12.4: Peroxides and Superoxides

Peroxides

A peroxide is a compound containing an oxygen–oxygen single bond or the peroxide anion, O2-2.^[1] The O−O group is called the peroxide group orperoxo group. In contrast to oxide ions, the oxygen atoms in the peroxide ion have an oxidation state of −1.

The simplest stable peroxide is hydrogen peroxide. Superoxides, dioxygenyls, ozones and ozonides are considered separately. Peroxide compounds can be roughly classified into organic and inorganic. Whereas the inorganic peroxides have an ionic, salt-like character, the organic peroxides are dominated by the covalent bonds. The oxygen-oxygen chemical bond of peroxide is unstable and easily split into reactive radicals via homolytic cleavage. For this reason, peroxides are found in nature only in small quantities, in water, atmosphere, plants, and animals. Peroxide ion formation has recently been highlighted as one of the main mechanisms by which oxides accommodate excess oxygen in ionic crystals and may have a large impact on a range of industrial applications including solid oxide fuel cells.

Superoxides

Superoxides are compounds in which the oxidation number of oxygen is −½. The O–O bond distance in O−2 is 1.33 Å, vs. 1.21 Å in O₂ and 1.49 Å in O2−2. 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 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):

\[4 O^{−2} + 2 H_2O → 3O_2 + 4 OH^−\]

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 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 can be dissolved 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. Salts also decompose in the solid state, but this process requires heating:

\[2 NaO_2 → Na_2O_2 + O_2\]

The derivatives of dioxygen, O₂, have characteristic O–O distances that correlate with the bond order of the O–O bond (Table 12.4.1).

Table 12.4.1: Bond Distances

Dioxygen compound	name	O–O distance in Å	O–O bond order
\(O^{+2}\)	dioxygenyl cation	1.12	2.5
\(O_2\)	dioxygen	1.21	2
\(O^{−2}\)	superoxide	1.28	1.5
\(O_2^{−2}\)	peroxide	1.49	1

Summary

A superoxide is a compound that contains the superoxide anion with the chemical formula \(O^{−2}\). The systematic name of the anion is dioxide(1−). Superoxide anion is particularly important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature.[2] Whereas molecular oxygen (dioxygen) is a diradical containing two unpaired electrons, the addition of a second electron fills one of its two degenerate molecular orbitals, leaving a charged ionic species with single unpaired electron and a net negative charge of −1. Both dioxygen and superoxide ion are free radicals that exhibit paramagnetism.

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