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Fluorosulfuric acid

Fluorosulfuric acid
Other names Fluorosulfonic acid,
Fluorosulfuric acid,
CAS number 7789-21-1
EINECS number 232-149-4
RTECS number LP0715000
Molecular formula FSO3H.
Molar mass 100.0645 g/mol
Appearance Colorless liquid
Density 1.84 g/cm-3, liquid
Melting point

-87.3 °C

Boiling point

165.5 °C

Solubility in water Soluble
Acidity (pKa) -10
Molecular shape tetrahedral
Dipole moment N/A
EU classification 1777 Corrosive material
R-phrases 20-35
S-phrases 26-45
Related Compounds
Related compounds Antimony pentafluoride
Trifluoromethanesulfonic acid
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

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.


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.


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]


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.


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


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.


  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.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fluorosulfuric_acid". A list of authors is available in Wikipedia.
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