Chlorosulfuric acid


Chlorosulfuric 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.
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 SO2Cl, but HSO3Cl is traditional. It is an intermediate, chemically and conceptually, between sulfuryl chloride and sulfuric acid. The compound is rarely obtained pure. Upon standing with excess sulfur trioxide, it decomposes to pyrosulfuryl chlorides:

Synthesis

The industrial synthesis entails the reaction of hydrogen chloride with a solution of sulfur trioxide in sulfuric acid:
It can also be prepared by chlorination of sulfuric acid, written here for pedagogical purposes as HSO3, vs. the usual format H2SO4:
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:
An early synthesis of saccharin begins with the reaction of toluene with ClSO2OH 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.

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