Selenium dioxide

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Selenium dioxide
Selenium dioxide
Ball-and-stick model of a chain in crystalline selenium dioxide
Selenium dioxide.jpg
Other names
Selenium(IV) oxide
Selenous anhydride
7446-08-4 YesY
3D model (Jmol) Interactive image
ChemSpider 22440 YesY
ECHA InfoCard 100.028.358
PubChem 24007
RTECS number VS8575000
UNII 9N3UK29E57 YesY
UN number 3283
Molar mass 110.96 g/mol
Appearance White, yellowish white,[1] or red[1] crystalline solid
Odor rotten radishes
Density 3.954 g/cm3, solid
Melting point 340 °C (644 °F; 613 K) (sealed tube)
Boiling point 350 °C (662 °F; 623 K) subl.
38.4 g/100 mL (20 °C)
39.5 g/100 ml (25 °C)
82.5 g/100 mL (65 °C)
Solubility soluble in benzene
Solubility in ethanol 6.7 g/100 mL (15 °C)
Solubility in acetone 4.4 g/100 mL (15 °C)
Solubility in acetic acid 1.11 g/100 mL (14 °C)
Solubility in methanol 10.16 g/100 mL (12 °C)
Vapor pressure 1.65 kPa (70 °C)
Acidity (pKa) 2.62; 8.32
> 1.76
see text
trigonal (Se)
Safety data sheet ICSC 0946
Toxic (T)
Dangerous for the environment (N)
R-phrases R23/25, R33, R50/53
S-phrases (S1/2), S20/21, S28, S45, S60, S61
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
5890 mg/m3 (rabbit, 20 min)
6590 mg/m3 (goat, 10 min)
6590 mg/m3 (sheep, 10 min)[2]
Related compounds
Other anions
Selenium disulfide
Other cations
Sulfur dioxide
Tellurium dioxide
Selenium trioxide
Related compounds
Selenous acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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


Solid SeO2 is a one-dimensional polymer, the chain consisting of alternating selenium and oxygen atoms. Each Se atom is pyramidal and bears a terminal oxide group. The bridging Se-O bond lengths are 179 pm and the terminal Se-O distance is 162 pm.[3] The relative stereochemistry at Se alternates along the polymer chain (syndiotactic). In the gas phase selenium dioxide is present as dimers and other oligomeric species, at higher temperatures it is monomeric.[4] The monomeric form adopts a bent structure very similar to that of sulfur dioxide with a bond length of 161 pm.[4] The dimeric form has been isolated in a low temperature argon matrix and vibrational spectra indicate that it has a centrosymmetric chair form.[3] Dissolution of SeO2 in selenium oxydichloride give the trimer [Se(O)O]3.[4] Monomeric SeO2 is a polar molecule, with the dipole moment of 2.62 D [5] pointed from the midpoint of the two oxygen atoms to the selenium atom.

The solid sublimes readily. At very low concentrations the vapour has a revolting odour, resembling decayed horseradishes. At higher concentrations the vapour has an odour resembling horseradish sauce and can burn the nose and throat on inhalation. Whereas SO2 tends to be molecular and SeO2 is a one-dimensional chain, TeO2 is a cross-linked polymer.[3]

SeO2 is considered an acidic oxide: it dissolves in water to form selenous acid.[4] Often the terms selenous acid and selenium dioxide are used interchangeably. It reacts with base to form selenite salts containing the SeO2−
anion. For example, reaction with sodium hydroxide produces sodium selenite:

SeO2 + 2 NaOH → Na2SeO3 + H2O


Selenium dioxide is prepared by oxidation of selenium by burning in air and nitric acid or by reaction with hydrogen peroxide, but perhaps the most convenient preparation is by the dehydration of selenous acid.

3 Se + 4 HNO3 + H2O → 3 H2SeO3 + 4 NO
2 H2O2 + Se → SeO2 + 2 H2O
H2SeO3 ⇌ SeO2 + H2O


The natural form of selenium dioxide, downeyite, is a very rare mineral. It is found in only a very few burning coal dumps.[6]


Organic synthesis[edit]

SeO2 is an important reagent in organic synthesis. Oxidation of paraldehyde (acetaldehyde trimer) with SeO2 gives glyoxal[7] and the oxidation of cyclohexanone gives cyclohexane-1,2-dione.[8] The selenium starting material is reduced to selenium, and precipitates as a red amorphous solid which can easily be filtered off.[8] This type of reaction is called a Riley oxidation. It is also renowned as a reagent for "allylic" oxidation,[9] a reaction that entails the following conversion

Allylic oxidation

This can be described more generally as;

R2C=CR'-CHR"2 + [O] → R2C=CR'-C(OH)R"2

where R, R', R" may be alkyl or aryl substituents.

As a colorant[edit]

Selenium dioxide imparts a red colour to glass. It is used in small quantities to counteract the colour due to iron 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 was also used as a toner in photographic developing.


Selenium is an essential element, but ingestion of more than 5 mg/day leads to nonspecific symptoms.[10]


  1. ^ a b
  2. ^ "Selenium compounds (as Se)". Immediately Dangerous to Life and Health. National Institute for Occupational Safety and Health (NIOSH). 
  3. ^ a b c Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium, Franceso A. Devillanova, Royal Society of Chemistry, 2007, ISBN 9780854043668
  4. ^ a b c d Holleman, A. F.; Wiberg, E. (2001), Inorganic Chemistry, San Diego: Academic Press, ISBN 0-12-352651-5 
  5. ^ Takeo, Harutoshi; Hirota, Eizi; Morino, Yonezo (1972). "Third-order potential constants and dipole moment of SeO2 by microwave spectroscopy". Journal of Molecular Spectroscopy. 41 (2): 420–422. doi:10.1016/0022-2852(72)90216-0. ISSN 0022-2852. 
  6. ^ Finkelman, Robert B.; Mrose, Mary E. (1977). "Downeyite, the first verified natural occurrence of SeO2" (PDF). American Mineralogist. 62: 316–320. 
  7. ^ Ronzio, A. R.; Waugh, T. D. (1955). "Glyoxal Bisulfite". Org. Synth.  ; Coll. Vol., 3, p. 438 
  8. ^ a b Hach, C. C. Banks, C. V.; Diehl, H. (1963). "1,2-Cyclohexanedione Dioxime". Org. Synth.  ; Coll. Vol., 4, p. 229 
  9. ^ Coxon, J. M.; Dansted, E.; Hartshorn, M. P. (1988). "Allylic Oxidation with Hydrogen Peroxide–Selenium Dioxide: trans-Pinocarveol". Org. Synth.  ; Coll. Vol., 6, p. 946 
  10. ^ Bernd E. Langner "Selenium and Selenium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a23_525

External links[edit]