The thionyl group is SO, a sulfur atom plus an oxygen atom. It occurs in compounds such as thionyl fluoride, SOF2. Thionyl chloride, SOCl2, is a common reagent used in organic synthesis to convert carboxylic acids to acyl chlorides. In organic chemistry, the thionyl group is known as a sulfoxide group or sulfinyl group, and has the general structure RS(=O)R'. See also Sulfuryl References Functional groups