Thionyl chloride is aninorganic compound with thechemical formulaSOCl2. It is a moderatelyvolatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as achlorinating 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 alsolisted under theChemical Weapons Convention as it may be used for the production ofchemical weapons.
Thionyl chloride is sometimes confused withsulfuryl chloride,SO2Cl2, but the properties of these compounds differ significantly. Sulfuryl chloride is a source ofchlorine whereas thionyl chloride is a source ofchloride ions.
The major industrial synthesis involves the reaction ofsulfur trioxide andsulfur 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]
The second of the above five reactions also affordsphosphorus 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]
Thionyl chloride has a long shelf life, however "aged" samples develop a yellow hue, possibly due to the formation ofdisulfur dichloride. It slowlydecomposes toS2Cl2,SO2 andCl2 at just above the boiling point.[9][12] Thionyl chloride is susceptible tophotolysis, 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]
Thionyl chloride is mainly used in the industrial production oforganochlorine compounds, which are often intermediates in pharmaceuticals and agrochemicals. It usually is preferred over other reagents, such asphosphorus pentachloride, as its by-products (HCl andSO2) 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.
By a similar process it also reacts withalcohols to formalkyl chlorides. If the alcohol ischiral the reaction generally proceeds via anSNi mechanism with retention of stereochemistry;[15] however, depending on the exact conditions employed, stereo-inversion can also be achieved. Historically the use ofSOCl2 withpyridine was called theDarzens halogenation, but this name is rarely used by modern chemists.
Conversion of a secondary alcohol to a chloroalkane by thionyl chloride
By an analogous process, secondaryamides will react with thionyl chloride to formimidoyl chlorides, with tertiary amides 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 theBischler–Napieralski reaction as a means of formingisoquinolines.
AsSOCl2 reacts with water it can be used to dehydrate various metal chloride hydrates, suchmagnesium chloride (MgCl2·6H2O),aluminium chloride (AlCl3·6H2O), andiron(III) chloride (FeCl3·6H2O).[9] This conversion involves treatment with refluxing thionyl chloride and follows the following general equation:[31]
MCln•xH2O + SOCl2 → MCln + x SO2 + 2x HCl
If an excess ofSOCl2 is used to dehydrate aluminium trichloride, it will form an adduct (1 molecule of thionyl chloride for each molecule of the aluminium trichloride dimer).
Thionyl chloride can engage in a range of different electrophilic addition reactions. It adds to alkenes in the presence ofAlCl3 to form an aluminium complex which can be hydrolysed to form asulfinic acid. Both aryl sulfinyl chlorides and diaryl sulfoxides can be prepared from arenes through reaction with thionyl chloride intriflic acid[32] or the presence of catalysts such asBiCl3,Bi(OTf)3,LiClO4 orNaClO4.[33][34]
In the laboratory, a reaction between thionyl chloride and an excess ofanhydrous alcohol can be used to produce anhydrous alcoholic solutions ofHCl.
Thionyl chloride undergoes halogen exchange reactions to give other thionyl species.
Thionyl chloride is a component of lithium–thionyl chloridebatteries,[37] where it acts as the positive electrode (in batteries:cathode) withlithium forming the negative electrode (anode); theelectrolyte is typicallylithium tetrachloroaluminate. The overall discharge reaction is as follows:
4 Li + 2 SOCl2 → 4 LiCl +1⁄8S8 + 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 very corrosive, theSO2 is toxic by inhalation, and the batteries therefore require special disposal procedures; additionally, they may explode if shorted. The technology was used on the 1997Sojourner Mars rover.
In 1849, the French chemistsJean-François Persoz and Bloch, and the German chemist Peter Kremers (1827–?), independently first synthesized thionyl chloride by reactingphosphorus pentachloride withsulfur dioxide.[40][41] However, their products were impure: both Persoz and Kremers claimed that thionyl chloride contained phosphorus,[42] and Kremers recorded its boiling point as 100 °C (instead of 74.6 °C). In 1857, the German-Italian chemistHugo Schiff subjected crude thionyl chloride to repeated fractional distillations and obtained a liquid which boiled at 82 °C and which he calledThionylchlorid.[43] In 1859, the German chemistGeorg Ludwig Carius noted that thionyl chloride could be used to makeacid anhydrides andacyl chlorides fromcarboxylic acids and to makealkyl chlorides fromalcohols.[44]
^Thionyl chloride in Linstrom, Peter J.; Mallard, William G. (eds.);NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg (MD) (retrieved 2014-05-11)
^Oka, Kitaro (1981). "Some Applications of Thionyl Chloride in Synthetic Organic Chemistry".Synthesis.1981 (9):661–681.doi:10.1055/s-1981-29563.S2CID94917739.
^Calderazzo, Fausto; Dell'Amico, Daniela Belli (April 1981). "Syntheses of carbonyl halides of late transition elements in thionyl chloride as solvent. Carbonyl complexes of palladium(II)".Inorganic Chemistry.20 (4):1310–1312.doi:10.1021/ic50218a072.
^Garber, E. B.; Pease, L. E. D.; Luder, W. F. (20 April 1953). "Titration of Aprotic Acids in Thionyl Chloride".Analytical Chemistry.25 (4):581–583.doi:10.1021/ac60076a012.
^Brauer, Georg, ed. (1963).Handbook of Preparative Inorganic Chemistry. Vol. 1. Translated by Scripta Technica; Reed, F. (2nd ed.). New York, NY: Academic Press. p. 383.ISBN978-0121266011.{{cite book}}:ISBN / Date incompatibility (help)
^Donovan, R. J.; Husain, D.; Jackson, P. T. (1969). "Spectroscopic and kinetic studies of the SO radical and the photolysis of thionyl chloride".Transactions of the Faraday Society.65: 2930.doi:10.1039/TF9696502930.
^Friedman, L.; Wetter, W. P. (1967). "Purification of thionyl chloride".Journal of the Chemical Society A: Inorganic, Physical, Theoretical: 36.doi:10.1039/J19670000036.
^Van Woerden, H. F. (1963). "Organic Sulfites".Chemical Reviews.63 (6):557–571.doi:10.1021/cr60226a001.
^Brenner, M.; Huber, W. (1953). "Herstellung von α-Aminosäureestern durch Alkoholyse der Methylester" [Manufacture of α-amino acid esters by alcoholysis of methyl esters].Helvetica Chimica Acta (in German).36 (5):1109–1115.doi:10.1002/hlca.19530360522.
^Arrieta, A.; Aizpurua, J. M.; Palomo, C. (1984). "N,N-Dimethylchlorosulfitemethaniminium chloride (SOCl2-DMF) a versatile dehydrating reagent".Tetrahedron Letters.25 (31):3365–3368.doi:10.1016/S0040-4039(01)81386-1.
^Hogan, P. J.; Cox, B. G. (2009). "Aqueous Process Chemistry: The Preparation of Aryl Sulfonyl Chlorides".Organic Process Research & Development.13 (5):875–879.doi:10.1021/op9000862.
^Pray, A. R.; Heitmiller, R. F.; Strycker, S.; Aftandilian, V. D.; Muniyappan, T.; Choudhury, D.; Tamres, M. (1990). "Anhydrous Metal Chlorides".Inorganic Syntheses. Vol. 28. pp. 321–323.doi:10.1002/9780470132593.ch80.ISBN978-0-470-13259-3.
^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.
^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.S2CID96348273.
^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.S2CID104552644.
^Rao, M. R. Aswathanarayana (March 1940). "Thionyl iodide: Part II. Rate of decomposition and spectroscopic".Proceedings of the Indian Academy of Sciences - Section A.11 (3):201–205.doi:10.1007/BF03046548.S2CID104752226.
^The German chemistGeorg Ludwig Carius noted that, when the reaction mixture that produced thionyl chloride was distilled, the crude mixture initially released substantial quantities of gas, so thatphosphoryl 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.)
^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 transformbenzoic acid (OC7H5OH) intobenzoyl chloride (ClC7H5O) and to transformsodium benzoate intobenzoic anhydride. On p. 96, he mentions that thionyl chloride will transformmethanol intomethyl chloride (Chlormethyl). Thionyl chloride behaves likephosphoryl 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.)
