Selenium dioxide

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

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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. (1955). "Glyoxal Bisulfite". Org. Synth.; Coll. Vol. 3: 438. 
3. ^ Hach, C. C. Banks, C. V.; Diehl, H. (1963). "1,2-Cyclohexanedione Dioxime". Org. Synth.; Coll. Vol. 4: 229. 
4. ^ 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 Edn.). Boca Raton (FL):CRC Press. ISBN 0-8493-0483-0.