Properties
SeO2 is a polar molecule, with the dipole moment pointed from the midpoint of the two oxygen atoms to the selenium atom. Solid SeO2 is a one-dimensional polymer, the chain consisting of alternating selenium 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. 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 terms selenous acid and selenium dioxide are used interchangeably.
Uses
Organic synthesis
SeO2 is an important reagent in organic synthesis. Oxidation of paraldehyde (acetaldehyde trimer) with SeO2 gives glyoxal and the oxidation of cyclohexanone gives cyclohexane-1,2-dione. The selenium starting material is reduced to selenium, and precipitates as a red amorphous solid which can easily be filtered off. This type of reaction is called a Riley oxidation. It is also renowned as a reagent for "allylic" oxidation, a reaction that entails the conversion: R2C=CR'-CHR"2 + [O] → R2C=CR'-C(OH)R"2
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.
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