Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year being produced during the early 1990s, but is occasionally also used as a solvent. It is toxic, reacts with water, and is also listed under the Chemical Weapons Convention as it may be used for the production of chemical weapons.
Thionyl chloride is sometimes confused with sulfuryl chloride, , but the properties of these compounds differ significantly. Sulfuryl chloride is a source of chlorine whereas thionyl chloride is a source of chloride ions.
Production
The major industrial synthesis involves the reaction of sulfur trioxide and sulfur dichloride: This synthesis can be adapted to the laboratory by heating oleum to slowly distill the sulfur trioxide into a cooled flask of sulfur dichloride. SO3 + SCl2 -> SOCl2 + SO2
Other methods include syntheses from: Phosphorus pentachloride: SO2 + PCl5 -> SOCl2 + POCl3 Chlorine and sulfur dichloride: SO2 + Cl2 + SCl2 -> 2 SOCl2 SO3 + Cl2 + 2SCl2 -> 3 SOCl2 Phosgene: SO2 + COCl2 -> SOCl2 + CO2
The second of the above four reactions also affords phosphorus oxychloride (phosphoryl chloride), which resembles thionyl chloride in many of its reactions.
Properties and structure
SOCl2 adopts a trigonal pyramidal molecular geometry with Cs molecular symmetry. This geometry is attributed to the effects of the lone pair on the central sulfur(IV) center.
In the solid state SOCl2 forms monoclinic crystals with the space group P21/c.
Stability
Thionyl chloride has a long shelf life, however "aged" samples develop a yellow hue, possibly due to the formation of disulfur dichloride. It slowly decomposes to S2Cl2, SO2 and Cl2 at just above the boiling point. Thionyl chloride is susceptible to photolysis, which primarily proceeds via a radical mechanism. Samples showing signs of ageing can be purified by distillation under reduced pressure, to give a colourless liquid.
Reactions
Thionyl chloride is mainly used in the industrial production of organochlorine compounds, which are often intermediates in pharmaceuticals and agrichemicals. It usually is preferred over other reagents, such as phosphorus pentachloride, as its by-products (HCl and ) are gaseous, which simplifies purification of the product.
Many of the products of thionyl chloride are themselves highly reactive and as such it is involved in a wide range of reactions.
With water and alcohols
Thionyl chloride reacts exothermically with water to form sulfur dioxide and hydrochloric acid: SOCl2 + H2O -> 2 HCl + SO2
By a similar process it also reacts with alcohols to form alkyl chlorides. If the alcohol is chiral the reaction generally proceeds via an SNi mechanism with retention of stereochemistry; however, depending on the exact conditions employed, stereo-inversion can also be achieved. Historically the use of with pyridine was called the Darzens halogenation, but this name is rarely used by modern chemists.
Reactions with an excess of alcohol produce sulfite esters, which can be powerful methylation, alkylation and hydroxyalkylation reagents.
SOCl2 + 2 R-OH -> (R-O)2SO + 2 HCl
For example, the addition of to amino acids in methanol selectively yields the corresponding methyl esters.
With carboxylic acids
Classically, it converts carboxylic acids to acyl chlorides:
SOCl2 + R-COOH -> R-COCl + SO2 + HCl
The reaction mechanism has been investigated:
With nitrogen species
With primary amines, thionyl chloride gives sulfinylamine derivatives (RNSO), one example being N-sulfinylaniline. Thionyl chloride reacts with primary formamides to form isocyanides and with secondary formamides to give chloroiminium ions; as such a reaction with dimethylformamide will form the Vilsmeier reagent. By an analogous process primary amides will react with thionyl chloride to form imidoyl chlorides, with secondary amides also giving chloroiminium ions. These species are highly reactive and can be used to catalyse the conversion of carboxylic acids to acyl chlorides, they are also exploited in the Bischler–Napieralski reaction as a means of forming isoquinolines.
Primary amides will continue on to form nitriles if heated (Von Braun amide degradation). Thionyl chloride has also been used to promote the Beckmann rearrangement of oximes.