Thionyl chloride is an inorganic compound with the formula SOCl₂. It is a reactive chemical used as a reagent in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure with a nauseating pungent odor that decomposes above 140 °C. Thionyl chloride is sometimes confused with sulfuryl chloride, SO₂Cl₂, but the properties of these compounds differ significantly. Approximately 45,000 tons per year of SOCl₂ were produced in the early 1990s.^[2]

Properties and structure

The molecule SOCl₂ is pyramidal, indicating the presence of a lone pair of electrons on the sulfur(IV) center. In contrast, the stoichiometrically related COCl₂ is planar. SOCl₂ reacts with water to release hydrogen chloride and sulfur dioxide.

SOCl₂ + H₂O → 2 HCl + SO₂

Production

The major industrial synthesis involves the reaction of sulfur trioxide and sulfur dichloride:^[3]

SO₃ + SCl₂ → SOCl₂ + SO₂

Other methods include syntheses from phosphorus pentachloride, chlorine, or phosgene:

SO₂ + PCl₅ → SOCl₂ + POCl₃

SO₂ + Cl₂ + SCl₂ → 2 SOCl₂

SO₃ + Cl₂ + 2 SCl₂ → 3 SOCl₂

SO₂ + COCl₂ → SOCl₂ + CO₂

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

Applications

Organic chemistry

Thionyl chloride is mainly used in the industrial production of organochlorine compounds, which are often intermediates in pharmaceuticals and agrichemicals. In some cases it requires purification prior to use.^[4]

Thionyl chloride is widely used in organic synthesis for the synthesis of organochlorides. It usually is preferred over other reagents, such as phosphorus pentachloride, as its by-products (HCl and SO₂) are gaseous, which simplifies purification of the product. Classically, it converts carboxylic acids to acyl chlorides^[5]

RCO₂H + SOCl₂ → RC(O)Cl + HCl + SO₂

By a similar stoichiometry it also converts alcohols to alkyl chlorides.

RCH₂OH + SOCl₂ → RCH₂Cl + HCl + SO₂

These reactions proceed via ester intermediates of the type RCH₂OS(O)Cl, which decompose via the SNi mechanism.^[6]

Many of the products of thionyl chloride are themselves highly reactive, which in combination with its easily removed by-products makes thionyl chloride well suited to one pot synthesis. Examples include the synthesis of oxazoline rings and the Darzens reaction^[7] (shown below) which proceeds via an internal nucleophilic substitution.