Thionyl chloride is an inorganic acid chloride mainly used as a reagent to prepare carboxylic acid chlorides from carboxylic acids.

Application
Thionyl chloride may be used in the following processes:

• To facilitate the synthesis of indenones from 3-hydroxyindanones via dehydroxylation.[2]
• To functionalize silica gel for thioacetalization of aldehydes.[3]
• Conversion of tert-butyl esters to acid chlorides.[4]
• Conversion of aliphatic and aromatic sulfoxides to sulfides in the presence of triphenylphosphine.[5]