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Manganese dioxide



Manganese(IV) oxide
IUPAC name Manganese dioxide
Manganese(IV) oxide
Other names Pyrolusite
Identifiers
CAS number 1313-13-9
EINECS number 215-202-6
Properties
Molecular formula MnO2
Molar mass 86.9368 g/mol (varies)
Appearance black solid
Density 5.026 g/cm3, solid
Melting point

535 °C decomp.

Solubility in water Insoluble
Thermochemistry
Std enthalpy of
formation
ΔfHo298
−520.9 kJ/mol
Standard molar
entropy
So298
53.1 J.K−1.mol−1
Hazards
EU classification Harmful (Xn)
NFPA 704
0
0
2
OX
R-phrases R20/22
S-phrases S2, S25
Related Compounds
Other anions Manganese disulfide
Other cations Technetium dioxide
Rhenium dioxide
Related compounds Manganese(II) oxide
Hausmannite
Manganese(III) oxide
Manganese trioxide
Manganese heptoxide
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Manganese(IV) oxide is the chemical compound MnO2, commonly called manganese dioxide. This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese. It is also present in manganese nodules. The principal use for MnO2 is for dry-cell batteries, such as the alkaline battery and the zinc-carbon battery. In 1976 this application accounted for 500,000 tonnes of pyrolusite.[1] MnO2 is also used for production of MnO4. It is used extensively as an oxidising agent in organic synthesis, for example, for the oxidation of allylic alcohols.

Additional recommended knowledge

Contents

MnO2 in organic synthesis

Manganese dioxide is used as an oxidant in organic synthesis. The effectiveness of the reagent depends on the method of preparation, a problem that is typical for other heterogeneous reagents where surface area, among other variables, is a significant factor.[1] The mineral pyrolusite makes a poor reagent. Usually, however, the reagent is generated by treatment of an aqueous solution KMnO4 with a Mn(II) salt, typically the sulfate at various pH’s.

The predominant application of MnO2 is for the oxidation of allylic alcohols to the corresponding aldehydes:

cis-RCH=CHCH2OH + MnO2 → cis-RCH=CHCHO + “MnO” + H2O

The configuration of the double bond is conserved in the reaction. The corresponding acetylenic alcohols are also suitable substrates, although the resulting propargylic aldehydes can be quite reactive. Benzylic and even unactivated alcohols are also good substrates. 1,2-Diols are cleaved by MnO2 to dialdehydes or diketones. Otherwise, the applications of MnO2 are numerous, being applicable to many kinds of reactions including amine oxidation, aromatization, oxidative coupling, and thiol oxidation.

Miscellaneous

Some examples of the use of MnO2 are given below:

  • MnO2 is used as a catalyst in the classical laboratory preparation of oxygen from potassium chlorate. A mixture of potassium chlorate and manganese dioxide is heated in a hard glass container and the oxygen gas collected over water.
  • Manganese dioxide also catalyses the decomposition of hydrogen peroxide to oxygen and water:
2 H2O2(aq) → O2(g) + 2 H2O(l)
  • Eighteenth-century British chemists referred to MnO2 simply as manganese. Elemental manganese was known as regulus of manganese.
  • Ancient cave painters used MnO2 as a black or brown pigment.
  • MnO2 was used for production of chlorine in the eighteenth century, before being displaced by electrolytic methods. The manganese dioxide was subsequently recovered by the Weldon process.
  • Earliest glassmakers used MnO2 to remove the natural bluish-green tint of glass, or to add a pinkish or purplish color.

Other oxides of manganese

The green salt potassium manganate is obtained in minutes when a solution of MnO2 in molten KOH or NaOH is treated with oxidizing agents such as potassium nitrate (KNO3), potassium perchlorate (KClO4), or even oxygen gas:

2 MnO2 + 4 OH + O2 → 2 MnO42– + 2 H2O

Potassium manganate converts into purple potassium permanganate in aqueous acidic solution: 3 MnO42– + 4 H+ → 2 MnO4 + MnO2(s) + 2 H2O

References

  1. ^ Attenburrow, J.; Cameron, A. F. B.; Chapman, J. H.; Evans, R. M.; Hems, B. A.; Jansen, A. B. A.; Walker, T. J. Chem. Soc. 1952, 1094.

Sources

  • Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.). Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4. 
  • Oosterhoeks Encyclopedie (Dutch)

Further reading

  • Cahiez, G.; Alami, M.; Taylor, R. J. K.; Reid, M.; Foot, J. S. "Manganese Dioxide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Manganese_dioxide". A list of authors is available in Wikipedia.
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