Chlorosulfuric acid (IUPAC name: sulfurochloridic acid) is the inorganic compound with the formula HSO<sub>3</sub>Cl. 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.
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.
The formula is more descriptively written SO<sub>2</sub>(OH)Cl, but HSO<sub>3</sub>Cl is traditional. It is an intermediate, chemically and conceptually, between sulfuryl chloride (SO<sub>2</sub>Cl<sub>2</sub>) and sulfuric acid (H<sub>2</sub>SO<sub>4</sub>). The compound is rarely obtained pure. Upon standing with excess sulfur trioxide, it decomposes to pyrosulfuryl chlorides:
The industrial synthesis entails the reaction of hydrogen chloride with a solution of sulfur trioxide in sulfuric acid:
It can also be prepared by the method originally used by acid's discoverer Alexander William Williamson in 1854, namely chlorination of sulfuric acid, written here for pedagogical purposes as HSO<sub>3</sub>(OH) vs. the usual format H<sub>2</sub>SO<sub>4</sub>:
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.
ClSO<sub>2</sub>OH is used to prepare alkyl sulfates, which are useful as detergents and as chemical intermediates:
One historical synthesis of saccharin begins with the reaction of toluene with ClSO<sub>2</sub>OH to give the ortho- and para-toluenesulfonyl chloride derivatives:
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, and to produce smoke screens.
ClSO<sub>3</sub>H reacts violently with water to yield sulfuric acid and hydrogen chloride, which are corrosive: