Manganese(IV) oxide

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Manganese(IV) oxide
General
Systematic name	Manganese dioxide Manganese(IV) oxide
Other names	Pyrolusite
Molecular formula	MnO₂
Molar mass	86.94 g/mol (varies)
Appearance	black solid
CAS number	^*
Properties
Density and phase	5.026 g/cm³, solid
Solubility in water	Insoluble
Melting point	535 °C decomp.
Thermodynamic data
Standard enthalpy of	−520.9 kJ/mol
Standard molar entropy S°_solid	53.1 J.K⁻¹.mol⁻¹
Hazards
EU classification	Harmful (Xn)
R-phrases	R20/22
S-phrases	S2, S25
NFPA 704
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Regulatory data	Flash point, RTECS number, etc.
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) Chemical infobox

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 (see Manganate)

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 K N O₃) or potassium perchlorate (K Cl O₄).

2MnO₂ + 4OH^- + O₂ → 2MnO₄^2- + 2H₂O

Potassium manganate converts into purple potassium permanganate in aqueous acidic solution: 3MnO₄^2- + 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 MnO_2-x(H₂O)_n where 02." 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.

References

Oosterhoeks Encyclopedie (Dutch)

External links

Manganese compounds | Oxides | Reagents for organic chemistry

Gourt Home::Gourt :: Manganese(IV) oxide

Other oxides of manganese (see Manganate)

MnO2 in organic synthesis

Miscellaneous lore

References

Further reading

External links

MnO₂ in organic synthesis