the process in the skin that leads to discolouration.

Q.2. What is an appropriate alternative for hydroquinone?

Answer. Mequinol (4-Hydroxyanisole) is the most common prescription substitute for hydroquinone. Methoxy phenol is also known as monomethyl ether with hydroquinone and p-hydroxyanisole. It is equally as effective as hydroquinone.

Q.3. What are the side-effects of hydroquinone?

Answer. The most common side-effects of hydroquinone are mild burning, stinging, redness, and dryness may occur. Unlikely but serious side effects that occur include blistering, skin cracking, blue-black darkening of the skin. Rare side-effects that may occur include symptoms of a serious allergic reaction, including rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing

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