Manganese dioxide

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 MnO42– + 2 H2O

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