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Manganese dioxide
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| Manganese(IV) oxide | |
|---|---|
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| IUPAC name | Manganese dioxide Manganese(IV) oxide |
| Other names | Pyrolusite |
| Identifiers | |
| CAS number | 1313-13-9 |
| PubChem | |
| EINECS number | |
| Properties | |
| Molecular formula | MnO2 |
| Molar mass | 86.9368 g/mol (varies) |
| Appearance | black solid |
| Density | 5.026 g/cm3, solid |
| Melting point |
535 °C decomp. |
| Solubility in water | Insoluble |
| Thermochemistry | |
| Std enthalpy of formation ΔfH |
−520.9 kJ/mol |
| Standard molar entropy S |
53.1 J.K−1.mol−1 |
| Hazards | |
| EU classification | Harmful (Xn) |
| NFPA 704 |
 0
2
1
OX
|
| R-phrases | R20/22 |
| S-phrases | S2, S25 |
| Related compounds | |
| Other anions | Manganese disulfide |
| Other cations | Technetium dioxide Rhenium dioxide |
| Related compounds | Manganese(II) oxide Hausmannite Manganese(II,III) oxide 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) Infobox references |
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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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[edit] Structure
A number of polymorphs of MnO2 have been identified. The most common is β-MnO2,(pyrolusite) which has the TiO2, rutile structure.[1] Others are α-MnO2, γ-MnO2, ε-MnO2, and λ-MnO2.[1][2] The structures of all of these forms are closely related and are built from linked MnO6 octahedra and "tunnels" capable of holding cations are formed in all except β-MnO2.[1][2] Interest in these polymorphs is due to their use in lithium ion batteries.
[edit] 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.[3] 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:
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- 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.
[edit] Example of Use
Some examples of the use of MnO2 are given below:
- MnO2 is used as a catalyst(A catalyst is used to speed up a chemical reaction) 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.
- MnO2 is used in combination with anthracite, sand, and gravel in industrial water treatment plants.
- Manganese dioxide also catalyses the decomposition of hydrogen peroxide to oxygen and water:
- 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.
- It is also used in batteries
[edit] 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
[edit] References
- ^ a b c Wells A. F., (1984) Structural Inorganic Chemistry 5th edition. Oxford Science Publications ISBN 0-19-855370-6
- ^ a b Fong G. C., Kennedy B. J., Elcombe M. M., "A powder neutron diffraction study of λ and γ manganese dioxide and of LiMn2O4". Zeitschrift für Kristallographie (1994), 209:12:941–945
- ^ Attenburrow, J.; Cameron, A. F. B.; Chapman, J. H.; Evans, R. M.; Hems, B. A.; Jansen, A. B. A.; Walker, T. J. Chem. Soc. (1952), 1094.
[edit] Sources
- Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.). Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4.
- Oosterhoeks Encyclopedie (Dutch)
[edit] 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.
