Chlorosulfuric acid

Names IUPAC name Other names Identifiers ChemSpider ECHA InfoCard 100.029.304 Edit this at Wikidata EC Number RTECS number UNII UN number 1754 Properties HSO₃Cl Molar mass 116.52 g mol⁻¹ Appearance colorless liquid, but commercial samples usually are pale brown 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) hydrolysis Solubility in other solvents reacts with alcohols
soluble in chlorocarbons Acidity (pK_a) −5.9 (in CF₃CO₂H)^[1] 1.433 Structure tetrahedral Hazards GHS labelling: GHS05: Corrosive GHS07: Exclamation mark Danger H314, H335 P260, P261, P264, P271, P280, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P312, P321, P363, P403+P233, P405, P501 NFPA 704 (fire diamond) Safety data sheet (SDS) ICSC 1039 Related compounds Sulfuryl chloride
Sulfuric acid

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. Commercial samples usually are pale brown or straw colored.^[3]

Salts and esters of chlorosulfuric acid are known as chlorosulfates.

Chlorosulfuric acid is a tetrahedral molecule. Its structure was debated for many decades until in 1941 Shrinivasa Dharmatti proved by magnetic susceptibility that chlorine is directly bonded to sulfur.^{[4] [5]}

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).^[6] The compound is rarely obtained pure. Upon standing with excess sulfur trioxide, it decomposes to pyrosulfuryl chlorides:^[7]

2 ClSO₃H + SO₃ → H₂SO₄ + S₂O₅Cl₂

The industrial synthesis entails the reaction of hydrogen chloride with a solution of sulfur trioxide in sulfuric acid:^[7]

HCl + SO₃ → ClSO₃H

It can also be prepared by the method originally used by acid's discoverer Alexander William Williamson in 1854,^[4] namely 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.

Williamson's discovery disproved then-popular hypothesis that sulfuric acid is a compound of water (which was incorrectly assumed to have formula of HO) and sulfur trioxide.^[8]

ClSO₂OH is used to prepare alkyl sulfates, which are useful as detergents and as chemical intermediates:^[7]

ROH + ClSO₃H → ROSO₃H + HCl

One historical 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.

Chlorosulfonic acid has been used as an anti-contrail agent in Ryan Model 147 reconnaissance drones,^[9] and to produce smoke screens.^{[10] [11]}

ClSO₃H reacts violently with water to yield sulfuric acid and hydrogen chloride, which are corrosive:

ClSO₃H + H₂O → H₂SO₄ + HCl

• 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.