Fluoroantimonic acid

Fluoroantimonic acid HSbF6 is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination affords the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2.

The reaction of hydrogen fluoride (HF) and SbF5 is exothermic. HF releases its proton (H+), and its conjugate base (F−) is sequestered by one of more molecules SbF5 to give the octahedral SbF6−. This anion is classified as noncoordinating, because it is both a very weak nucleophile and a very weak base. The proton effectively becomes "naked", which accounts for the system's extreme acidity. Fluoroantimonic acid is 2×1019 times stronger than 100% sulfuric acid. The reaction of hydrofluoric acid and antimony pentafluoride proceeds as follows



Structure
Two related products have been crystallised from HF-SbF5 mixtures, and both have been analyzed by single crystal X-ray crystallography. These salts have the formulas [H2F+][Sb2F11−] and [H3F2+][Sb2F11−]. In both salts the anion is Sb2F11−. As mentioned above, SbF6− is classified as weakly basic; the larger monoanion Sb2F11− would be expected to be still weaker.

Comparison with other acids
The following values are based upon the Hammett acidity function. Acidity is indicated by large negative values of H0.
 * Fluoroantimonic acid (1990) (H0 Value = −31.3)
 * Magic acid (1974) (H0 Value = −19.2)
 * Carborane superacid (1969) (H0 Value = −18.0)
 * Fluorosulfuric acid (1944) (H0 Value = −15.1)
 * Triflic acid (1940) (H0 Value = −14.9)

Applications
This extraordinarily strong acid protonates nearly all organic compounds. In 1967, Bickel and Hogeveen showed that HF-SbF5 will remove H2 from isobutane and methane from neopentane:
 * (CH3)3CH +  H+  →  (CH3)3C+  +  H2
 * (CH3)4C +  H+  →  (CH3)3C+  +  CH4

Safety
HF-SbF5 is rapidly and explosively decomposed by water. It reacts with virtually all known solvents. Solvents that have been proven to be compatible with HF-SbF5 are SO2ClF and liquified sulfur dioxide. Chlorofluorocarbons have also been used as solvents. Containers for HF-SbF5 are made of PTFE.