Thionyl chloride is the mixed-acid anhydride of sulfurous and hydrochloric acids. It is strongly electrophilic and reacts with weak Brönsted acids to yield the corresponding chlorides:

ROH + SOCl2 → RCl + SO2 + HCl, (21.1)

RCOOH + SOCl2 → RCOCl + SO2 + HCl. (21.2)

The equilibrium is favorably shifted by gas evolution, and unlike PCl5 and POCl3, there are no nonvolatile by-products. Thionyl chloride reacts vigorously with water (R = H in Reaction 21.1), although often with a considerable delay; hence, the transportation of thionyl chloride is regulated, and it is a sea-freight item.