Benzoyl peroxide

Benzoyl peroxide (BPO) /ˈbɛnzɔɪl pəˈrɒksaɪd/ is an organic compound in the peroxide family. It consists of two benzoyl groups bridged by a peroxide link. Its structural formula is [C₆H₅C(O)]₂O₂. It is one of the most important organic peroxides in terms of applications and the scale of its production. Benzoyl peroxide is used as an acne treatment, for improving flour, for bleaching hair and teeth, for cross-linking polyester resins, and many other uses.

It is on the World Health Organization's List of Essential Medicines, a list of the most important medication needed in a basic health system.

Uses[edit]

Most benzoyl peroxide is used as a radical initiator to induce polymerizations.^[3] Other major applications include its antiseptic and bleaching properties.

Acne treatment[edit]

Benzoyl peroxide is effective for reducing the number and severity of acne lesions. BPO has a bactericidal effect onPropionibacterium acnes bacteria associated with acne and does not induce antibiotic resistance.^[4][5] It may be combined withsalicylic acid, sulfur, erythromycin or clindamycin (antibiotics), and adapalene (a synthetic retinoid). Two common combination drugsinclude benzoyl peroxide/clindamycin and adapalene/benzoyl peroxide, a unique formulation considering most retinoids are deactivated by peroxides. Combination products such as benzoyl peroxide/clindamycin and benzoyl peroxide/salicylic acid appear to be slightly more effective than benzoyl peroxide alone for the treatment of acne lesions.^[5]

Benzoyl peroxide for acne treatment is typically applied to the affected areas in gel or cream form, in concentrations of 2.5% increasing through 5%, and up to 10%.^[4] There is no strong evidence to support the idea that higher concentrations of benzoyl peroxide are more effective than lower concentrations.^[4]

Benzoyl peroxide commonly causes initial dryness and sometimes irritation, although the skin develops tolerance after a week or so. A small percentage of people are much more sensitive to it and liable to suffer burning, itching, peeling and possiblyswelling.^{[citation needed]} It is sensible to apply the lowest concentration and build up as appropriate. Once tolerance is achieved, increasing the quantity or concentration and gaining tolerance at a higher level may give better subsequent acne clearance.^[6]Irritation can also be reduced by avoiding harsh facial cleansers and wearing sunscreen prior to sun exposure

Other uses[edit]

Other common uses for benzoyl peroxide include

• Dyeing hair
• Tooth whitening systems
• The preparation of flour
• A initiator and catalyst for polyester thermoset resins, as an alternative to the much more hazardous methyl ethyl ketone peroxide.^{[citation needed]}
• A hardener in order to start the polymerization process in resins. For instance, PMMA resins can be polymerized with benzoyl peroxide.^[7]
• Removing ink and dye stains on vinyl dolls and other playscale figures.^[8]

In the U.S., the typical concentration for benzoyl peroxide is 2.5% to 10% for both prescription and over-the-counter drugpreparations that are used in treatment for acne. Higher concentrations are used for hair bleach and teeth whitening. Benzoyl peroxide, like most peroxides, is a powerful bleaching agent. Contact with fabrics or hair can cause permanent color dampening almost immediately. Even secondary contact can cause bleaching; for example, contact with a towel that has been used to wash off benzoyl peroxide-containing hygiene products.

Side effects[edit]

Concentrated benzoyl peroxide is potentially explosive,^[10] and can cause fires without external ignition. The hazard is acute for the pure material, and for this reason, the compound is generally used as a solution or a paste. For example, cosmetics contain only a small percent of benzoyl peroxide and pose no explosion risk.

Studies have highlighted the carcinogenic potential of benzoyl peroxide. A 1981 study from the journal Science concluded that "caution should be recommended in the use of this and other free radical-generating compounds".^[11]

In a 1977 study using a human maximization test, 76% of subjects acquired a contact sensitization to benzoyl peroxide. Formulations of 5% and 10% were used.^[12]

Synthesis, structure and physical properties[edit]

Benzoyl peroxide was the first organic peroxide prepared by intentional synthesis. It was prepared by treating benzoyl chloride with barium peroxide,^[13] a reaction that probably follows this equation:

2 C₆H₅C(O)Cl + BaO₂ → [C₆H₅C(O)]₂O₂ + BaCl₂

Benzoyl peroxide is usually prepared by treating hydrogen peroxide with benzoyl chloride.

The oxygen-oxygen bond in peroxides is weak. Thus benzoyl peroxide readily undergoes homolysis (symmetrical fission), forming free radicals:

[C₆H₅C(O)]₂O₂ → 2 C₆H₅CO₂^•

The symbol ^• indicates that the products are radicals; i.e., they contain at least one unpaired electron. Such species are highly reactive. The homolysis is usually induced by heating. The half-life of benzoyl peroxide is one hour at 92 °C. At 131 °C, the half-life is one minute.^[14]

Benzoyl peroxide breaks down in contact with skin, producing benzoic acid and oxygen, neither of which is significantly toxic.^[15] It is important to note that the safety of the decomposition products does not mean that the substance itself is safe, as it is benzoyl peroxide's action as an oxidizing agent that is of importance. Hydrogen peroxide can be corrosive due to its oxidizing properties, but decomposes to form water and oxygen. Sodium hypochlorite (commonly known as "bleach") also shares these properties, but disproportionates to form harmless products such as sodium chloride.