|Molar mass||242.23 g mol−1|
103–105 °C decomp.
|Solubility in water||poor|
|EU classification||Explosive (E)
|R-phrases||, , ,|
|S-phrases||, , ,|
|Supplementary data page|
|n, εr, etc.|
Solid, liquid, gas
|Spectral data||UV, IR, NMR, MS|
(what is this?) |
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Benzoyl peroxide (pronounced /ˈbɛnzɔɪl pəˈrɒksaɪd/) is an organic compound in the organic peroxide family. It consists of two benzoyl groups joined by a peroxide group. 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 polymerising polyester and many other uses.
Benzoyl peroxide was the first organic peroxide prepared by intentional synthesis. It was prepared by treating benzoyl chloride with barium peroxide, a reaction that probably follows this stoichiometry:
The oxygen-oxygen bond in peroxides is weak. Thus benzoyl peroxide readily undergoes homolysis (symmetrical scission), forming free radicals:
The symbol • indicates that the products are radicals, i.e. they contain an odd number of electrons. 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.
Benzoyl peroxide breaks down in contact with skin, producing benzoic acid and oxygen, neither of which is significantly toxic. 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 decomposes to form harmless products such as sodium chloride.
Benzoyl peroxide for acne treatment is typically applied to the affected areas in gel or cream form, in concentrations of 2.5% increasing through the usually effective 5% to up to 10%. Research suggests that 5 and 10% concentrations are not significantly more effective than 2.5% and 2.5% is usually better tolerated. It 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 possibly swelling. It is sensible to apply the lowest concentration and build up as appropriate. Once tolerance is achieved, increasing the quantity or concentration a second time and gaining tolerance at a higher level usually gives better subsequent acne clearance. Benzoyl peroxide works as a peeling agent, increasing skin turnover and clearing pores, thus reducing the bacterial count there as well as directly as an antimicrobial.
Other common uses for benzoyl peroxide include dyeing hair, and as an active ingredient in teeth whitening systems. It is also used in the preparation of flour, and can be used as an initiator and catalyst for polyester thermoset resins (as an alternative to the much more hazardous methyl ethyl ketone peroxide).
In the U. S., the typical concentration for benzoyl peroxide is 2.5% to 10% for both prescription and over the counter preparations 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.
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.
Unlike most organic compounds, benzoyl peroxide is potentially explosive and hence it can cause fires without external ignition. The hazard is acute for the pure material, and for this reason, in commerce the compound is usually used as a solution or a paste. For example, cosmetics contain only a few percent of benzoyl peroxide and thus pose no explosion risk.