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Manganese oxide

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Manganese(IV) oxide is the chemical compound MnO2, more usually 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. 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.

Other oxides of manganese

The green salt potassium manganate is obtained after about 10 minutes when a solution of MnO2 in molten KOH or NaOH is treated with oxidizing agents such as saltpeter (potassium nitrate KNO3) or potassium perchlorate (KClO4).

2MnO2 + 4OH- + O2 → 2MnO42- + 2H2O

Potassium manganate converts into purple potassium permanganate in aqueous acidic solution:

3MnO42- + 4H+ → 2 MnO4- + MnO2(s) + 2H2O

MnO2 in organic synthesis

Manganese dioxide is widely used as an oxidant in organic synthesis. Forms of MnO2 useful as an oxidizing reagent is not a straightforwardly stoichiometric reagent, far from it. The method of reagent preparation, sample history, and solvents are serious considerations. These problems are typical for many heterogeneous reagents where surface area, among other variables, is a significant variable. Compounding this problem, MnO2 exists in various crystalline forms and is not stoichiometric, being better described as MnO2-x(H2O)n where 0<x<0.5 and n can be nonzero. The mineral pyrolusite makes a poor reagent. A variety of recipes exist for "MnO2." 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 brown precipitate of “manganese dioxide” is dried and activated thermally. It is used in anywhere from 5-50x excess relative to the substrate. Typical solvents are aromatic hydrocarbons, chlorocarbons, diethyl ether, THF, acetone, and ethyl acetate.

The predominant application of MnO2 is for the oxidation of allylic alcohols to the corresponding aldehydes. The configuration of the double bond is conserved:

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

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 lore

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

MnO2is used as a catalyst in the laboratory preparation of oxygen from potassium chlorate, a classical experiments of elementary chemistry classes. The procedure consists of merely heating potassium chlorate and manganese dioxide together in a hard glass container and collecting the gas over water. See oxygen, manganese and potassium chlorate for precautions. 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 sometimes used MnO2 as black paint.

The MnO2 was used for production of chlorine in the eighteenth century, before being displaced by eletrolytic methods. The Mn recovery process used in connection with such production is called the Weldon process.

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