The Chemical Properties of Benzoyl Peroxide

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Benzoyl peroxide has the chemical formula C14H10O4, and can be envisioned as a pair of benzoic acid molecules, joined together at the oxygen atoms from which the acidic hydrogen atoms are missing. 

Benzoyl peroxide belongs to the general class of organic peroxides.  Its reactive properties are a result of the central oxygen-oxygen bond that defines all peroxides.  The peroxide bond cleaves easily (with heating, for example), producing a pair of radicals - highly reactive, neutral chemical structures that have a lone, unpaired electron.  So far as radicals go, the benzoyl radical is relatively stable.  (Its structure permits multiple resonance forms, accounting for the added stability.) 

Like all peroxides, benzoyl peroxide is a strong oxidizer.  As both an oxidizer and a fuel, it has the potential to ignite and explode if enough energy is present.  A spark can set it on fire at as little as 40°C (104°F), and it can ignite spontaneously once it reaches 80°C (176°F).  This peroxide reacts (to produce fire) readily with combustibles and reducing agents.  Shock or friction can generate enough energy to cause it to detonate.  Dry samples may even explode spontaneously. (1)

Benzoyl peroxide is an irritant to humans.  Skin contact is not typically life-threatening, although long-term exposure can cause sensitization, and other people may exhibit allergic responses.  Inhalation is more dangerous, as it not only inflames the membranes, but breaks down into toxic components.  This can be fatal. (1)

These hazards are most pronounced when benzoyl peroxide is in its pure form, or in high concentrations.  In dilute amounts, it poses much less of a risk.  In fact, small amounts of benzoyl peroxide are common in day-to-day life.

Because it is a strong oxidizer, benzoyl peroxide has found use as a bleaching, or whitening agent.  It is the “peroxide” commonly referred to in tooth whiteners and hair dyes.  It works as a bleaching agent when it attacks the double bond structures that are present in most colored molecules.  By destroying these structures, the molecules can no longer absorb certain wavelengths of visible light, so they appear to have no color.  Another common use of benzoyl peroxide is in the treatment of acne.  While a small amount is only irritating to human skin, it proves quite lethal to acne-causing bacteria.

To chemists, the most important use of benzoyl peroxide is as a radical initiator.  Because it breaks down easily into fairly stable radicals, it provides a reliable source of radicals at a known concentration that can be used to start a radical polymerization reaction.  The article “What is a polymer?” describes this process further.

Basic chemical and physical data on benzoyl peroxide follows.

Empirical Formula:        C14H10O4        (2)

Molecular Weight:        242.23 amu         (2)

Appearance:                 White, crystalline (1)

Specific Gravity:           1.334                      (1)

Melting Point:               105°C                     (2)

Boiling Point:                NA - explodes       (2)

Solubility:                      Slightly soluble in water, soluble in ethanol, ether and acetone (2)

Refractive Index:           1.543                      (2)


1.  MSDS - Benzoyl Peroxide, J. T. Baker, Feb 21, 2008  (accessed Feb 21, 2010)

2.  CRC Handbook of Chemistry and Physics, 82nd Edition, 2001-2002

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