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Hydroquinone is an aromatic organic compound with the chemical formula C6H4(OH)2, also known as benzene-1,4-diol or quinol.
- It is a type of phenol as well as a benzene derivative.
- Hydroquinone has two hydroxyl groups bonded in a para position to the benzene ring.
- It is a white granular solid.
- The substituted derivatives of this parent compound are also known as hydroquinones.
- In 1843, Friedrich Wöhler coined the term "hydroquinone."
Read more: Common chemical formulas
| Table of Content |
Key terms: Hydroquinone, Phenol, Benzene, Quinol, Oxidation reaction, Hydroxyl groups, Ions
What is Hydroquinone?
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Hydroquinone is a type of phenol. It occurs naturally in various plant species and is also used in photographic processing.
- The formula for hydroquinone is C6H6O2, and it is widely used as a biomarker for benzene exposure.
- This compound's antioxidant properties can cause toxicity in several organs, particularly the kidney.
- Hydroquinone is even used to treat hyperpigmentation of the skin. It is a key ingredient in a variety of cosmetic products.
- The compound is primarily metabolized to glutathione conjugates.
- Hydroquinone is obtained in normal people through the catabolism of substrates such as tyrosine.
- Others are acquired through gut bacteria, dietary ingestion, cigarette smoking, and consumption of arbutin-containing foods.
- The presence of Hydroquinone in the normal human body can also be caused by the use of various over-the-counter medications.
Structure of hydroquinone
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|---|---|---|
| Benzene hexachloride | Benzene reactions | Alcohol, Phenols & Ethers |
| Halonalkanes & haloarenes important questions | Redox titration | Oxidation State Group 15 |
Structure of Hydroquinone
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A couple of hydroxyl groups are linked to one benzene ring in the para position in the structure of hydroquinone.
- Under pressure and at room temperature, the compound appears as a white granular solid.
- The hydroxyl groups found in the structure of Hydroquinone are weakly acidic.
- The compound can form a mono-phenolate ion by losing one H+ from one of its hydroxyls. It can even lose an H+ ion from both in order to form a phenolate ion.
Hydroquinone: weak acid
This melanin synthesis inhibitor, also known as Quinol or 4-diol, has a variety of applications.
- The applications of hydroquinone are generally associated with how it behaves when used as a water-soluble reducing agent.
- One of the most important components of photographic developers is hydroquinone, which is made up of a compound called Metol.
- Silver halides can be reduced to fundamental silver by hydroquinone.
Structure of metol
Hydroquinone is manufactured using a variety of methods. However, cumene and phenol hydroxylation are the most widely used industrial processes. There are also less common methods of producing Hydroquinone. These include phenol oxidation, quinic acid dry distillation, aniline oxidation by manganese dioxide, and the use of iron pentacarbonyl and acetylene. The compound is found naturally not only in various plant species but also in the defensive glands of bombardier beetles.
Structure of hydroquinone
Read more: Hydrogen
Production of Hydroquinone
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Hydroquinone is typically manufactured in two ways in the industrial sector. In the reaction mechanism, the most commonly used technique is identical to the cumene process and includes the dialkylation of benzene with propene to produce 1,4-diisopropyl benzene. When the compound reacts with air, it produces bis hydroperoxide, which is structurally similar to cumene hydroperoxide, and it rearranges in acid to produce acetone and hydroquinone.
The second method involves hydroxylation of phenol in the presence of a catalyst. Hydroquinone and catechol (benzene-1,2-diol) are produced using hydrogen peroxide in this process:
C6H5OH + H2O2 → C6H4(OH)2 + H2O
Other methods for producing hydroquinone include:
Hydroquinone and its derivatives can be produced by the oxidation of various phenols. Elbs persulfate oxidation and Dakin oxidation are two examples of such methods.
Elbs persulfate oxidation reaction
In 1820, French chemists Pelletier and Caventou discovered hydroquinone through the dry distillation of quinic acid.
Hydroquinone synthesis from quinic acid
Read more: Clemmensen Reduction
Properties of Hydroquinone
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Hydroquinone's physical and chemical properties are as follows:
- Chemical Formula: C2H6O2
- Density: 1.3 g cm3
- Molar Mass/ molecular weight: 110.11 g/mol
- Melting point: 172 degrees Celsius.
- Boiling point: 287 degrees Celsius
Hydroquinone Uses
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Hydroquinone's Uses include:
- Hydroquinone is employed as a reducing agent.
- The compound has the potential to be extremely effective in the treatment of melasma.
- Hydroquinone can also be used to treat acne scars.
- It can also be used to prevent the formation of methyl methacrylate.
- Hydroquinone is a common and active ingredient in a variety of cosmetic products.
- It can aid in skin whitening.
- In the case of benzene exposure, hydroquinone is also used as a biomarker.
- The substance is also found in photographic developers.
Read more:
| Relevant concepts | ||
|---|---|---|
| Resorcinol | Hydrogen Peroxide | Phenolic Acid |
| Oxidation & reduction reactions | Oppenauer Oxidation | Alcohol Structure of Hydroxyl group |
Hydroquinone Applications
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Hydroquinone has a variety of applications related to its function as a water-soluble reducing agent such as:
- It is a major component of most black and white photographers for film and paper, where it transforms silver halides into elemental silver using the compound metol.
- Its power-saving properties have a variety of other applications.
- Using its antioxidant properties, hydroquinone acts as a polymerization barrier, inhibiting the polymerization of acrylic acid, methyl methacrylate, cyanoacrylate, and other monomers susceptible to radical-initiated polymerization.
- Hydroquinone improves the shelflife of light-sensitive resins such as preceramic polymers by acting as a free-radical scavenger. By losing a hydrogen cation from both hydroxyl groups, hydroquinone can form a diphenolate ion.
Read more: Epoxides
Hydroquinone Side Effects
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Despite the fact that hydroquinone has a wide range of applications, it does have some drawbacks. The following are the side effects:
- Stinging
- Mild charring
- Dryness
- Skin discolouration.
These are some of the most common side effects of Hydroquinone use. They may go away on their own, but if they persist and worsen over time, seeking immediate medical attention is advised. Other serious side effects of using Hydroquinone as a medicine are as follows:
- Cracking of the skin
- Blistering
- Darkening of the skin to a blue or black color
- Although allergic reactions to Hydroquinone are uncommon, symptoms such as itching, swelling, rash, difficulty breathing, and severe dizziness must be addressed as soon as possible.
Things to Remember
- Before using this compound, you should always consult a pharmacist or a doctor because it can have serious side effects and cause allergies.
- It contains inactive ingredients such as sulfites, which can cause allergies and other problems.
- Pregnant women should avoid using Hydroquinone, and if they do, they should consult with their health care provider.
- Hydroquinone is used as the biomarker in case of benzene exposure.
- It can also be found in photographic developers.
Sample Questions
Ques. What are Hydroquinone Alternatives, and is Oxidized Hydroquinone Effective? (3 marks)
Ans. Mequinol, also known as 4-Hydroxyanisole, is a popular substitute for Hydroquinone. It is also referred to as monomethyl ether, methoxyphenol with Hydroquinone, and p-hydroxyanisole. According to research, Mequinol is as effective as Hydroquinone.
Is it possible to tell whether oxidized Hydroquinone is effective or not? The answer is that oxidized Hydroquinone is neither effective nor harmful. Because oxidized hydroquinone turns brown, it should not be used for any other purpose.
Ques: Is it safe to use hydroquinone for an extended period of time? (5 marks)
Ans: There are some side effects to using Hydroquinone for an extended period of time. Complications include finger discoloration, Ochronosis, Cornea degeneration, and conjunctival melanosis. In this context, Ochronosis is one of the most common medical conditions caused by long-term Hydroquinone use.
- The recommended concentration of this compound is less than 4%. Patients taking Hydroquinone should avoid exposure to sunlight.
- Hydroquinone should only be used for four to five months at a time.
- Hydroquinone therapy also necessitates a period of complete skin relaxation after the treatment is completed.
- This skin relaxation period lasts two to three months on average.
- Doctors do not recommend using Hydroquinone for an extended period of time, owing to the compound's antioxidant properties.
- Nonetheless, the compound is safe to use for a limited time.
Ques. How is aluminum sulfate produced? (2 marks)
Ans. After the intermediate formation of hydroquinone, 10 g of finely powdered quinone is suspended in cold water and saturated liquid with sulfur dioxide until full solution and discoloration have occurred.
Ques. How is hydroquinone used to treat hyperpigmentation? (2 marks)
Ans. Hydroquinone is effective in lightening dark areas of skin, also known as hyperpigmentation, melasma, "liver marks," "aging spots," and freckles, which occur as a result of pregnancy, birth control pills, hormone therapy, or skin damage. This medication works by inhibiting the process in the skin that causes discolouration.
Ques. What are the consequences of hydroquinone? (2 marks)
Ans. Mild burning, stinging, redness, and dryness are the most common side effects of hydroquinone. Blistering, skin cracking, and blue-black darkening of the skin are all possible but serious side effects. Symptoms of a serious allergic reaction, such as rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, and difficulty breathing, are rare side effects that may occur.
Ques. List some of the uses of Hydroquinone. (4 marks)
Ans. Hydroquinone has the following applications:
- As a diluting agent
- For the prevention of methyl methacrylate.
- In terms of skin whitening.
- To treat acne and scars
- Is used widely in cosmetic products,
Ques. Describe the hydroxylation of phenol over a catalyst process. (2 marks)
Ans. In the presence of a catalyst, phenol is hydroxylated. Hydroquinone and catechol (benzene-1,2-diol) are produced as a result of the process, which employs hydrogen peroxide:
C6H5OH + H2O2 → C6H4(OH)2 + H2O
Ques. Explain the cumene procedure for the production of hydroquinone. (3 marks)
Ans. Hydroquinone is typically manufactured in two ways in the industrial sector. The most commonly used reaction mechanism is identical to the cumene process and includes the dialkylation of benzene with propene to produce 1,4-diisopropyl benzene. When the compound reacts with air, it produces bis hydroperoxide, which has a similar structure to cumene hydroperoxide, and it rearranges in acid to produce acetone and hydroquinone.
Ques. What is an alternative to hydroquinone? (2 marks)
Ans. Mequinol (4-Hydroxyanisole) is the principal alternative to hydroquinone for a prescription.Methoxy phenol, monomethyl ether with hydroquinone, and p-hydroxyanisole are other names for this substance. It was discovered to be equally potent to hydroquinone.
Ques. Can hydroquinone be used over long periods of time? (2 marks)
Ans. More concerning are the side effects of hydroquinone use that remain over time. These side effects include ochronosis, discoloration of the fingers, conjunctival melanosis, and corneal degeneration. The clinical issue most frequently linked to prolonged hydroquinone usage is ochromia.
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