Thionyl chloride (or thionyl dichloride) is an inorganic compound with the formula SOCl₂. It is a reactive chemical reagent used in chlorination reactions. It is a liquid at room temperature and pressure. It is odorous, toxic, corrosive, lachrymatory, and a skin and inhalation hazard. It decomposes when heated above 140°C.

Reactions

Thionyl chloride is used both on a laboratory and an industrial scale. It reacts with water to release hydrogen chloride (HCl) and sulfur dioxide (SO₂), and it is not found in nature.

HO + O=SCl₂ → SO₂ + 2 HCl

Thionyl chloride reacts with carboxylic acids to produce acyl chlorides,

R-CO-OH + O=SCl₂ → R-CO-Cl + SO₂ + HCl

and with alcohols to produce alkyl chlorides.

R-OH + O=SCl₂ → R-Cl + SO₂ + HCl

Uses

Thionyl chloride is used inside lithium-thionyl chloride batteries as the positive active material with lithium as the negative active material. It is also used as a reagant for the production of other chemical compounds or materials. A major application is in the preparation of acid chlorides from carboxylic acids, including the important organic reactant acetyl chloride made via the following reaction:

H₃C-COO-H + O=SCl₂ → H₃C-COCl + SO₂ + H-Cl

Synthesis of Thionyl Chloride

The major industrial process is from the reaction of sulfur trioxide and sulfur dichloride ^[1]:

SO₃ + SCl₂ → SOCl₂ + SO₂

Other methods include:

SO₂ + PCl₅ → SOCl₂ + POCl₃

SO₂ + Cl₂ + SCl₂ → 2 SOCl₂

SO₃ + Cl₂ + 2 SCl₂ → 3 SOCl₂

References

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, 1984.