3Organic 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 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).