Selenium oxydichloride

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.

Selenium oxydichloride can be prepared by several methods, and a common one involves the conversion of selenium dioxide to dichloroselenious acid followed by dehydration:

SeO₂ + 2 HCl → Se(OH)₂Cl₂

Se(OH)₂Cl₂ → SeOCl₂ + H₂O

The original synthesis involved the redistribution reaction of selenium dioxide and selenium tetrachloride.

Pure selenium oxydichloride autoionizes to a dimer:

SeOCl₂ ↔ (SeO)₂Cl⁺
₃ + Cl⁻

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.

• Selenium oxybromide SeOBr₂
• Selenous acid H₂SeO₃