selenium dioxide

Selenium dioxide (SeO₂) is a poisonous, white crystalline compound that sublimes into a yellow-green vapor. It is produced by oxidizing selenium with nitric acid or air, or by dehydrating selenious acid.1

In organic chemistry, selenium dioxide is primarily used as a catalyst and oxidizing agent. A key application is its ability to oxidize a methylene group adjacent to a carbonyl group into a second carbonyl group.1

Selenium itself is a rare chemical element, classified as a metalloid, found in Group 16 of the periodic table. It shares properties with sulfur and tellurium. While selenium is not inherently poisonous, many of its compounds, including selenium dioxide, are toxic.2