- Manganese dioxide
Manganese dioxide Identifiers CAS number PubChem ChemSpider EC number Jmol-3D images Image 1 Properties Molecular formula MnO2 Molar mass 86.9368 g/mol Appearance Brown-black solid Density 5.026 g/cm3 Melting point
535 °C decomp
Solubility in water insoluble Thermochemistry Std enthalpy of
o298 −520.9 kJ/mol Standard molar
o298 53.1 J K−1 mol−1 Hazards MSDS ICSC 0175 EU Index 025-001-00-3 EU classification Harmful (Xn)
R-phrases S-phrases , NFPA 704 Flash point 535 °C Related compounds Other anions Manganese disulfide Other cations Technetium dioxide
Related manganese oxides Manganese(II) oxide
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Manganese(IV) oxide is the inorganic compound with the formula MnO2. This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for MnO2 is for dry-cell batteries, such as the alkaline battery and the zinc-carbon battery. MnO2 is also used as a pigment and as a precursor to other manganese compounds, such as KMnO4. It is used as a reagent in organic synthesis, for example, for the oxidation of allylic alcohols.
Several polymorphs of MnO2 are claimed, as well as a hydrated form. Like many other dioxides, MnO2 crystallizes in the rutile crystal structure (this polymorph is called β-MnO2), with three-coordinate oxide and octahedral metal centres. MnO2 is characteristically nonstoichiometric, being deficient in oxygen. The complicated solid-state chemistry of this material is relevant to the lore of "freshly prepared" MnO2 in organic synthesis.
Naturally occurring manganese dioxide contains impurities and a considerable amount of manganese in its 3+ oxidation state. Only a limited number of deposits contain the γ modification in purity sufficient for the battery industry. The production of ferrite also requires high purity manganese dioxide. Therefore the production of synthetic manganese dioxide is important. Two groups of methods are used, yielding "chemical manganese dioxide" (CMD) and "electrolytical manganese dioxide" (EMD). The CMD is mostly used for the production of ferrites, whereas EMD is used for the production of batteries.
Chemical manganese dioxide
One of the two chemical methods starts from natural manganese dioxide and converts it using dinitrogen tetroxide and water to manganese(II) nitrate solution. Evaporation of the water, leaves the crystalline nitrate salt. At temperatures of 400 °C, the salt decomposes, releasing N2O4 and leaving a residue of purified manganese dioxide. These two steps can be summarized as:
- MnO2 + N2O4 → Mn(NO3)2
- Mn(NO3)2 → MnO2 + N2O4
In the other chemical process, manganese dioxide ore is reduced by heating with oil or coal. The resulting manganese(II) oxide is dissolved in sulfuric acid, and the filtered solution is treated with ammonium carbonate to precipitate MnCO3. The carbonate is calcined in air to give a mixture of manganese(II) and manganese(IV) oxides. To complete the process, a suspension of this material in sulfuric acid is treated with sodium chlorate. Chloric acid, which forms in situ, converts any Mn(III) and Mn(II) oxides to the dioxide, releasing chlorine as a by-product.
Electrolytical manganese dioxide
The important reactions of MnO2 are associated with its redox, both oxidation and reduction.
- MnO2 + 2 C → Mn + 2 CO
The key reactions of MnO2 in batteries is the one-electron reduction:
- MnO2 + e- + H+ → MnO(OH)
MnO2 catalyses several reactions that form O2. In a classical laboratory demonstration, heating a mixture of potassium chlorate and manganese dioxide produces oxygen gas. Manganese dioxide also catalyses the decomposition of hydrogen peroxide to oxygen and water:
- 2 H2O2 → 2 H2O + O2
Manganese dioxide decomposes above about 530 °C to manganese(III) oxide and oxygen. At temperatures close to 1000 °C, the mixed-valence compound Mn3O4 forms. Higher temperatures give MnO.
- 2 MnO2 + 2 H2SO4 → 2 MnSO4 + O2 + 2 H2O
- MnO2 + 4 HCl → MnCl2 + Cl2 + 2 H2O
The standard electrode potentials for the half reactions indicate that the reaction is endothermic at pH = 0 (1 M [H+]), but it is favoured by the lower pH as well as the evolution (and removal) of gaseous chlorine.
- 2 MnO2 + 4 KOH + O2 → 2 K2MnO4 + 2 H2O
Potassium manganate is the precursor to potassium permanganate, a common oxidant.
The predominant application of MnO2 is as a component of dry cell batteries, so called Leclanché cell, or zinc–carbon batteries. Approximately 500,000 tonnes are consumed for this application annually. Other industrial applications include the use of MnO2 as an inorganic pigment in ceramics and in glassmaking.
A specialized use of manganese dioxide is as 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. The mineral pyrolusite makes a poor reagent. Usually, however, the reagent is generated in situ by treatment of an aqueous solution KMnO4 with a Mn(II) salt, typically the sulfate. MnO2 oxidizes allylic alcohols to the corresponding aldehydes or ketones:
- 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.
Manganese dioxide was one of the earliest natural substances used by human ancestors. It was used as a pigment at least from the middle paleolithic. It was possibly used first for body painting, and later for cave painting. Some of the most famous early cave paintings in Europe were executed by means of manganese dioxide.
Manganese dioxide can slightly stain human skin if it is damp or in a heterogeneous mixture, but the stains can be washed off quite easily with some rubbing.
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- ^ Cahiez, G.; Alami, M.; Taylor, R. J. K.; Reid, M.; Foot, J. S. (2004), "Manganese Dioxide", in Paquette, Leo A., Encyclopedia of Reagents for Organic Synthesis, New York: J. Wiley & Sons .
- ^ Attenburrow, J.; Cameron, A. F. B.; Chapman, J. H.; Evans, R. M.; Hems, B. A.; Jansen, A. B. A.; Walker, T. (1952), J. Chem. Soc.: 1094 .
- ^ Leo A. Paquette and Todd M. Heidelbaugh, "(4S)-(−)-tert-Butyldimethylsiloxy-2-cyclopen-1-one", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv9p0136 ; Coll. Vol. 9: 136 (this procedure illustrates the use of MnO2 for the oxidation of an allylic alcohol.
- REACH Mn Consortium
- Index of Organic Synthesis procedures utilizing MnO2
- Example Reactions with Mn(IV) oxide
- National Pollutant Inventory - Manganese and compounds Fact Sheet
- PubChem summary of MnO2
- International Chemical Safety Card 0175
- Potters Manganese Toxicity by Elke Blodgett
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