Thionyl chloride

Thionyl chloride

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Thionyl chloride (or thionyl dichloride) is an inorganic compound with the formula SOCl2. SOCl2 is a reactive chemical reagent used in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure that decomposes above 140 °C. SOCl2 is sometimes confused with sulfuryl chloride, SO2Cl2, but the chemical properties of these S(IV) and S(VI) compounds differ significantly. Approximately 45,000 tons/y were produced in the early 1990's.

Properties and structure

The molecule SOCl2 is pyramidal, indicating the presence of a lone pair of electrons on the S(IV) center (in contrast, the stoichiometrically related species COCl2 is planar). SOCl2 reacts with water to release hydrogen chloride and sulfur dioxide.
H2O + O=SCl2SO2 + 2 HCl


The major industrial synthesis involves the reaction of sulfur trioxide and sulfur dichloride:
SO3 + SCl2 → SOCl2 + SO2.

Other methods include

SO2 + PCl5 → SOCl2 + POCl3

SO2 + Cl2 + SCl2 → 2 SOCl2

SO3 + Cl2 + 2 SCl2 → 3 SOCl2

The first of the above three reactions also affords phosphorus oxychloride (phosphoryl chloride), which resembles thionyl chloride in many of its reactions.


Thionyl chloride is main used in the industrial production of organochlorine compounds, which are often intermediates in pharmaceuticals and agrichemicals.

Organic chemistry

Thionyl chloride is used to convert carboxylic acids to acyl chlorides

and alcohols to the corresponding alkyl chlorides via an internal nucleophilic substitution.

It is preferred over other reagents such as phosphorus pentachloride because the products of the thionyl chloride reactions, HCl and SO2 are gaseous, simplifying the purification of the product. Excess thionyl chloride may be removed by distillation. An example of a reaction of thionyl chloride with an alcohol is in a synthesis of bicifadine :

Sulfonic acids react with thionyl chloride to produce sulfonyl chlorides. Likewise, thionyl chloride will transform sulfinic acids into sulfinyl chlorides and phosphonic acids into phosphoryl chlorides.

Thionyl chloride will react with primary formamides to form isocyanides.

Amides will react with thionyl chloride to form imidoyl chlorides. However, primary amides under heating with thionyl chloride will continue on to form nitriles.

Inorganic chemistry

Anhydrous metal chlorides may be obtained from hydrated metal chlorides by refluxing in freshly distilled thionyl chloride:
MCln·xH2O + x SOCl2 → MCln + x SO2 + 2x HCl

Other applications

Thionyl chloride is a component of lithium-thionyl chloride batteries as the positive active material with lithium as the negative active material. In military usage, thionyl chloride is used in the "di-di" method of producing G-series nerve agents.


SOCl2 is a reactive compound that can explosively release dangerous gases upon contact with water and other reagents. Industrial production of thionyl chloride is controlled under the Chemical Weapons Convention, where it is listed in schedule 3.


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