Selenium dioxide

Selenium dioxide
Other names	Selenium(IV) oxide Selenous anhydride
Identifiers
CAS number	7446-08-4 Y
PubChem	24007
UN number	3283
RTECS number	VS8575000
Properties
Molecular formula	SeO2
Molar mass	110.96 g/mol
Appearance	White crystalline solid
Density	3.95 g/cm3, solid
Melting point	340 ºC (sealed tube)
Boiling point	315 °C subl.
Solubility in water	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)
Vapor pressure	1.65 kPa (70 ºC)
Acidity (pKa)	2.62; 8.32
Structure
Crystal structure	see text
Coordination geometry	trigonal (Se)
Hazards
MSDS	ICSC 0946
EU Index	034-002-00-8
EU classification	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	0 3 0
Flash point	Non-flammable
Related compounds
Other anions	Selenium disulfide
Other cations	Sulfur dioxide Tellurium dioxide
Related selenium oxides	Selenium trioxide
Related compounds	Selenous acid
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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.

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. The vapour has an odour resembling horseradish sauce and can burn the nose and throat on inhalation. 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 SeO2−3. For example, reaction with sodium hydroxide produces sodium selenite:

SeO₂ + 2 NaOH → Na₂SeO₃ + H₂O

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.

3 Se + 4 HNO₃ + H₂O → 3 H₂SeO₃ + 4 NO

H₂O₂ + Se → SeO₂ + H₂ (United States Patent #2,616,791)

H₂SeO₃ ⇌ SeO₂ + H₂O

Occurrence

Very rare mineral downeyite is the natural form of selenium dioxide and is derived from very few burning coal dumps.

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]. The selenium starting material is reduced to selenium, and precipitates as a red amorphous solid which can easily be filtered off.^[3] This type of reaction is called a Riley oxidation. 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

External links

Wikimedia Commons has media related to: selenium dioxide

• International Chemical Safety Card 0946