Manganese dioxide

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.

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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 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

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.