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.

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. 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 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

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.

It is also renown as a reagent for "allylic" oxidation, 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.