Chromium(VI) oxide peroxide

Chromium(VI) oxide peroxide
Names
Other names
chromium(VI) oxide peroxide,
chromium pentoxide
Identifiers
3D model (JSmol)
Properties
CrO5
Molar mass 131.99 g·mol−1
soluble (decomposes without stabilisers)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Chromium(VI) peroxide (CrO5) or chromium oxide peroxide is an unstable compound formed by the addition of acidified hydrogen peroxide solutions to solutions of metal chromates or dichromates, such as sodium chromate or potassium dichromate. The generally yellow chromates or orange dichromates turn to dark blue as chromium(VI) peroxide is formed. Chromate or dichromate reacts with hydrogen peroxide and an acid to give chromium peroxide and water.

CrO42− + 2 H2O2 + 2 H+ → CrO5 + 3 H2O

After a few seconds, the chromium(VI) peroxide decomposes to turn green as chromium(III) compounds are formed.[1] To avoid this decomposition, it is possible to stabilize chromium(VI) oxide peroxide in a water-immiscible organic solvent such as diethyl ether, butan-1-ol or amyl acetate by adding a layer of the organic solvent above the chromate/dichromate solution and shaking during the addition of hydrogen peroxide. In this way, the chromium(VI) peroxide (unstable in the aqueous phase in which newly formed) is dissolved in the immiscible organic solvent. In this condition it can be observed over a much longer period.

2 CrO5 + 7 H2O2 + 6 H+ → 2 Cr3+ + 10 H2O + 7 O2

This compound contains one oxo ligand and two peroxo ligands, making a total of five oxygen atoms per chromium atom.

CrO₅ is unstable and decomposes on standing to form CrO₃. It is incompatible with ketones and primary alcohols and oxidizes them to aldehydes and carboxylic acids and itself gets reduced to Cr³⁺ ion.Therefore, CrO₅ is a good oxidizing agent (even better than Chromates and Dichromates due to the presence of two reactive peroxo ligands) but due to its low stability it is never used in organic syntheses.

Derivatives

Structure of the complex of CrO5 with pyridine

The etherate, bipyridyl and pyridyl complexes of this compound have been found to be effective oxidants in organic chemistry.[2] The structure of the pyridyl complex has been determined crystallographically.[3]

References

  1. ↑ Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils; (1985). "Chromium" (in German). Lehrbuch der Anorganischen Chemie (91–100 ed.). Walter de Gruyter. pp. 1081–1095. ISBN 3-11-007511-3."
  2. ↑ Firouzabadi, H.; Iranpoor, N.; Kiaeezadeh, F.; Toofan, J. (1986). "Chromium(VI) based oxidants-1 Chromium peroxide complexes as versatile, mild, and efficient oxidants in organic synthesis". Tetrahedron. 42: 719. doi:10.1016/S0040-4020(01)87476-7.
  3. ↑ Stomberg, Rolf (1962). "Crystal Structure of Peroxochromates, CrO5⋅C5H5N". Nature. 196 (4854): 570. doi:10.1038/196570b0.


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