In organic chemistry, peroxide is a specific functional group or a molecule containing an oxygen-oxygen single bond (R-O-O-R'). When the other oxygen bears a hydrogen, it is called a hydroperoxide (R-O-O-H). The radical HOO· is known as hydroperoxide radical, and is thought to be involved in combustion of hydrocarbons in air.
Organic peroxides tend to decompose easily to free radicals of the form:
However, the same property also means that organic peroxides can accidentally initiate explosive polymerization in materials with unsaturated chemical bonds. Since peroxides can form spontaneously in some materials, some caution must be exercised with such "peroxide-forming materials." Triacetone triperoxide (TATP) and hexamethylene triperoxide diamine are explosive organic peroxide compounds; TATP may be formed accidentally as a waste product in some reactions. In addition, many liquid ethers in the presence of air, light, and metal slowly (over a period of months) form ether peroxides (e.g., diethyl ether peroxide), which are extremely unstable. As a consequence, it is recommended that ether be stored over potassium hydroxide, which not only destroys peroxides but also acts as a powerful desiccant. Extreme care must be taken with samples showing signs of crystal growth or precipitates.
TATP is an easily synthesized, inexpensive, explosive compound that is difficult to detect by normal screening methods. Consequently, it is an explosive favored by terrorists. TATP was used in the 2005 London Underground bombings and the 2001 "shoe bomber." In 2002, a simple mass spectroscopy screening method was developed.[1]
The peroxide ion contains two electrons more than the oxygen molecule. These two electrons, according to the molecular orbital theory, complete the two π* antibonding orbitals. This has as result a weakening of the bond strength of the peroxide ion and a greater length for the bond O-O : Li2O2 130 pm to BaO2 147 pm. Furthermore, the peroxide ion is diamagnetic.
The peroxides of the alkali metals and Ca, Sr and Ba are ionic. The peroxides of a number of electropositive metals such as Mg, the lanthanides and the uranyl-ion show an intermediary character, between ionic and covalent. The peroxides of metals such as Zn, Cd and Hg are mainly covalent.
Peroxides are powerful oxidizers, and usually fairly unstable. Ionic peroxides react with water and diluted acids to form hydrogen peroxide. Organic compounds are oxidized to carbonates, even at normal temperatures. Sodium peroxide is a powerful oxidator of metals, such as iron.
