Chlorosulfuric acid

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Chlorosulfuric acid
Chlorosulfuric acid
Chlorosulfuric acid
IUPAC name
Sulfurochloridic acid
Other names
Chlorosulfuric acid,
Chlorosulfonic acid,
Chlorosulphonic acid,
Chlorinesulfonic acid,
Chlorinesulphonic acid,
Chloridosulfonic acid,
Chloridosulphonic acid,
Sulfuric chlorohydrin
CAS number 7790-94-5 YesY
ChemSpider 23040 YesY
Jmol-3D images Image
PubChem 24638
RTECS number FX5730000
UN number 1754
Molar mass 116.52 g mol−1
Appearance colorless liquid that fumes in air
Density 1.753 g cm−3
Melting point −80 °C (−112 °F; 193 K)
Boiling point 151 to 152 °C (304 to 306 °F; 424 to 425 K) (755 mmHg or 100.7 kPa)
Solubility in other solvents reacts with alcohols
soluble in chlorocarbons
Molecular shape tetrahedral
EU Index 016-017-00-1
EU classification Corrosive (C)
R-phrases R14, R35, R37
S-phrases (S2), S26, S45
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 4: Very short exposure could cause death or major residual injury. E.g., VX gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
Related compounds
Related compounds
Sulfuryl chloride
Sulfuric acid
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Chlorosulfuric acid (IUPAC name: sulfurochloridic acid) is the inorganic compound with the formula HSO3Cl. It is also known as chlorosulfonic acid, being the sulfonic acid of chlorine. It is a distillable, colorless liquid which is hygroscopic and a powerful lachrymator.[1]

Structure and properties[edit]

Chlorosulfuric acid is a tetrahedral molecule. The formula is more descriptively written SO2(OH)Cl, but HSO3Cl is traditional. It is an intermediate, chemically and conceptually, between sulfuryl chloride (SO2Cl2) and sulfuric acid (H2SO4).[2] The compound is rarely obtained pure. Upon standing with excess sulfur trioxide, it decomposes to pyrosulfuryl chlorides:[3]

2 ClSO3H + SO3 → H2SO4 + S2O5Cl2


The industrial synthesis entails the reaction of hydrogen chloride with a solution of sulfur trioxide in sulfuric acid:[3]

HCl + SO3 → ClSO3H

It can also be prepared by chlorination of sulfuric acid, written here for pedagogical purposes as SO2(OH)2, vs. the usual format H2SO4:

PCl5 + SO2(OH)2 → ClSO3H + POCl3 + HCl

The latter method is more suited for laboratory-scale operations.


ClSO2OH is used to prepare sulfonic acids, which are useful in detergents and as chemical intermediates:


An early synthesis of saccharin begins with the reaction of toluene with ClSO2OH to give the ortho- and para-toluene sulfonyl chloride derivatives:

CH3C6H5 + 2ClSO2OH → CH3C6H4SO2Cl + H2SO4

Oxidation of the ortho isomer gives the benzoic acid derivative that then is cyclized with ammonia and neutralized with base to afford saccharin.

Chlorosulphonic acid has been used as an anti-contrail agent in Ryan Model 147 reconnaissance drones, and to produce smoke screens.[4]


ClSO3H reacts violently with water to yield sulfuric acid and HCl, commonly seen as vapors fuming from the liquid. Precautions, such as proper ventilation, associated with HCl should be observed.

Related halosulfuric acids[edit]

  • FSO2OH is a related strong acid with a diminished tendency to evolve HF.
  • Bromosulfonic acid, BrSO2OH, is unstable, decomposing at its melting point of 8 °C to give Br2, SO2, and H2SO4.
  • Iodosulfonic acid is unknown.


  1. ^ Cremlyn, R. J. (2002). Chlorosulfonic Acid. Royal Society of Chemistry. ISBN 978-0-85404-498-6. 
  2. ^ Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. pp. 549–550. 
  3. ^ a b Maas, J.; Baunack, F. (2002). "Chlorosulfuric Acid". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a07_017. 
  4. ^ The Royal Navy at War (DVD). London: Imperial War Museum. 2005. 

See also[edit]