Thionyl chloride

Production

The major industrial synthesis involves the reaction of sulfur trioxide and sulfur dichloride:

SO₃ + SCl₂ → SOCl₂ + SO₂

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

SO₂ + PCl₅ → SOCl₂ + POCl₃SO₂ + Cl₂ + SCl₂ → 2 SOCl₂SO₃ + Cl₂ + 2 SCl₂ → 3 SOCl₂SO₂ + 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.

Properties and structure

SOCl₂ adopts a pyramidal molecular geometry with C_s molecular symmetry. This geometry is attributed to the effects of the lone pairs on the sulfur(IV) centre.

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 clear 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 SO₂) 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 oxygen species

Thionyl chloride reacts with water to form sulfur dioxide and hydrochloric acid. This process is highly exothermic.

SOCl₂ + H₂O → 2 HCl + SO₂

Classically, it converts carboxylic acids to acyl chlorides.

SOCl₂ + RCO₂H → RC(O)Cl + SO₂ + HCl

By a similar process it also converts alcohols to 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 SOCl₂ in combination with a tertiary amine such as pyridine was called the Darzens halogenation however 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 ROH → (RO)₂SO + 2 HCl

For example, the addition of SOCl₂ to amino acids in methanol selectively yields the corresponding methyl esters.

With nitrogen species

With primary amines, thionyl chloride gives the 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.

With sulphur species

Thionyl chloride will transform sulfinic acids into sulfinyl chlorides

Sulfonic acids react with thionyl chloride to produce sulfonyl chlorides. Sulfonyl chlorides have also been prepared from the direct reaction of the corresponding diazonium salt with thionyl chloride.

Thionyl chloride can be used in variations of the Pummerer rearrangement.

With phosphorus species

Thionyl chloride converts phosphonic acids and phosphonates into phosphoryl chlorides. It is for this type of reaction that thionyl chloride is listed as a Schedule 3 compound, as it can be used in the "di-di" method of producing G-series nerve agents. For example, thionyl chloride converts dimethyl methylphosphonate into methylphosphonic acid dichloride, which can be used in the production of sarin and soman.

With metals

As SOCl₂ reacts vigorously with water it can be used to dehydrate various metal chloride hydrates, for example MgCl₂⋅6H₂O, AlCl₃⋅6H₂O, FeCl₃⋅6H₂O etc. This conversion involves treatment with refluxing thionyl chloride and follows the following general equation:

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

Other reactions

Thionyl chloride can engage in a range of electrophilic addition reactions. It adds to alkenes in the presence of AlCl₃ to form an aluminium complex which can be hydrolysed to form a sulfinic acid. Both aryl sulfinyl chlorides and diaryl sulfoxides can be prepared from arenes through reaction with thionyl chloride in triflic acid or the presence of catalysts such as BiCl₃, Bi(OTf)₃, LiClO₄ or NaClO₄.

In the laboratory, a reaction between thionyl chloride and an excess of anhydrous alcohol can be used to produce anhydrous alcoholic solutions of HCl.

Thionyl chloride undergoes halogen exchange reactions to give compounds such as thionyl bromide and thionyl fluoride

Batteries

Thionyl chloride is a component of lithium-thionyl chloride batteries, where it acts as the positive electrode (cathode) with lithium forming the negative electrode (anode); the electrolyte is typically lithium tetrachloroaluminate. The overall discharge reaction is as follows:

4Li + 2 SOCl₂ → 4 LiCl + S + SO₂

These non rechargeable batteries have many advantages over other forms of lithium battery such as a high energy density, a wide operational temperature range and long storage and operational lifespans. However, their high cost and safety concerns have limited their use. The contents of the batteries are highly toxic and require special disposal procedures, additionally they may explode if shorted.

Safety

SOCl₂ is a reactive compound that can violently and/or explosively release dangerous gases upon contact with water and other reagents. Thionyl chloride is controlled under the Chemical Weapons Convention, where it is listed in Schedule 3. Thionyl chloride is used in the "di-di" method of producing G-series nerve agents.