Thionyl chloride

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Thionyl chloride
Thionyl chloride beaker bottle by ChemicalForce.jpg
Thionyl chloride sample, fuming in open air.
IUPAC name
Sulfurous dichloride
Other names
Sulfinyl chloride
Sulfinyl dichloride
Sulfur monoxide dichloride
Sulfur oxide dichloride
Sulfurous oxychloride
Sulfuryl(IV) chloride
Thionyl dichloride
Jmol-3D images Image
Molar mass 118.97 g/mol
Appearance Colorless fuming liquid (fresh)
Yellowish fuming liquid (old)
Odor Suffocating, sulfur dioxide-like
Density 1.638 g/cm3 (at 20 °C)
Melting point −104.5 °C (−156.1 °F; 168.7 K)
Boiling point 74.6 °C (166.3 °F; 347.8 K)
Solubility Reacts with alcohols, aldehydes, carboxylic acids, ketones, DMF, DMSO
Miscible with benzene, carbon tetrachloride, chloroform, ethers, toluene
Vapor pressure 15.7 kPa (25 °C)
4.7 kPa (0 °C)
384 Pa (-40 °C)
309.8 kJ/mol (gas)
-245.6 kJ/mol (liquid)
Safety data sheet ScinceLab
Flash point Non-flammable
Related compounds
Related compounds
Sulfuryl chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Thionyl chloride is an inorganic compound with the chemical formula SOCl2. It is an extremely useful compound in chemistry, but it is extremely dangerous and toxic.

Thionyl chloride is sometimes confused with sulfuryl chloride, SO2Cl2, but the properties of these compounds differ significantly: Thionyl chloride is a source of chloride ions, while sulfuryl chloride is a source of chlorine.



Thionyl chloride reacts on contact with water to form sulfur dioxide and hydrogen chloride/hydrochloric acid mist, process which is extremely exothermic:

SOCl2 + H2O → 2 HCl + SO2

Thionyl chloride will convert carboxylic acids to acyl chlorides:

SOCl2 + RCO2H → RC(O)Cl + SO2 + HCl

This reaction can be used to synthesize acetic anhydride.


Thionyl chloride is a colorless oily liquid that fumes in air. Old samples have a yellowish color, due to decomposition. Light accelerates its decomposition. Thionyl chloride reacts with water, carboxylic acids, alcohols and all protic solvents. It is however miscible and soluble in most aprotic solvents, such as benzene, carbon tetrachloride, chloroform, diethyl ether, toluene, xylene. Thionyl chloride melts at −104.5 °C and boils at 74.6 °C. It has a density of 1.638 g/cm3 at 20 °C.


Thionyl chloride is sold by various chemical suppliers, but due to its hazards it's generally not available to the amateur chemist. Its sale is regulated in many places.

Certain batteries contain small amounts of thionyl chloride, but it's impractical to extract it.

Thionyl chloride is listed on schedule 3 of the Chemical Weapons Convention and ordering large amounts may draw attention of the authorities.


Unlike its similar compound sulfuryl chloride, thionyl chloride is much more difficult to synthesize.

Industrially it's prepared by reacting sulfur trioxide with sulfur dichloride:

SO3 + SCl2 → SOCl2 + SO2

Oleum can be used instead of sulfur trioxide, and this is usually preferred, as pure sulfur trioxide is a nightmare to produce, purify, store and use, while oleum is much easier to handle, even though it's not a pleasant experience either. The higher the sulfur trioxide concentration, the better the reaction yield is.

The resulting thionyl chloride is extracted via fractional distillation, best in low light conditions, under dry atmosphere.

SM user Magpie has made a small amount of thionyl chloride.

A more dangerous method involves the reaction between sulfur dioxide and phosgene:

SO2 + COCl2 → SOCl2 + CO2

However, due to phosgene's extreme toxicity, it's best to avoid this reaction all together.


  • Make acyl chlorides
  • Make anhydrous metal chlorides
  • Make anhydrous alcoholic solutions of HCl



Thionyl chloride is extremely toxic and should not be handled unless you're a professional. It reacts violently with water to give sulfur dioxide and hydrochloric acid, which are toxic and corrosive.


Thionyl chloride is best stored in sealed bottles, in a special cabinet, away from any light and moisture.


Thionyl chloride can be safely neutralized by adding it to an alkaline slurry or with dry calcium or sodium carbonate or bicarbonate. Neutralization must be done outside, or in a fumehood, as the neutralization releases a corrosive mist of HCl.


Relevant Sciencemadness threads