Manganese dioxide

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Overview

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.

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. 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

MnO₂ has found many applications and investigations inside and beyond the laboratory. The examples below illustrate some:

• MnO₂ is used as a catalyst in the laboratory preparation of oxygen from potassium chlorate, a classical experiment of elementary chemistry classes. The procedure consists of heating a mixture potassium chlorate and manganese dioxide in a hard glass container and collecting the gas 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.
• The MnO₂ was used for production of chlorine in the eighteenth century, before being displaced by eletrolytic methods. Manganese was recovered from such production by the so-called Weldon process.
• Earliest glassmakers used manganese 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 MnO₄^2– + 2 H₂O

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

References

1. Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.). Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4.CS1 maint: Multiple names: authors list (link)
2. 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.

External links

• Index of Organic Synthesis procedures utilizing MnO₂
• National Pollutant Inventory - Manganese and compounds Fact Sheet
• PubChem summary of MnO₂
• International Chemical Safety Card 0175