peroxide, chemical compound containing two oxygen atoms, each of which is bonded to the other and to a radical or some element other than oxygen; e.g., in hydrogen peroxide, H₂O₂, the atoms are joined together in the chainlike structure H-O-O-H. Peroxides are powerful oxidizing agents. They are unstable, releasing oxygen when heated. Peroxides may be formed directly by reaction of an element or compound with oxygen. In dry, carbon-dioxide-free air, sodium or barium metal reacts to form its peroxide. In moist air, zinc metal is oxidized and hydrogen peroxide is formed. When a metal peroxide is treated with a dilute acid, a solution of hydrogen peroxide and a metal salt is formed. Ethers can react with oxygen from the air to form peroxides. This creates a special hazard, since the peroxides are often so unstable that they decompose explosively if heated.

Any of a class of chemical compounds in which two oxygen atoms are linked by a single covalent bond. Several organic (see organic compound) and inorganic (see inorganic compound) peroxides are useful as bleaching and oxidizing agents (see oxidation-reduction), as initiators of polymerization reactions, and in the preparation of hydrogen peroxide (a mild bleach and antiseptic) and other oxygen compounds. The peroxide anion (chemical formula O₂²⁻) is present in peroxides of inorganic compounds.

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A peroxide is a compound containing an oxygen-oxygen single bond. The simplest stable peroxide is hydrogen peroxide. Superoxides, dioxygenyls, ozones and ozonides compound are considered separately.

Organic chemistry

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:

RO·

This makes them useful as catalysts for some types of polymerisation, such as the polyester resins used in glass-reinforced plastics. MEKP (methyl ethyl ketone peroxide) is commonly used for this purpose.

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.

Inorganic chemistry

In inorganic chemistry, peroxide is the anion O₂²⁻. It is highly basic, and present in ionic compounds. Pure peroxides (containing only cations and the peroxide anions) are usually formed by burning alkali metals or alkaline earth metals in air or oxygen. Sodium peroxide Na₂O₂ is a typical example.

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.

The oxides, peroxides and superoxides are closely related, forming a chain of oxygen ions of progressively higher oxidation number.

Barium peroxide is used in pyrotechnics and tracer ammunition, and was once used in the manufacture of hydrogen peroxide. Sodium peroxide is used as a carbon dioxide absorber and oxygen regenerator (e.g. in some submarines), through the reaction:

2Na₂O₂ + 2CO₂ → 2Na₂CO₃ + O₂

See also

• catalase
• oxygen
• ozone
• peroxidases
• hydrogen peroxide
• carbamide peroxide
• sodium percarbonate
• calcium peroxide
• magnesium peroxide
• potassium monopersulfate
• sodium perborate monohydrate
• ozonide, O₃⁻
• superoxide, O₂⁻
• oxide, O²⁻
• dioxygenyl, O₂⁺