Chlorosulfuric acid

Chlorosulfuric acid

Names
IUPAC name Sulfurochloridic acid
Other names Chlorosulfuric acid, Chlorosulfonic acid, Chlorosulphonic acid, Chlorinesulfonic acid, Chlorinesulphonic acid, Chloridosulfonic acid, Chloridosulphonic acid, Sulfuric chlorohydrin
Identifiers
CAS Number	7790-94-5 Y
3D model (JSmol)	Interactive image
ChemSpider	23040 Y
ECHA InfoCard	100.029.304
PubChem CID	24638
RTECS number	FX5730000
UN number	1754
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
SMILES ClS(=O)(=O)O
Properties
Chemical 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 (−112 °F; 193 K)
Boiling point	151 to 152 °C (304 to 306 °F; 424 to 425 K) (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
Safety data sheet	ICSC 1039
EU classification (DSD) (outdated)	Corrosive (C)
R-phrases (outdated)	R14, R35, R37
S-phrases (outdated)	(S2), S26, S45
NFPA 704	0 3 2 W OX
Related compounds
Related compounds	Sulfuryl chloride Sulfuric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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 which is hygroscopic and a powerful lachrymator.^[1]

Salts and esters of chlorosulfuric acid are known as chlorosulfates.

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 HSO₃(OH), vs. the usual format H₂SO₄:

PCl₅ + HSO₃(OH) → HSO₃Cl + POCl₃ + HCl

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

Applications

ClSO₂OH is used to prepare alkyl sulfates, which are useful as 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-toluenesulfonyl chloride derivatives:

CH₃C₆H₅ + 2 ClSO₂OH → CH₃C₆H₄SO₂Cl + H₂SO₄ + HCl

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

Chlorosulphonic acid has been used as an anti-contrail agent in Ryan Model 147 reconnaissance drones, and to produce smoke screens.^[4][5]

Safety

ClSO₃H reacts violently with water to yield sulfuric acid and hydrogen chloride, commonly seen as vapors fuming from the liquid. Precautions, such as proper ventilation, associated with HCl should be observed.

Related halosulfuric acids

• Fluorosulfonic acid, FSO₂OH, is a related strong acid with a diminished tendency to evolve hydrogen fluoride.
• Bromosulfonic acid, BrSO₂OH, is unstable, decomposing at its melting point of 8 °C to give bromine, sulfur dioxide, and sulfuric acid.
• Iodosulfonic acid is not known to occur.

References