Selenium dioxide
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223215650
2008-07-03T02:30:48Z
206.192.172.22
/* Preparation */
{{Chembox new
| Name = Selenium dioxide
| ImageFile = Selenium dioxide.png
<!-- | ImageSize = 200px -->
| ImageName = Selenium dioxide
| ImageFile1 = Selenium-dioxide-chain-3D-balls.png
<!-- | ImageSize1 = 300px -->
| ImageName1 = Ball-and-stick model of a chain in crystalline selenium dioxide
| OtherNames = Selenium(IV) oxide<br />Selenium dioxide
| Section1 = {{Chembox Identifiers
| CASNo = 7446-08-4
| RTECS = WS4550000
}}
| Section2 = {{Chembox Properties
| Formula = [[Selenium|Se]][[Oxygen|O]]<sub>2</sub>
| MolarMass = 110.96 g/mol
| Appearance = White crystalline solid
| Density = 3.95 g/cm<sup>3</sup>, solid
| Solubility = 3.950 g/100 ml (25 °C)
| BoilingPt = 315 °C ''sublimes''
| pKa = 2.62 (H<sub>2</sub>SeO<sub>3</sub> {{unicode|⇌}} HSeO<sub>3</sub><sup>−</sup> + H<sup>+</sup>)<br />8.32 (HSeO<sub>3</sub><sup>−</sup> {{unicode|⇌}} SeO<sub>3</sub><sup>2−</sup> + H<sup>+</sup>)
}}
| Section3 = {{Chembox Structure
| Coordination = trigonal
| CrystalStruct = chain structure
}}
| Section7 = {{Chembox Hazards
| EUClass = Toxic ('''T''')<br />Dangerous for<br />the environment ('''N''')
| NFPA-H = 3
| NFPA-F =
| NFPA-R =
| RPhrases = {{R23/25}}, {{R33}}, {{R50/53}}
| SPhrases = {{S1/2}}, {{S20/21}}, {{S28}}<br />{{S45}}, {{S60}}, {{S61}}
| FlashPt = non-flammable
}}
| Section8 = {{Chembox Related
| OtherCations = [[Sulfur dioxide]]<br />[[Tellurium dioxide]]
| OtherCpds = [[Selenium trioxide]]<br />[[Selenous acid]]
}}
}}
'''Selenium dioxide''' is the [[chemical compound]] with the [[chemical formula|formula]] SeO<sub>2</sub>. This colorless solid is one of the most frequently encountered compounds of [[selenium]].
==Properties==
Solid SeO<sub>2</sub> is a one-dimensional [[polymer]], the chain consisting of alternating Se and [[oxygen]] atoms. Each Se atom, which is pyramidal, bears a terminal oxide group. The relative [[stereochemistry]] at Se alternates along the polymer chain ([[syndiotactic]]). The solid sublimes readily. Gaseous selenium dioxide adopts a bent structure very similar to that of [[sulfur dioxide]]. Dissolution of SeO<sub>2</sub> in [[selenium oxydichloride]] give the trimer [Se(O)O]<sub>3</sub>.<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref> Whereas SO<sub>2</sub> tends to be molecular and SeO<sub>2</sub> is a one-dimensional chain, [[Tellurium dioxide|TeO<sub>2</sub>]] is a cross-linked polymer.
SeO<sub>2</sub> is considered an [[acidic oxide]]: it dissolves in water to form selenous (selenious) acid. Often the term selenous (selenious) acid and selenium dioxide are used interchangeably. It reacts with base to form selenite SeO<sub>3</sub><sup>2-</sup>:
:SeO<sub>2</sub> + 2 NaOH → [[sodium selenite|Na<sub>2</sub>SeO<sub>3</sub>]] + H<sub>2</sub>O
==Preparation==
Selenium dioxide is prepared by oxidation of [[selenium]] by burning in air, nitric acid or by reaction with [[hydrogen peroxide]], but perhaps the most convenient preparation is by the dehydration of [[selenous acid]].
:3Se + 4HNO<sub>3</sub> + H<sub>2</sub>O → 3H<sub>2</sub>SeO<sub>3</sub> + 4NO
:H<sub>2</sub>O<sub>2</sub> + Se → SeO<sub>2</sub> + H<sub>2</sub> United States Patent #2,616,791
:H<sub>2</sub>SeO<sub>3</sub> {{unicode|⇌}} SeO<sub>2</sub> + H<sub>2</sub>O
==Uses==
====Organic synthesis====
SeO<sub>2</sub> is an important reagent in [[organic synthesis]]. Oxidation of [[paraldehyde]] (acetaldehyde trimer) with SeO<sub>2</sub> gives [[glyoxal]]<ref>{{OrgSynth | author = Ronzio, A. R.; Waugh, T. D. | title = Glyoxal Bisulfite | collvol = 3 | collvolpages = 438 | year = 1955| prep = CV3P0438}}</ref> and the oxidation of [[cyclohexanone]] gives cyclohexane-1,2-dione.<ref>{{OrgSynth | author = Hach, C. C. Banks, C. V.; Diehl, H. | title = 1,2-Cyclohexanedione Dioxime | collvol = 4 | collvolpages = 229 | year = 1963 | prep = CV4P0229}}</ref>. This type of reaction is called a [[Riley oxidation]]. It is also renown as a reagent for "allylic" oxidation,<ref>{{OrgSynth | author = Coxon, J. M.; Dansted, E.; Hartshorn, M. P. | title = Allylic Oxidation with Hydrogen Peroxide–Selenium Dioxide: trans-Pinocarveol | collvol = 6 | collvolpages = 946 | year = 1988 | prep = CV6P0946}}</ref> a reaction that entails the conversion
:R<sub>2</sub>C=CR'-CHR"<sub>2</sub> + [O] → R<sub>2</sub>C=CR'-C(OH)R"<sub>2</sub>
(where R, R', R" are [[alkyl]] or [[aryl]]).
====As a colorant====
Selenium dioxide imparts a red colour to [[glass]]: it is used in small quantities to counteract the blue colour due to [[cobalt]] impurities and so to create (apparently) colourless glass. In larger quantities, it gives a deep ruby red colour.
Selenium dioxide is the active ingredient in some [[Bluing (steel)|cold-blueing]] solutions.
It is also used as a toner in [[photographic developing]].
{{commonscat|selenium dioxide}}
==References==
{{reflist}}
==Further reading==
*{{Cotton&Wilkinson6th}}
*{{RubberBible83rd}}
==External links==
*{{ICSC|0946|09}}
[[Category:Selenium compounds]]
[[Category:Oxides]]
[[Category:Photographic chemicals]]
[[Category:Acidic oxides]]
[[de:Selendioxid]]
[[ko:이산화 셀레늄]]
[[it:Diossido di selenio]]
[[ja:二酸化セレン]]
[[fi:Seleenidioksidi]]
[[zh:二氧化硒]]