A superoxide, also known by the obsolete name hyperoxide , is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide(1−). It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature. With one unpaired electron, the superoxide ion is a free radical, and, like dioxygen, it is paramagnetic.
Superoxides are compounds in which the oxidation number of oxygen is −½ and the valence ½. The O-O bond distance in O2− is 1.33 Å, vs. 1.21 Å in O2 and 1.49 Å in O22−.
The salts CsO2, RbO2, KO2, and NaO2 are prepared by the direct reaction of O2 with the respective alkali metal. The overall trend corresponds to a reduction in the bond order from 2 (O2), to 1.5 (O2−), to 1 (O22−).
The alkali salts of O2− are orange-yellow in color and quite stable, provided they are kept dry. Upon dissolution of these salts in water, however, the dissolved O2− undergoes disproportionation (dismutation) extremely rapidly:
In this process O2− acts as a Brønsted base, initially forming the radical HO2·. The pKa of its conjugate acid, hydrogen superoxide (HO2·, also known as "hydroperoxyl" or "perhydroxy radical"), is 4.88 so that at neutral pH 7 the vast majority of superoxide is in the anionic form, O2−.
Salts also decompose in the solid state, but this process requires heating:
This reaction is the basis of the use of potassium superoxide as an oxygen source in chemical oxygen generators, such as those used on the space shuttle and on submarines. Superoxides are also used in firefighters' oxygen tanks in order to provide a readily available source of oxygen.
Superoxide in biology
Superoxide is biologically quite toxic and is deployed by the immune system to kill invading microorganisms. In phagocytes, superoxide is produced in large quantities by the enzyme NADPH oxidase for use in oxygen-dependent killing mechanisms of invading pathogens. Mutations in the gene coding for the NADPH oxidase cause an immunodeficiency syndrome called chronic granulomatous disease, characterized by extreme susceptibility to infection, especially catalase positive organisms. In turn, micro-organisms genetically engineered to lack superoxide dismutase (SOD), lose virulence. Superoxide is also deleteriously produced as a byproduct of mitochondrial respiration (most notably by Complex I and Complex III), as well as several other enzymes, for example xanthine oxidase.
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