Fluorosulfuric acid


Fluorosulfuric acid is the inorganic compound with the chemical formula HSO3F. It is one of the strongest acids commercially available. The formula HSO3F emphasizes its relationship to sulfuric acid, H2SO4; HSO3F is a tetrahedral molecule. It is a colourless liquid, although commercial samples are often yellow.

Chemical properties

Fluorosulfuric acid is a free-flowing colorless liquid. It is soluble in polar organic solvents, but poorly soluble in nonpolar solvents such as alkanes. Reflecting its strong acidity, it dissolves almost all organic compounds that are even weak proton acceptors. HSO3F hydrolyzes slowly to HF and sulfuric acid. The related triflic acid retains the high acidity of HSO3F but is more hydrolytically stable. The self-ionization of fluorosulfonic acid also occurs:

Production

Fluorosulfuric acid is prepared by the reaction of HF and sulfur trioxide:
Alternatively, KHF2 or CaF2 can be treated with oleum at 250 °C. Once freed from HF by sweeping with an inert gas, HSO3F can be distilled in a glass apparatus.

Super-acids

HSO3F is one of the strongest known simple Brønsted acids, although carborane-based acids are still stronger. It has an H0 value of −15.1 compared to −12 for sulfuric acid. The combination of HSO3F and the Lewis acid antimony pentafluoride produces "Magic acid", which is a far stronger protonating agent. These acids all fall into the category of "superacids", acids stronger than 100% sulfuric acid.

Applications

HSO3F is useful for regenerating mixtures of HF and H2SO4 for etching lead glass.
HSO3F isomerizes alkanes and the alkylation of hydrocarbons with alkenes, although it is unclear if such applications are of commercial importance. It can also be used as a laboratory fluorinating agent.

Safety

Fluorosulfuric acid is considered to be highly toxic and corrosive. It hydrolyzes to release HF. Addition of water to HSO3F can be violent, similar to the addition of water to sulfuric acid but much more violent.