Thionyl chloride

Thionyl chloride is an inorganic compound with the chemical formula SOCl₂. It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately 45,000 tonnes (50,000 short tons) per year being produced during the early 1990s,^[5] but is occasionally also used as a solvent.^[6][7][8] 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

Names
IUPAC name Thionyl chloride
Other names Thionyl dichloride Sulfurous oxychloride Sulfinyl chloride Sulfinyl dichloride Dichlorosulfoxide Sulfur oxide dichloride Sulfur monoxide dichloride Sulfuryl(IV) chloride
Identifiers
CAS Number	7719-09-7 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29290 Y
ChemSpider	22797 Y
ECHA InfoCard	100.028.863
EC Number	231-748-8
PubChem CID	24386
RTECS number	XM5150000
UNII	4A8YJA13N4 N
UN number	1836
CompTox Dashboard (EPA)	DTXSID3064778
InChI InChI=1S/Cl2OS/c1-4(2)3 Y Key: FYSNRJHAOHDILO-UHFFFAOYSA-N Y InChI=1/Cl2OS/c1-4(2)3 Key: FYSNRJHAOHDILO-UHFFFAOYAN
SMILES ClS(Cl)=O
Properties
Chemical formula	SOCl2
Molar mass	118.97 g/mol
Appearance	Colourless liquid (yellows on ageing)
Odor	Pungent and unpleasant
Density	1.638 g/cm3, liquid
Melting point	−104.5 °C (−156.1 °F; 168.7 K)
Boiling point	74.6 °C (166.3 °F; 347.8 K)
Solubility in water	Reacts
Solubility	Soluble in most aprotic solvents: toluene, chloroform, diethyl ether. Reacts with protic solvents such as alcohols
Vapor pressure	384 Pa (−40 °C) 4.7 kPa (0 °C) 15.7 kPa (25 °C)[1]
Refractive index (nD)	1.517 (20 °C)[2]
Viscosity	0.6 cP
Structure
Molecular shape	pyramidal
Dipole moment	1.44 D
Thermochemistry
Heat capacity (C)	121.0 J/mol (liquid)[3]
Std molar entropy (S⦵298)	309.8 kJ/mol (gas)[3]
Std enthalpy of formation (ΔfH⦵298)	−245.6 kJ/mol (liquid)[3]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Very toxic, corrosive, releases HCl on contact with water
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H302, H314, H331
Precautionary statements	P261, P280, P305+P351+P338, P310
NFPA 704 (fire diamond)	4 0 2 W
Flash point	Non-flammable
NIOSH (US health exposure limits):
PEL (Permissible)	None[4]
REL (Recommended)	C 1 ppm (5 mg/m3)[4]
IDLH (Immediate danger)	N.D.[4]
Related compounds
Related Thionyl halides	Thionyl fluoride Thionyl bromide Thionyl iodide
Related compounds	Sulfuryl chloride Selenium oxydichloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Thionyl chloride is sometimes confused with sulfuryl chloride, SO₂Cl₂, 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.^[9] This synthesis can be adapted to the laboratory by heating oleum to slowly distill the sulfur trioxide into a cooled flask of sulfur dichloride.^[10]

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 five reactions also affords phosphorus oxychloride (phosphoryl chloride), which resembles thionyl chloride in many of its reactions. They may be separated by distillation, since thionyl chloride boils at a much lower temperature than phosphoryl chloride.^{[citation needed]}

Properties and structure

Crystal structure of solid SOCl2

SOCl₂ adopts a trigonal pyramidal molecular geometry with C_s 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.^[11]

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.^[9][12] Thionyl chloride is susceptible to photolysis, which primarily proceeds via a radical mechanism.^[13] Samples showing signs of ageing can be purified by distillation under reduced pressure, to give a colourless liquid.^[14]

Impure thionyl chloride, appearing slightly yellow

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 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 S_Ni mechanism with retention of stereochemistry;^[15] however, depending on the exact conditions employed, stereo-inversion can also be achieved. Historically the use of SOCl₂ with pyridine was called the Darzens halogenation, but this name is rarely used by modern chemists.

Conversion of a secondary alcohol to a chloroalkane by thionyl chloride

Reactions with an excess of alcohol produce sulfite esters, which can be powerful methylation, alkylation and hydroxyalkylation reagents.^[16]

SOCl₂ + 2 R−OH → (R−O)₂SO + 2 HCl

For example, the addition of SOCl₂ to amino acids in methanol selectively yields the corresponding methyl esters.^[17]

With carboxylic acids

Classically, it converts carboxylic acids to acyl chlorides:^[18][19][20]

SOCl₂ + R−COOH → R−COCl + SO₂ + HCl

The reaction mechanism has been investigated:^[21]

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^[22] and with secondary formamides to give chloroiminium ions; as such a reaction with dimethylformamide will form the Vilsmeier reagent.^[23]

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;^[24] 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).^[25]

Thionyl chloride has also been used to promote the Beckmann rearrangement of oximes.

With sulfur species

• Thionyl chloride will transform sulfinic acids into sulfinyl chlorides^[26][27]
• Sulfonic acids react with thionyl chloride to produce sulfonyl chlorides.^[28][29] Sulfonyl chlorides have also been prepared from the direct reaction of the corresponding diazonium salt with thionyl chloride.^[30]
• 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 with water it can be used to dehydrate various metal chloride hydrates, such magnesium chloride (MgCl₂·6H₂O), aluminium chloride (AlCl₃·6H₂O), and iron(III) chloride (FeCl₃·6H₂O).^[9] This conversion involves treatment with refluxing thionyl chloride and follows the following general equation:^[31]

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

Other reactions

• Thionyl chloride can engage in a range of different 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^[32] or the presence of catalysts such as BiCl₃, Bi(OTf)₃, LiClO₄ or NaClO₄.^[33][34]
• 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 other thionyl species.

Reactions with fluorinating agents such as antimony trifluoride give thionyl fluoride:

3 SOCl₂ + 2 SbF₃ → 3 SOF₂ + 2 SbCl₃

A reaction with hydrogen bromide gives thionyl bromide:

SOCl₂ + 2 HBr → SOBr₂ + 2 HCl

Thionyl iodide can likewise be prepared by a reaction with potassium iodide, but is reported to be highly unstable.^[35][36]

Batteries

A selection of lithium–thionyl chloride batteries

Thionyl chloride is a component of lithium–thionyl chloride batteries,^[37] where it acts as the positive electrode (in batteries: cathode) with lithium forming the negative electrode (anode); the electrolyte is typically lithium tetrachloroaluminate. The overall discharge reaction is as follows:

4 Li + 2 SOCl₂ → 4 LiCl + 1⁄8 S₈ + SO₂

These non-rechargeable batteries have advantages over other forms of lithium batteries such as a high energy density, a wide operational temperature range, and long storage and operational lifespans. However, their high cost, non-rechargeability, 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. The technology was used on the 1997 Sojourner Mars rover.

Safety

SOCl₂ is highly reactive and can violently release hydrochloric acid upon contact with water and alcohols. It is also a controlled substance under the Chemical Weapons Convention, where it is listed as a Schedule 3 substance, since it is used in the manufacture of G-series nerve agents^{[citation needed]} and the Meyer and Meyer–Clarke methods of producing sulfur-based mustard gases.^[38]

History

In 1849, the French chemists Jean-François Persoz and Bloch, and the German chemist Peter Kremers (1827–?), independently first synthesized thionyl chloride by reacting phosphorus pentachloride with sulfur dioxide.^[39][40] However, their products were impure: both Persoz and Kremers claimed that thionyl chloride contained phosphorus,^[41] and Kremers recorded its boiling point as 100 °C (instead of 74.6 °C). In 1857, the German-Italian chemist Hugo Schiff subjected crude thionyl chloride to repeated fractional distillations and obtained a liquid which boiled at 82 °C and which he called Thionylchlorid.^[42] In 1859, the German chemist Georg Ludwig Carius noted that thionyl chloride could be used to make acid anhydrides and acyl chlorides from carboxylic acids and to make alkyl chlorides from alcohols.^[43]

See also

• Oxalyl chloride
• Phosphorus pentachloride
• Phosgene
• Sulfur dichloride
• Thionyl bromide

References

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33. Peyronneau, M.; Roques, N.; Mazières, S.; Le Roux, C. (2003). "Catalytic Lewis Acid Activation of Thionyl Chloride: Application to the Synthesis of Aryl Sulfinyl Chlorides Catalyzed by Bismuth(III) Salts". Synlett (5): 0631–0634. doi:10.1055/s-2003-38358.
34. Bandgar, B. P.; Makone, S. S. (2004). "Lithium/Sodium Perchlorate Catalyzed Synthesis of Symmetrical Diaryl Sulfoxides". Synthetic Communications. 34 (4): 743–750. doi:10.1081/SCC-120027723. S2CID 96348273.
35. Rao, M. R. Aswathanarayana (March 1940). "Thionyl iodide: Part I. Formation of thionyl iodide". Proceedings of the Indian Academy of Sciences - Section A. 11 (3): 185–200. doi:10.1007/BF03046547. S2CID 104552644.
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37. Gangadharan, R.; Namboodiri, P.N.N.; Prasad, K.V.; Viswanathan, R. (January 1979). "The lithium—thionyl chloride battery — a review". Journal of Power Sources. 4 (1): 1–9. Bibcode:1979JPS.....4....1G. doi:10.1016/0378-7753(79)80032-4.
38. Institute of Medicine (1993). "Chapter 5: Chemistry of Sulfur Mustard and Lewisite". Veterans at Risk: The Health Effects of Mustard Gas and Lewisite. The National Academies Press. ISBN 0-309-04832-X.
39. See:

    • Persoz; Bloch (1849). "Des composés binaires formés par les métalloïdes, et, en particulier, de l'action du chloride phosphorique sur les acides sulfureux, sulfurique, phosphorique, chromique, etc" [Binary compounds formed by metalloids and in particular the action of phosphorus pentachloride on sulfurous acid, sulfuric acid, phosphoric acid, chromic acid, etc.]. Comptes rendus (in French). 28: 86–88.
    • Persoz; Bloch (1849). "Addition à une Note précédente concernant les combinaisons du chloride phosphorique avec les acides phosphorique, sulfurique et sulfureux" [Addition to a preceding note concerning compounds of phosphorus pentachloride with phosphoric acid, sulfuric acid, and sulfurous acid]. Comptes rendus (in French). 28: 389.
40. Kremers, P. (1849). "Ueber schwefligsaures Phosphorchlorid" [On sulfurous phosphorus pentachloride]. Annalen der Chemie und Pharmacie (in German). 70 (3): 297–300. doi:10.1002/jlac.18490700311.
41. The German chemist Georg Ludwig Carius noted that, when the reaction mixture that produced thionyl chloride was distilled, the crude mixture initially released substantial quantities of gas, so that phosphoryl chloride (POCl₃) was carried into the receiver. Carius, L. (1859). "Ueber die Chloride des Schwefels und deren Derivate" [On sulfur chloride and its derivatives]. Annalen der Chemie und Pharmacie (in German). 111: 93–113. doi:10.1002/jlac.18591110111. From p. 94: " … dabei ist jedoch die Vorsicht zu gebrauchen, … und nie reines Chlorthionyl erhalten wird." ( … however, during that [i.e., the fractional distillation], caution must be used, [so] that one carefully avoids a concentration of hydrogen chloride or excess sulfurous acid in the liquid that is to be distilled, as otherwise, by the evolution of gas that occurs at the start of the distillation, much phosphoryl chloride is transferred and pure thionyl chloride is never obtained.)
42. Schiff, Hugo (1857). "Ueber die Einwirkung des Phosphorsuperchlorids auf einige anorganische Säuren" [On the reaction of phosphorus pentachloride with some inorganic acids]. Annalen der Chemie und Pharmacie (in German). 102: 111–118. doi:10.1002/jlac.18571020116. The boiling point of thionyl chloride which Schiff observed, appears on p. 112. The name Thionylchlorid is coined on p. 113.
43. Carius, L. (1859). "Ueber die Chloride des Schwefels und deren Derivate" [On sulfur chloride and its derivatives]. Annalen der Chemie und Pharmacie (in German). 111: 93–113. doi:10.1002/jlac.18591110111. On p. 94, Carius notes that thionyl chloride can be " … mit Vortheil zur Darstellung wasserfreier Säuren verwenden." ( … used advantageously for the preparation of acid anhydrides.) Also on p. 94, Carius shows chemical equations in which thionyl chloride is used to transform benzoic acid (OC₇H₅OH) into benzoyl chloride (ClC₇H₅O) and to transform sodium benzoate into benzoic anhydride. On p. 96, he mentions that thionyl chloride will transform methanol into methyl chloride (Chlormethyl). Thionyl chloride behaves like phosphoryl chloride: from pp. 94-95: "Die Einwirkung des Chlorthionyls … die Reaction des Chlorthionyls weit heftiger statt." (The reaction of thionyl chloride with [organic] substances containing oxygen proceeds in general parallel to that of phosphoryl chloride; where the latter exerts an effect, thionyl chloride usually does so also, only in nearly all cases the reaction occurs far more vigorously.)