As reacts with water it can be used to dehydrate various metal chloride hydrates, such magnesium chloride (), aluminium chloride (), and iron(III) chloride (). This conversion involves treatment with refluxing thionyl chloride and follows the following general equation: MCl_\mathit{n}\! .\! \mathit{x}\ H2O{} + \mathit{x}\ SOCl2 -> MCl_\mathit{n}{} + \mathit{x}\ SO2{} + 2\! \mathit{x}\ HCl Other reactions Thionyl chloride can engage in a range of different electrophilic addition reactions. It adds to alkenes in the presence of to form an aluminium complex which can be hydrolysed to form a sulfinic acid. Both aryl sulfinyl chlorides and diaryl sulfoxides can be prepared from arenes through reaction with thionyl chloride in triflic acid or the presence of catalysts such as , , or . In the laboratory, a reaction between thionyl chloride and an excess of anhydrous alcohol can be used to produce anhydrous alcoholic solutions of HCl. Thionyl chloride undergoes halogen exchange reactions to give compounds such as antimony trifluoride and antimony trichloride: 3 SOCl2 + 2 SbF3 -> 3 SOF2 + 2 SbCl3 or hydrogen bromide and hydrogen chloride: SOCl2 + 2 HBr -> SOBr2 + 2 HCl Batteries Thionyl chloride is a component of lithium–thionyl chloride batteries, where it acts as the positive electrode (in batteries: cathode) with lithium forming the negative electrode (anode); the electrolyte is typically lithium tetrachloroaluminate. The overall discharge reaction is as follows: 4 Li + 2 SOCl2 -> 4 LiCl + 1/8 S8 + SO2 These non-rechargeable batteries had many advantages over other forms of lithium batteries such as a high energy density, a wide operational temperature range, and long storage and operational lifespans. However, their high cost, non-rechargeability, and safety concerns have limited their use. The contents of the batteries are highly toxic and require special disposal procedures; additionally, they may explode if shorted. The technology was used on the Sojourner Mars rover. Safety SOCl2 is highly reactive and can violently release hydrochloric acid upon contact with water and alcohols. It is also a controlled substance under the Chemical Weapons Convention, where it is listed as a Schedule 3 substance, since it is used in the "di-di" method of producing G-series nerve agents and the Meyer and Meyer–Clarke methods of producing sulfur-based mustard gases. History In 1849, the French chemists Jean-François Persoz and Bloch, and the German chemist Peter Kremers (1827-?), independently first synthesized thionyl chloride by reacting phosphorus pentachloride with sulfur dioxide.