Superoxide

A superoxide is a compound that contains the superoxide anion, which has the chemical formula O−2. The systematic name of the anion is dioxide(1−). The reactive oxygen anion superoxide is particularly important as the product of the one-electron reduction of dioxygen O2, 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. A superoxide is a compound that contains the superoxide anion, which has the chemical formula O−2. The systematic name of the anion is dioxide(1−). The reactive oxygen anion superoxide is particularly important as the product of the one-electron reduction of dioxygen O2, 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. The superoxide anion, O−2, and its protonated form, the hydroperoxyl radical HO2, are in equilibrium in an aqueous solution: Given that the hydroperoxyl radical has a pKa of around 4.8, at neutral pH superoxide predominantly exists in the anionic form. Superoxides form salts with alkali metals and alkaline earth metals. The salts CsO2, RbO2, KO2, and NaO2 are prepared by the reaction of O2 with the respective alkali metal. The alkali salts of O−2 are orange-yellow in color and quite stable, provided they are kept dry. Upon dissolution of these salts in water, however, the dissolved O−2 undergoes disproportionation (dismutation) extremely rapidly (in a pH-dependent manner): 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 in order to provide a readily available source of oxygen. In this process O−2 acts as a Brønsted base, initially forming the radical HO2·. But 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 all but 0.3% of superoxide is in the anionic form, O−2. Potassium superoxide is soluble in dimethyl sulfoxide (facilitated by crown ethers) and is stable as long as protons are not available. Superoxide can also be generated in aprotic solvents by cyclic voltammetry.