Peroxy acid

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General formulae of an organic peroxy acid (top) compared with a carboxylic acid (bottom).

A peroxy acid (often spelled as one word, peroxyacid, and sometimes called peracid) is an acid which contains an acidic –OOH group. The two main classes are those derived from conventional mineral acids, especially sulfuric acid, and the organic derivatives of carboxylic acids. They are generally strong oxidizers.

Inorganic peroxy acids

See also: Peroxymonosulfuric acid and Peroxynitric acid

Peroxymonosulfuric acid (Caro's acid) is probably the most important inorganic peracid, at least in terms of the scale. It is used for the bleaching of pulp and for the detoxification of cyanide in the mining industry. It is produced by treating sulfuric acid with hydrogen peroxide. Peroxyphosphoric acid (H₃PO₅) is prepared similarly.

Organic peracids

Several organic peroxyacids are commercially useful. They can be prepared in several ways. Most commonly, peracids are generated by treating the corresponding carboxylic acid with hydrogen peroxide:

RCO₂H + H₂O₂ $\overrightarrow{\leftarrow}$ RCO₃H + H₂O

A related reaction involves treatment of the carboxylic anhydride:

(RCO)₂O + H₂O₂ → RCO₃H + RCO₂H

This method is popular for converting cyclic anhydrides to the corresponding monoperoxyacids, for example monoperoxyphthalic acid.
The third method involves treatment of acid chlorides:

RC(O)Cl + H₂O₂ → RCO₃H + HCl

meta-chloroperoxybenzoic acid (mCPBA) is prepared in this way.

Properties and uses

Peroxycarboxylic acids are about 1000× weaker than the parent carboxylic acid, owing the absence of resonance stabilization of the anion. For similar reasons, their pK_a values tend also to be relatively insensitive to the substituent R.

The largest use of organic peroxy acids is for the conversion of alkenes to epoxides. Certain cyclic ketones are converted to the ring-expanded esters using peracids in a Baeyer-Villiger oxidation. They are also used for the oxidation of amines and thioethers to amine oxides and sulfoxides. The laboratory applications of the valued reagent mCPBA illustrate these reactions. It is used as a reagent in the Baeyer-Villiger oxidation and in oxidation of carbon-carbon double bonds in alkenes to generate epoxides (oxiranes). Reaction of peroxycarboxylic acids with acid chlorides affords diacyl peroxides:

RC(O)Cl + RC(O)O₂H → (RC(O))₂O₂ + HCl

See also

• Organic peroxide
• Meta-Chloroperoxybenzoic acid
• Peracetic acid
• Peroxyacyl nitrates