Selenium oxydichloride is the inorganic compound with the formula SeOCl2. It is a colorless liquid. With a high dielectric constant (55) and high specific conductance, it is an attractive solvent. Structurally, it is a close chemical relative of thionyl chloride SOCl2, being a pyramidal molecule.

Preparation and reactions

Selenium oxydichloride can be prepared by several methods, and a common one involves the conversion of selenium dioxide to dichloroselenious acid followed by dehydration: The original synthesis involved the redistribution reaction of selenium dioxide and selenium tetrachloride. Pure selenium oxydichloride autoionizes to a dimer: The SeOCl2 is generally a labile Lewis acid and solutions of sulfur trioxide in SeOCl2 likely form [SeOCl]+[SO3Cl]− the same way. The compound hydrolyzes readily to form hydrogen chloride and selenium dioxide, and very few organic compounds dissolve in it without reaction. At elevated temperatures, it is a strong oxidizer, yielding a chloride, selenium dioxide, and diselenium dichloride.