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Concept
Superoxide
Summary
In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of the one-electron reduction of dioxygen , which occurs widely in nature. Molecular oxygen (dioxygen) is a diradical containing two unpaired electrons, and superoxide results from the addition of an electron which fills one of the two degenerate molecular orbitals, leaving a charged ionic species with a single unpaired electron and a net negative charge of −1. Both dioxygen and the superoxide anion are free radicals that exhibit paramagnetism. Superoxide was historically also known as "hyperoxide".
Superoxide forms salts with alkali metals and alkaline earth metals. The salts caesium superoxide (), rubidium superoxide (), potassium superoxide (), and sodium superoxide () are prepared by the reaction of with the respective alkali metal.
The alkali salts of are orange-yellow in color and quite stable, if they are kept dry. Upon dissolution of these salts in water, however, the dissolved undergoes disproportionation (dismutation) extremely rapidly (in a pH-dependent manner):
2 O2- + H2O -> 3/2 O2 + 2 OH-
This reaction (with moisture and carbon dioxide in exhaled air) 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 to provide a readily available source of oxygen. In this process, acts as a Brønsted base, initially forming the hydroperoxyl radical ().
The superoxide anion, , and its protonated form, hydroperoxyl, are in equilibrium in an aqueous solution:
O2- + H2O HO2 + OH-
Given that the hydroperoxyl radical has a pKa of around 4.8, superoxide predominantly exists in the anionic form at neutral pH.
Potassium superoxide is soluble in dimethyl sulfoxide (facilitated by crown ethers) and is stable as long as protons are not available.
Official source
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