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Chlorosulfuric acid

Chlorosulfuric acid
IUPAC name chlorosulfonic acid
Other names chloridosulfonic acid,

sulfuric chlorohydrin

CAS number 7790-94-5
RTECS number FX5730000
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 mm Hg)

Solubility in other solvents reacts with water, alcohols
soluble in chlorocarbons
Refractive index (nD) 1.433
Molecular shape tetrahedral
Main hazards corrosive
evolves HCl upon hydrolysis
NFPA 704
R-phrases 14-35-37
S-phrases 26-45
Related Compounds
Related compounds sulfuryl chloride
sulfuric acid
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Chlorosulfuric acid is ClSO2OH. This tetrahedral molecule is an intermediate, chemically and conceptually, between SO2Cl2 and H2SO4. It is also known as chlorosulfonic acid. It is a distilable, colorless liquid that should be handled with care. It is a hygroscopic and a powerful lachrymator.



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

HCl + SO3 → ClSO2OH

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 → ClSO2OH + POCl3 + HCl

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

Related compounds

  • FSO2OH is a strong acid and shows only a mild 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.


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

R-H + ClSO2OH → RSO2OH + HCl

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

CH3C6H5 + ClSO2OH → CH3C6H4SO2Cl + HCl

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

Safety considerations

ClSO2OH reacts violently with water to release HCl. Inhalation of ClSO2OH would generate HCl in the lungs, which is very dangerous. Thus, precautions associated with HCl should be observed.


    • Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001, pages 549-550 (discussion of XSO2OH for X = F, Cl, Br, I)
    • R.J. Cremlyn, "Chlorosulfonic Acid" Springer-Verlag New York, Inc. (Royal Society of Chemistry, 2002, 300 pp. ISBN 978-0-85404-498-6
    This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Chlorosulfuric_acid". A list of authors is available in Wikipedia.
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