Chlorosulfuric acid
Chlorosulfuric acid (IUPAC name: sulfurochloridic acid) is the inorganic compound with the formula HSOCl. 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. Structure and properties. Chlorosulfuric acid is a tetrahedral molecule. The formula is more descriptively written SO(OH)Cl, but HSOCl is traditional. It is an intermediate, chemically and conceptually, between sulfuryl chloride (SOCl) and sulfuric acid (HSO). 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 HSO(OH), vs. the usual format HSO: The latter method is more suited for laboratory-scale operations. Applications. ClSOOH 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 ClSOOH 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. ClSOH 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.