Manganese dioxide
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2008-04-30T16:43:37Z
Axiosaurus
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added structure section with references
{{Chembox new
| Name = Manganese(IV) oxide
| ImageFile = Manganese(IV) oxide.jpg
| ImageSize = 200px
| ImageName = Manganese(IV) oxide
| IUPACName = Manganese dioxide<br />Manganese(IV) oxide
| OtherNames = [[Pyrolusite]]
| Section1 = {{Chembox Identifiers
| CASNo = 1313-13-9
| EINECS = 215-202-6
}}
| Section2 = {{Chembox Properties
| Formula = MnO<sub>2</sub>
| MolarMass = 86.9368 g/mol (''varies'')
| Appearance = black solid
| Density = 5.026 g/cm<sup>3</sup>, solid
| Solubility = Insoluble
| MeltingPt = 535 °C ''decomp.''
}}
| Section4 = {{Chembox Thermochemistry
| DeltaHf = −520.9 kJ/mol
| Entropy = 53.1 J.K<sup>−1</sup>.mol<sup>−1</sup>
}}
| Section7 = {{Chembox Hazards
| EUClass = Harmful ('''Xn''')
| RPhrases = R20/22
| SPhrases = S2, S25
| NFPA-H =2
| NFPA-F =0
| NFPA-R =1
| NFPA-O =Ox
}}
| Section8 = {{Chembox Related
| OtherAnions = [[Manganese disulfide]]
| OtherCations = [[Technetium dioxide]]<br />[[Rhenium dioxide]]
| OtherCpds = [[Manganese(II) oxide]]<br />[[Hausmannite]]<br />[[Manganese(III) oxide]]<br />[[Manganese trioxide]]<br />[[Manganese heptoxide]]
}}
}}
'''Manganese(IV) oxide''' is the [[chemical compound]] MnO<sub>2</sub>, 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 nodule]]s. The principal use for MnO<sub>2</sub> is for dry-cell [[battery (electricity)|batteries]], such as the [[alkaline battery]] and the [[zinc-carbon battery]]. In 1976 this application accounted for 500,000 [[tonne]]s of pyrolusite.<sup>[1]</sup> MnO<sub>2</sub> is also used for production of [[Potassium permanganate|MnO<sub>4</sub><sup>–</sup>]]. It is used extensively as an [[redox|oxidising agent]] in [[organic synthesis]], for example, for the oxidation of [[allylic]] [[alcohol]]s.
==Structure==
A number of [[polymorphism (materials science)|polymorphs]] of MnO<sub>2</sub> have been identified. The most common is β-MnO<sub>2</sub>,([[pyrolusite]]) which has the [[titanium dioxide|TiO<sub>2</sub>]], rutile structure.<ref name = "Wells"> Wells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications ISBN 0-19-855370-6 </ref> Others are α-MnO<sub>2</sub> , γ-MnO<sub>2</sub>, ε-MnO<sub>2</sub> and λ-MnO<sub>2</sub>.<ref name = "Wells"/><ref name = "Fong">A powder neutron diffraction study of λ and γ manganese dioxide and of LiMn<sub>2</sub>O<sub>4</sub> Fong G C. ; Kennedy B. J. ; Elcombe M. M.; Zeitschrift für Kristallographie, 1994, 209, 12, 941-945</ref> The structures of all of these forms are closely related and are built from linked MnO<sub>6</sub> octahedra and "tunnels" capable of holding cations are formed in all except β-MnO<sub>2</sub>.<ref name = "Wells"/><ref name = "Fong"/> Interest in these [[polymorphism (materials science)|polymorphs]] is due to their use in [[lithium ion]] batteries.
==MnO<sub>2</sub> 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.<ref>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.</ref> The mineral pyrolusite makes a poor reagent. Usually, however, the reagent is generated by treatment of an aqueous solution KMnO<sub>4</sub> with a Mn(II) salt, typically the sulfate at various pH’s.
The predominant application of MnO<sub>2</sub> is for the oxidation of allylic alcohols to the corresponding aldehydes:
::cis-RCH=CHCH<sub>2</sub>OH + MnO<sub>2</sub> → cis-RCH=CHCHO + “MnO” + H<sub>2</sub>O
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 MnO<sub>2</sub> to dialdehydes or diketones. Otherwise, the applications of MnO<sub>2</sub> are numerous, being applicable to many kinds of reactions including amine oxidation, aromatization, oxidative coupling, and thiol oxidation.
==Example of Use==
Some examples of the use of MnO<sub>2</sub> are given below:
*MnO<sub>2</sub> is used as a [[catalyst]] 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.
*MnO<sub>2</sub> 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]]:
:2 H<sub>2</sub>O<sub>2</sub>([[aqueous|aq]]) → O<sub>2</sub>([[gas|g]]) + 2 H<sub>2</sub>O([[liquid|l]])
*Eighteenth-century British chemists referred to MnO<sub>2</sub> simply as ''manganese''. Elemental manganese was known as ''regulus of manganese''.
*Ancient [[Cave painting|cave painters]] used MnO<sub>2</sub> as a black or brown pigment.
*MnO<sub>2</sub> 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 MnO<sub>2</sub> to remove the natural bluish-green tint of glass, or to add a pinkish or purplish color.
*It is also used in batteries
==Other oxides of manganese==
The green salt [[potassium manganate]] is obtained in minutes when a solution of MnO<sub>2</sub> in molten KOH or NaOH is treated with [[oxidizing agent]]s such as [[potassium nitrate]] ([[Potassium|K]][[Nitrogen|N]]O<sub>3</sub>), [[potassium perchlorate]] (K[[Chlorine|Cl]]O<sub>4</sub>), or even oxygen gas:
:2 MnO<sub>2</sub> + 4 OH<sup>–</sup> + O<sub>2</sub> → 2 MnO<sub>4</sub><sup>2–</sup> + 2 H<sub>2</sub>O
Potassium manganate converts into purple [[potassium permanganate]] in [[aqueous]] [[acid]]ic solution:
3 MnO<sub>4</sub><sup>2–</sup> + 4 H<sup>+</sup> → 2 MnO<sub>4</sub><sup>–</sup> + MnO<sub>2</sub>(s) + 2 H<sub>2</sub>O
==References==
{{reflist}}
==Sources==
* {{cite book | author=Greenwood, N. N.; & Earnshaw, A. |
title=Chemistry of the Elements (2nd Edn.) |
location=Oxford | publisher=Butterworth-Heinemann | year=1997 | id=ISBN 0-7506-3365-4}}
* Oosterhoeks Encyclopedie (Dutch)
==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.
==External links==
* [http://www.orgsyn.org/orgsyn/chemname.asp?nameID=33430 Index of Organic Synthesis procedures utilizing MnO<sub>2</sub>]
* [http://www.organic-chemistry.org/chemicals/oxidations/manganese(IV)oxide.shtm Example Reactions with Mn(IV) oxide]
* [http://www.npi.gov.au/database/substance-info/profiles/52.html National Pollutant Inventory - Manganese and compounds Fact Sheet]
* [http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=14801 PubChem summary of MnO<sub>2</sub>]
* [http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc01/icsc0175.htm International Chemical Safety Card 0175]
[[Category:Manganese compounds]]
[[Category:Oxides]]
[[Category:Oxidizing agents]]
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