Chlorosulfuric acid

Chlorosulfuric acid
Chlorosulfuric acid
General
Systematic name	chlorosulfonic acid
Other names	chloridosulfonic acid, sulfuric chlorohydrin
Molecular formula	HSO3Cl
SMILES	?
Molar mass	116.52 g/mol
Appearance	colorless liquid that fumes in air
CAS number	7790-94-5
Properties
Density and phase	1.753 g/cm3
Solubility	reacts with water, alcohols soluble in chlorocarbons
Melting point	-80 °C
Boiling point	151-152 °C (755 mm Hg)
Viscosity	? cP at ? °C
Structure
Molecular shape	tetrahedral
Dipole moment	? D
Hazards
MSDS	External MSDS
Main hazards	corrosive evolves HCl upon hydrolysis
NFPA 704
R/S statement	R: 14-35-37 S: 26-45
RTECS number	FX5730000
Supplementary data page
Structure and properties	n, 1.433
Thermodynamic data	Phase behaviour Solid, liquid, gas
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.

Synthesis

The industrial synthesis entails the reaction of Hydrogen chloride with a solution of suflur 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.

Applications

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.

References

• 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 0854044981