Thionyl bromide is the chemical compound SOBr2. It is less stable and less widely used than its chloride analogue, thionyl chloride, but engages in similar reactions.

Chemistry

It is prepared by the action of hydrogen bromide on thionyl chloride, although the corresponding reaction at higher pH (i.e. alkali bromides) proceeds only with difficulty: Phosphorus trichlorodibromide (but not phosphorus pentabromide!) converts sulfur dioxide to thionyl bromide. Thionyl chlorobromide appears to be a key intermediate in these syntheses, but has not been isolated. Thionyl bromide will convert alcohols to alkyl bromides and carboxylic acids to acyl bromides. Unlike with thionyl chloride, stoichiometric bases are problematic activating agents, because free bromide anions decompose thionyl bromide to tribromide, sulfur dioxide, and sulfur. It can be used for brominations of certain α,β-unsaturated carbonyl compounds. It may occasionally be used as a solvent.

Safety

SOBr2 hydrolyzes readily in air to release dangerous fumes of sulfur dioxide and hydrogen bromide. Decomposition to bromine and sulfur monoxide does not occur except at elevated temparatures.