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.

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:


 * 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 and alcohols  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. 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&middot;xH2O + x SOCl2 &rarr; MCln + 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.