|Jmol-3D images||Image 1|
|Molar mass||116.52 g mol−1|
|Appearance||colorless liquid that fumes in air|
|Density||1.753 g cm−3|
|Solubility in water||hydrolysis|
|Solubility in other solvents||reacts with alcohols
soluble in chlorocarbons
|Refractive index (nD)||1.433|
|EU classification||Corrosive (C)|
|R-phrases||R14, R35, R37|
|S-phrases||(S2), S26, S45|
|Related compounds||Sulfuryl chloride
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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 that should be handled with care. It is hygroscopic and a powerful lachrymator.
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). The compound is rarely obtained pure. Upon standing with excess sulfur trioxide, it decomposes to pyrosulfuryl chlorides:
- 2 ClSO3H + SO3 → H2SO4 + S2O5Cl2
- 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:
- ROH + ClSO3H → ROSO3H + 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 + 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, commonly seen as vapors fuming from the liquid. Precautions, such as proper ventilation, 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.
- Cremlyn, R. J. (2002). Chlorosulfonic Acid. Royal Society of Chemistry. ISBN 978-0-85404-498-6.
- Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. pp. 549–550.
- Maas, J.; Baunack, F. (2002). "Chlorosulfuric Acid". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a07_017.
- The Royal Navy at War (DVD). London: Imperial War Museum. 2005.