water and alcohols Thionyl chloride reacts exothermically with water to form sulfur dioxide and hydrochloric acid: SOCl2 + H2O -> 2 HCl + SO2 By a similar process it also reacts with alcohols to form alkyl chlorides. If the alcohol is chiral the reaction generally proceeds via an SNi mechanism with retention of stereochemistry; however, depending on the exact conditions employed, stereo-inversion can also be achieved. Historically the use of with pyridine was called the Darzens halogenation, but this name is rarely used by modern chemists. Reactions with an excess of alcohol produce sulfite esters, which can be powerful methylation, alkylation and hydroxyalkylation reagents. SOCl2 + 2 R-OH -> (R-O)2SO + 2 HCl For example, the addition of to amino acids in methanol selectively yields the corresponding methyl esters. With carboxylic acids Classically, it converts carboxylic acids to acyl chlorides: SOCl2 + R-COOH -> R-COCl + SO2 + HCl The reaction mechanism has been investigated: With nitrogen species With primary amines, thionyl chloride gives sulfinylamine derivatives (RNSO), one example being N-sulfinylaniline. Thionyl chloride reacts with primary formamides to form isocyanides and with secondary formamides to give chloroiminium ions; as such a reaction with dimethylformamide will form the Vilsmeier reagent. By an analogous process primary amides will react with thionyl chloride to form imidoyl chlorides, with secondary amides also giving chloroiminium ions. These species are highly reactive and can be used to catalyse the conversion of carboxylic acids to acyl chlorides, they are also exploited in the Bischler–Napieralski reaction as a means of forming isoquinolines. Primary amides will continue on to form nitriles if heated (Von Braun amide degradation). Thionyl chloride has also been used to promote the Beckmann rearrangement of oximes. With sulfur species Thionyl chloride will transform sulfinic acids into sulfinyl chlorides Sulfonic acids react with thionyl chloride to produce sulfonyl chlorides. Sulfonyl chlorides have also been prepared from the direct reaction of the corresponding diazonium salt with thionyl chloride. Thionyl chloride can be used in variations of the Pummerer rearrangement. With phosphorus species Thionyl chloride converts phosphonic acids and phosphonates into phosphoryl chlorides. It is for this type of reaction that thionyl chloride is listed as a Schedule 3 compound, as it can be used in the "di-di" method of producing G-series nerve agents. For example, thionyl chloride converts dimethyl methylphosphonate into methylphosphonic acid dichloride, which can be used in the production of sarin and soman. With metals