Sulfur oxide dichloride
Sulfur monoxide dichloride
|Molar mass||118.97 g/mol|
|Appearance||clear, 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||soluble in most aprotic solvents: toluene, chloroform, diethyl ether|
|Vapor pressure||384 Pa (-40 °C)
4.7 kPa (0 °C)
15.7 kPa (25 °C)
Refractive index (nD)
|1.517 (20 °C) |
|Dipole moment||1.44 D|
|121.0 kJ/mol (liquid)|
|309.8 kJ/mol (gas)|
Std enthalpy of
|-245.6 kJ/mol (liquid)|
Reacts violently with water to release toxic gas
|GHS signal word||Danger|
|H302, H314, H331|
|P261, P280, P305+351+338, P310|
|R-phrases||R14, R20/22, R29, R35|
|S-phrases||(S1/2), S26, S30, S36/37/39, S45|
Related thionyl halides
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
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 tons per year being produced during the early 1990s. It is toxic and will react violently with water to produce toxic gases, it is also listed as a Schedule 3 compound, as in addition to its legitimate uses it may be use for the production of chemical weapons.
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.
- SO3 + SCl2 → SOCl2 + SO2
- SO2 + PCl5 → SOCl2 + POCl3
- SO2 + Cl2 + SCl2 → 2 SOCl2
- SO3 + Cl2 + 2 SCl2 → 3 SOCl2
- SO2 + 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
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 by purified by distillation under reduced pressure, to give a clear colourless liquid.
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
- SOCl2 + H2O → 2 HCl + SO2
- 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, stero-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.
- SOCl2 + 2 ROH → (RO)2SO + 2 HCl
With nitrogen species
Thionyl chloride will react 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.
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.
As SOCl2 reacts vigously 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:
- MCln·xH2O + x SOCl2 → MCln + x SO2 + 2x HCl
- 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
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 diposal procedures, additionally they may explode if shorted.
SOCl2 is a reactive compound that can violently and/or explosively release dangerous gases upon contact with water and other reagents. Industrial production of 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.
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