Peroxide-Type Compounds

a class of chemical compounds that contain oxygen atoms directly bound to each other.

Inorganic peroxide-type compounds. The simplest and most important widespread inorganic peroxide is hydrogen peroxide (H₂O₂). The crystalline lattice of inorganic peroxide-type compounds consists of metal ions in combination with one of the molecular oxyanions, O₂^2–, O₂^–, or O₃^–. According to which of these anions is present, a distinction is made between peroxides, which contain the anion O₂^2–; superoxides, with the anion O₂^–”; and ozonides, with the anion O₃^–. Oxidizing strength differs among peroxide-type compounds, which decompose to release oxygen upon mild thermal or chemical treatment.

Peroxides and superoxides of alkali metals—such as the peroxides Na₂O₂ and K₂O₂ and the superoxides KO₂, RbO₂, and CSO₂—are prepared most simply by roasting the metal in air or in an oxygen atmosphere. Peroxides and superoxides of metals are salts of the weak acids, hydrogen peroxide (H₂O₂) and hydrogen superoxide (HO₂), respectively. Hydrogen superoxide is an active species that rapidly rearranges to give H₂O₂ and O₂ at normal temperatures. It is an intermediate of most combustion reactions and oxidations by oxygen and H₂O₂. The action of ozone (O₃) on hydroxides or superoxides gives ozonides of alkali metals, with the formula MO₃ (where M represents any alkali metal), for example, KO₃. Thermal instability, oxidizing strength, and the amount of oxygen capable of being released are higher in superoxides than in peroxides and higher still in ozonides.

Hydrolysis of inorganic peroxides yields oxidizing agents of various strengths, which can either be saturated compounds, such as H₂O₂, or such species as OH:

M₂O₂ + 2H₂O = 2MOH + H₂O₂

MO₂ + H₂O = MOH + HO₂

MO₃ + H₂O = MOH + OH + O₂

The hydroperoxidates are compounds that contain a hydrogen peroxide of crystallization instead of a water of crystallization, for example, K₂CO₃·H₂O₂; they also include hydroperoxidates of peroxides, for example, CaO₂·2H₂O₂.

The peroxy group, —O—O—, is found in peroxy acids, or peracids, and in binuclear complexes. Examples are the peroxy-sulfuric acids, peroxymonosulfuric acid (HOSO₂—OOH) and peroxydisulfuric acid (HOSO₂—O—O—SO₂OH). Analogous peroxy derivatives are known for carbonic acid and several other acids. These compounds are obtained either by electrolysis of the common acids or by the reaction of concentrated acids and H₂O₂. Binuclear complexes that contain the peroxy group are known to exist for many metals; the complexes of cobalt have been most extensively studied. Many of these cobalt complexes are obtained by the reaction of oxygen with cobalt salts, which can be either in solution or in the crystalline state. Most peroxy compounds are hydrolyzed by water, resulting in the formation of H₂O₂.

Peroxide-type compounds are used in industry as oxidizing agents (peroxydisulfuric acid and sodium peroxide), bleaching agents (peroxyborates, and peroxycarbonates), and convenient sources of oxygen for reconstituting air in a process that is equivalent to converting CO₂ into O₂ (the superoxides NaO₂ and KO₂). Several complex peroxy salts reversibly add oxygen and, upon heating or a change in the acidity of the solution, release oxygen. These salts can thus be used as “oxygen batteries,” as carriers of oxygen and in the separation of mixtures of nitrogen and oxygen. The structure of inorganic peroxide-type compounds determines their reactivity and physical properties as well as the feasibility of their use under various specified conditions.

Organic peroxide-type compounds. Organic peroxide-type compounds contain the peroxy group bound to one or two carbon atoms. The major organic peroxide-type compounds are alkyl and aryl peroxides, R—O—O—R, where R is either an alkyl or aryl group; acyl peroxides, RCO—O—O—COR; organic hydroperoxides, R—O—O—H; and organic peroxy acids, or peracids, RCO—O—O—H. The last type is related to those compounds in which the peroxide grouping is bound to a heteroatom, for example, R₃Si—O—O—Li and R₂B—O—OR, and to the ozonides, which contain the —O—O—O— grouping, for example, CF₃—O—O—O—CF₃. Organic peroxide-type compounds are obtained mainly by hydrogen-peroxide or oxygen-mediated oxidation of various organic compounds, for example, saturated hydrocarbons, olefins, alcohols, aldehydes, ketones, and organometallic compounds (the oxygen-mediated process is often photochemical): O₂

Acyl peroxides and peroxy acids are produced by reacting carboxylic acids or carboxylic-acid derivatives with hydrogen peroxide in the presence of a base:

Dimethyl peroxide (CH₃—O—O—CH₃) is a gas that boils at –13°C, while di-tert-butyl peroxide boils at 70°C (at 197 mm Hg). Acetyl peroxide (CH₃COO)₂ melts at 27°C and boils at 63°C (at 21 mm Hg), benzoyl peroxide (C₆H₅COO)₂ melts between 106° and 108°C, and perbenzoic acid melts between 41° and 43°C. Polymeric peroxide-type compounds with the repeating unit

are known to exist.

The oxygen-oxygen bond of organic peroxide-type compounds can be cleaved by heat or ultraviolet irradiation, resulting in the formation of free radicals of the type RO· or RCO—O·. The subsquent fate of these radicals, and thus the overall direction of the reaction, depends on the nature of R. Of all radicals, alkoxy or acyloxy radicals most often decompose further to give free hydrocarbon radicals:

The free radicals thus formed may initiate a chain reaction by which the peroxide decomposes, and for this reason many peroxides, especially those of lower molecular weight, are explosive. This must be taken into account when working with olefins, dienes, and ethers, all of which easily form peroxides under the influence of atmospheric oxygen. The stability of peroxide-type compounds increases with the electronegativity of the substitu-ents on the peroxy group; furthermore, stability increases as the alkyl substituents go from primary to secondary and tertiary.

The organic peroxide-type compounds benzoyl peroxide, acetyl peroxide, and di-tert-butyl peroxide are widely used to initiate free-radical polymerization and to vulcanize rubber; they are also used in such reactions as oxidation, halogenation addition to a double bond, and telomerization. Peroxide-type compounds, especially peroxyacids, are used in the textile industry as bleaching agents and in organic synthesis as oxidizing agents, for example, to synthesize olefin epoxides by the Prilezhaev reaction. Peroxide-type compounds are intermediate products of many industrially important reactions, for example, the synthesis of phenol and acetone by the oxidation of cumene; they also play a large role in combustion processes and in biochemical oxidations.