Fluorosulfuric acid

Fluorosulfuric acid is FSO₃H; it is one of the strongest acids commercially available. It is also known by the alternative name, fluorosulfonic acid. The molecule is better described by the formula FSO₂OH, which emphasizes its relationship to sulfuric acid, H₂SO₄. FSO₃H is a tetrahedral molecule.

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Chemical properties

Fluorosulfuric acid is a free-flowing colorless liquid. It is soluble in polar organic solvents (e.g. nitrobenzene, acetic acid, and ethyl acetate), 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.^[1] FSO₃H hydrolyzes slowly to HF and sulfuric acid. The related triflic acid CF₃SO₃H retains the high acidity of FSO₃H but is hydrolytically stable.

Production

Fluorosulfuric acid is prepared by the reaction of HF and sulfur trioxide

SO₃ + HF → FSO₃H

Alternatively, KHF₂ or CaF₂ can be treated with oleum at 250 °C. Once freed from HF by sweeping with an inert gas, FSO₃H can be distilled in a glass apparatus.^[2]

Super-acids

FSO₃H is one of the strongest known simple Brønsted acids, although recent work on carborane-based acids have led to still stronger acids.^[3] It has an H₀ value of −15.1 compared to −12 for sulfuric acid. The combination of FSO₃H 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

FSO₃H isomerizes alkanes and the alkylation of hydrocarbons with alkenes.^[4] It can also be used as a laboratory fluorinating agent.^[2]

Safety

Fluorosulfuric acid is considered to be highly toxic and corrosive. It hydrolyzes to release HF. Addition of water to FSO₃H can be violent, similar to the addition of water to sulfuric acid.

References

1. ^ Encyclopedia of Reagents for Organic Synthesis 2001 John Wiley & Sons, Ltd
2. ^ ^{a b} Cotton, F.A.; Wilkinson, G. (1980) Advanced Inorganic Chemistry: A Comprehensive Text, 4th Edition, p.246, Wiley-Interscience Publications.
3. ^ M. Juhasz, S. Hoffmann, E. Stoyanov, K.-C. Kim, C. A. Reed, Angewandte Chemie International Edition 43 5352 - 5355 2004.
4. ^ Olah, G.; Farooq, O.; Husain, A.; Ding, N.; Trivedi, N.; Olah, J. (1991) Superacid FSO₃H/HF-Catalyzed Butane Isomerisation. Catalysis Letters 10, 239-248.