Chlorosulfuric acid

Chlorosulfuric acid
Chlorosulfuric acid
CAS number 7790-94-5 YesY
PubChem 24638
ChemSpider 23040 YesY
UN number 1754
RTECS number FX5730000
Jmol-3D images Image 1
Molecular formula HSO3Cl
Molar mass 116.52 g mol−1
Appearance colorless liquid that fumes in air
Density 1.753 g cm−3
Melting point

−80 °C

Boiling point

151–152 °C
(755 mmHg or 100.7 kPa)

Solubility in water hydrolysis
Solubility in other solvents reacts with alcohols
soluble in chlorocarbons
Refractive index (nD) 1.433
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
NFPA 704.svg
Related compounds
Related compounds Sulfuryl chloride
Sulfuric acid
 YesY acid (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Chlorosulfuric acid (IUPAC name: sulfurochloridic acid) is the inorganic compound with the formula HSO3Cl. It is also known as chlorosulfonic acid. It is a distillable, colorless liquid that should be handled with care. It is a hygroscopic and a powerful lachrymator.[1]


Structure and properties

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.


ClSO3H reacts violently with water to yield sulfuric acid and HCl. Precautions associated with HCl should be observed.

Related halosulfuric acids

  • 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. ^ R.J. Cremlyn, "Chlorosulfonic Acid" Springer-Verlag New York, Inc. (Royal Society of Chemistry, 2002, 300 pp. ISBN 978-0-85404-498-6
  2. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001, pages 549-550 (discussion of XSO2OH for X = F, Cl, Br, I)
  3. ^ a b Joachim Maas, Fritz Baunack "Chlorosulfuric Acid" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim.doi:10.1002/14356007.a07_017

See also

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