Thionyl chloride

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.

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Properties and structure

The molecule SOCl₂ is pyramidal, indicating the presence of a lone pair of electrons on the S(IV) center. In contrast, COCl₂ is planar.

SOCl₂ reacts with water to release hydrogen chloride and sulfur dioxide.

H₂O + O=SCl₂ → SO₂ + 2 HCl

Because of its high reactivity toward water, SOCl₂ would not be expected to occur in nature.

Production

The major industrial synthesis involves 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.

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

Applications

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 reagent for the production of other chemical compounds or materials.

In military usage, thionyl chloride is used in the "di-di" method of producing G-series nerve agents.

Organic chemistry

Thionyl chloride is widely used to convert carboxylic acids^[2][3] and alcohols^[4][5] to the corresponding acyl chlorides and alkyl chlorides respectively 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.

RC(O)OH + O=SCl₂ → RC(O)Cl + SO₂ + HCl

R-OH + O=SCl₂ → R-Cl + SO₂ + HCl

Sulfonic acids react with thionyl chloride to produce sulfonyl chlorides.^[6][7] Likewise, thionyl chloride will transform sulfinic acids into sulfinyl chlorides^[8][9] and phosphonic acids into phosphoryl chlorides.

Thionyl chloride will react with primary formamides to form isocyanides.^[10]

Amides will react with thionyl chloride to form imidoyl chlorides. However, primary amides under heating with thionyl chloride will continue on to form nitriles.^[11]

Inorganic chemistry

Anhydrous metal chlorides may be obtained from hydrated metal chlorides by refluxing in freshly distilled thionyl chloride:^[12]

MCl_n·xH₂O + x SOCl₂ → MCl_n + x SO₂ + 2x HCl

Safety

SOCl₂ is toxic, corrosive, and lachrymatory. It is a skin and inhalation hazard, as well as being odorous. It can react explosively when exposed to high concentrations of nucleophiles such as phosphites.

Industrial production of thionyl chloride is controlled under the Chemical Weapons Convention, where it is listed in schedule 3.

References