Carboxylic acids react with Thionyl Chloride (\(SOCl_2\)) to form acid chlorides. During the reaction the hydroxyl group of the carboxylic acid is converted to a chlorosulfite intermediate making it a better leaving group. The chloride anion produced during the reaction acts a nucleophile.
1) Nucleophilic attack on Thionyl Chloride
2) Removal of Cl leaving group
3) Nucleophilic attack on the carbonyl
4) Leaving group removal
Prof. Steven Farmer (Sonoma State University)