Peroxy acid

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General formulas 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

oxidizers
.

Inorganic peroxy acids

See also: Peroxymonosulfuric acid, Peroxynitric acid, and Peroxymonophosphoric acid

Peroxymonosulfuric acid (Caro's acid) is probably the most important inorganic peracid, at least in terms of its production scale.[1] 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. Peroxymonophosphoric acid (H3PO5) is prepared similarly.[2]

Some peroxy acids are only hypothetical, but their anions are known. This is the case for peroxycarbonate[3] and perborate (see sodium perborate).[4]

Organic peracids

Several organic peroxyacids are commercially useful.[5] They can be prepared in several ways. Most commonly, peracids are generated by treating the corresponding carboxylic acid with hydrogen peroxide:[6]

RCO2H + H2O2 ⇌ 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.[7]

A related method starts with the peroxyanhydride.[8]

Aromatic

aldehydes can be autoxidized
to give peroxycarboxylic acids:

Ar-CHO + O2 → Ar-COOOH (Ar =
aryl
group)

The products, however, react with the initial aldehyde forming the carboxylic acid:

Ar-COOOH + Ar-CHO → 2 Ar-COOH

Properties and uses

In terms of acidity, peroxycarboxylic acids are about 1000 times weaker than the parent carboxylic acid, due to the absence of resonance stabilization of the anion. For similar reasons, their pKa values tend also to be relatively insensitive to substituents.

The most common use of organic peroxy acids is for the conversion of alkenes to epoxides, the

thioethers to amine oxides and sulfoxides
. The laboratory applications of the valued reagent mCPBA illustrate these reactions.

Reaction of peroxycarboxylic acids with acid chlorides affords diacyl peroxides:

RC(O)Cl + RC(O)O2H → (RC(O))2O2 + HCl

The oxidizing tendency of peroxides is related to the

acidity
of the O−H bond. Thus, the order of oxidizing power is CF3CO3H > CH3CO3H > H2O2.

See also

References

  1. ISBN 978-3527306732
    .
  2. ^ Creaser, I.I.; Edwards, J.O. (1972). "?". Topics in phosphorus chemistry. 7: 379–435.
  3. doi:10.1021/jp035749l
    .
  4. ^ "perborate(2-) (CHEBI:30175)". www.ebi.ac.uk.
  5. ISBN 978-3527306732
    .
  6. doi:10.15227/orgsyn.044.0081
    .
  7. doi:10.15227/orgsyn.050.0015
    .
  8. doi:10.15227/orgsyn.008.0030
    .
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