Chlorosulfuric acid

Chlorosulfuric acid is ClSO2OH. This tetrahedral molecule is an intermediate, chemically and conceptually, between SO2Cl2 and H2SO4. It is also known as chlorosulfonic acid. It is a distilable, colorless liquid that should be handled with care. It is a hygroscopic and a powerful lachrymator.

Synthesis
The industrial synthesis entails the reaction of hydrogen chloride with a solution of sulfur trioxide in sulfuric acid:
 * HCl + SO3 → ClSO2OH

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 → ClSO2OH + POCl3 + HCl

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

Related compounds

 * FSO2OH is a strong acid and shows only a mild 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.

Applications
ClSO2OH is used to prepare sulfonic acids, which are useful in detergents and as chemical intermediates.
 * R-H + ClSO2OH → RSO2OH + 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 + ClSO2OH → CH3C6H4SO2Cl +  HCl

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

Safety considerations
ClSO2OH reacts violently with water to release HCl. Inhalation of ClSO2OH would generate HCl in the lungs, which is very dangerous. Thus, precautions associated with HCl should be observed.