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Selenium dioxide
Selenium dioxide Ball-and-stick model of a chain in crystalline selenium dioxide
General
Other names	Selenium(IV) oxide Selenium dioxide
Molecular formula	SeO2
Molar mass	110.96 g/mol
Appearance	White crystalline solid
CAS number	[7446-08-4]
Properties
Density and phase	3.95 g/cm3, solid
Solubility in water	3.950 g/100 ml (25 °C)
Boiling point	315 °C ''sublimes''
Acidity (p''K''a)	2.62 (H2SeO3 HSeO3− + H+) 8.32 (HSeO3− SeO32− + H+)
Structure
Coordination geometry	trigonal
Crystal structure	chain structure
'Hazards'
EU classification	Toxic ('T') Dangerous for the environment ('N')
NFPA 704
R-phrases	, ,
S-phrases	, , , ,
U.S. Permissible Exposure Limit	5 ppm
IDLH (NIOSH)	100 ppm
Flash point	non-flammable
RTECS number	WS4550000
Supplementary data page
Structure and properties	''n'', ''εr'', etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other cations	Sulfur dioxide Tellurium dioxide
Related compounds	Selenium trioxide Selenous acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

'Selenium dioxide' is the chemical compound with the formula SeO₂. This colorless solid is one of the most frequently encountered compounds of selenium.

Contents

Properties

Preparation

Uses

Organic synthesis

As a colorant

References

Further reading

External links

Properties

Solid SeO₂ 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 SeO₂ in selenium oxydichloride give the trimer [Se(O)O]₃.^[1] Whereas SO₂ tends to be molecular and SeO₂ is a one-dimensional chain, TeO₂ is a cross-linked polymer.
SeO₂ 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 SeO₃^2-:
:SeO₂ + 2 NaOH → Na₂SeO₃ + H₂O

Preparation

Selenium dioxide is prepared by oxidation of selenium by burning in air or by reaction with hydrogen peroxide, but perhaps the most convenient preparation is by the dehydration of selenous acid.
:3Se + 4HNO₃ + H₂O → 3H₂SeO₃ + 4NO
:H₂SeO₃ SeO₂ + H₂O

Uses

Organic synthesis

SeO₂ is an important reagent in organic synthesis. Oxidation of paraldehyde (acetaldehyde trimer) with SeO₂ gives glyoxal^[2] and the oxidation of cyclohexanone gives cyclohexane-1,2-dione.^[3]
It is also renown as a reagent for "allylic" oxidation,^[4] a reaction that entails the conversion
:R₂C=CR'-CHR"₂ + [O] → R₂C=CR'-C(OH)R"₂
(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 cold-blueing solutions.
It is also used as a toner in photographic developing.

References

1. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. Ronzio, A. R.; Waugh, T. D. “Glyoxal Bisulfite” Organic Syntheses, Collected Volume 3, p.438 (1955) http://www.orgsyn.org/orgsyn/pdfs/CV3P0438.pdf
3. Hach, C. C. Banks, C. V.; Diehl, H. “1,2-Cyclohexanedione Dioxime” Organic Syntheses, Collected Volume 4, p.229 (1963). http://www.orgsyn.org/orgsyn/pdfs/CV4P0229.pdf
4. Coxon, J. M.; Dansted, E.; Hartshorn, M. P. “Allylic Oxidation with Hydrogen Peroxide–Selenium Dioxide: trans-Pinocarveol” Organic Syntheses, Collected Volume 6, p.946 (1988). http://www.orgsyn.org/orgsyn/pdfs/CV6P0946.pdf

Further reading

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External links

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