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| Selenium dioxide | |
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| Other names | Selenium(IV) oxide Selenium dioxide |
| Identifiers | |
| CAS number | 7446-08-4 |
| RTECS number | WS4550000 |
| Properties | |
| Molecular formula | SeO2 |
| Molar mass | 110.96 g/mol |
| Appearance | White crystalline solid |
| Density | 3.95 g/cm3, solid |
| Boiling point |
315 °C sublimes |
| Solubility in water | 3.950 g/100 ml (25 °C) |
| Acidity (pKa) | 2.62 (H2SeO3 ⇌ HSeO3− + H+) 8.32 (HSeO3− ⇌ SeO32− + H+) |
| Structure | |
| Crystal structure | chain structure |
| Coordination geometry |
trigonal |
| Hazards | |
| EU classification | Toxic (T) Dangerous for the environment (N) |
| NFPA 704 |
 0
3
0
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| R-phrases | R23/25, R33, R50/53 |
| S-phrases | (S1/2), S20/21, S28 S45, S60, S61 |
| Flash point | non-flammable |
| 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) Infobox references |
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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
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 SeO32-:
- 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.
- 3Se + 4HNO3 + H2O → 3H2SeO3 + 4NO
- 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 glyoxal2 and the oxidation of cyclohexanone gives cyclohexane-1,2-dione.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
- ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- ^ Ronzio, A. R.; Waugh, T. D. (1955). "Glyoxal Bisulfite". Org. Synth.; Coll. Vol. 3: 438.
- ^ Hach, C. C. Banks, C. V.; Diehl, H. (1963). "1,2-Cyclohexanedione Dioxime". Org. Synth.; Coll. Vol. 4: 229.
- ^ Coxon, J. M.; Dansted, E.; Hartshorn, M. P. (1988). "Allylic Oxidation with Hydrogen Peroxide–Selenium Dioxide: trans-Pinocarveol". Org. Synth.; Coll. Vol. 6: 946.
Further reading
- Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.
- Lide, D. R. (ed.) (2002). CRC Handbook of Chemistry and Physics, 83rd ed., Boca Raton, FL: CRC Press. ISBN 0-8493-0483-0.
External links
