Chlorosulfuric acid

Chlorosulfuric acid
IUPAC name Sulfurochloridic acid
Other names Chlorosulfuric acid, Chlorosulfonic acid, Chloridosulfonic acid sulfuric chlorohydrin
Identifiers
CAS number	7790-94-5 Y
PubChem	24638
ChemSpider	23040 Y
UN number	1754
RTECS number	FX5730000
Jmol-3D images	Image 1
SMILES ClS(=O)(=O)O
InChI InChI=1S/ClHO3S/c1-5(2,3)4/h(H,2,3,4) Y Key: XTHPWXDJESJLNJ-UHFFFAOYSA-N Y InChI=1/ClHO3S/c1-5(2,3)4/h(H,2,3,4) Key: XTHPWXDJESJLNJ-UHFFFAOYAO
Properties
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
Structure
Molecular shape	tetrahedral
Hazards
MSDS	ICSC 1039
EU Index	016-017-00-1
EU classification	Corrosive (C)
R-phrases	R14, R35, R37
S-phrases	(S2), S26, S45
NFPA 704	0 4 2 W
Related compounds
Related compounds	Sulfuryl chloride Sulfuric acid
Y acid (verify) (what is: Y/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

Synthesis

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.

Applications

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.

Safety

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.

References

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