Thionyl chloride reacts exothermically with water to form sulfur dioxide and hydrochloric acid:
By a similar process it also reacts with alcohols to form alkyl chlorides. If the alcohol is chiral the reaction generally proceeds via an Si 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.
For example, the addition of  to amino acids in methanol selectively yields the corresponding methyl esters.