Fluorosulfuric acid

Fluorosulfuric acid

Chembox new
Name = Fluorosulfuric acid
ImageFile = Fluorosulfuric-acid-2D.png ImageName = Fluorosulfuric acid
ImageFile1 = Fluorosulfuric-acid-3D-vdW.png ImageName1 = Fluorosulfuric acid
OtherNames = Fluorosulfonic acid,
Fluorosulfuric acid,

Section1 = Chembox Identifiers
CASNo = 7789-21-1
EINECS = 232-149-4
RTECS = LP0715000

Section2 = Chembox Properties
Formula = FSO3H.
MolarMass = 100.0645 g/mol
Appearance = Colorless liquid
Density = 1.84 g/cm-3, liquid
Solubility = Soluble
MeltingPt = -87.3 °C
BoilingPt = 165.5 °C
pKa = -10

Section3 = Chembox Structure
MolShape = tetrahedral
Dipole = N/A

Section7 = Chembox Hazards
ExternalMSDS =
EUClass = 1777 Corrosive material
RPhrases = 20-35
SPhrases = 26-45

Section8 = Chembox Related
OtherCpds = Antimony pentafluoride
Trifluoromethanesulfonic 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.

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. [ Encyclopedia of Reagents for Organic Synthesis 2001 John Wiley & Sons, Ltd] 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 → FSO3HAlternatively, 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.Cotton, F.A.; Wilkinson, G. (1980) "Advanced Inorganic Chemistry: A Comprehensive Text", 4th Edition, p.246, Wiley-Interscience Publications.]


FSO3H is one of the strongest known simple Brønsted acids, although recent work on carborane-based acids have led to still stronger acids. [M. Juhasz, S. Hoffmann, E. Stoyanov, K.-C. Kim, C. A. Reed, Angewandte Chemie International Edition 43 5352 - 5355 2004.] It has an "H"0 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. [Olah, G.; Farooq, O.; Husain, A.; Ding, N.; Trivedi, N.; Olah, J. (1991) Superacid FSO3H/HF-Catalyzed Butane Isomerisation. Catalysis Letters 10, 239-248.] It can also be used as a laboratory fluorinating agent.


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.


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