Superacid

According to the classical definition, a superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function (H₀) of −12. According to the modern definition, superacid is a medium in which the chemical potential of the proton is higher than in pure sulfuric acid. Commercially available superacids include trifluoromethanesulfonic acid (CF₃SO₃H), also known as triflic acid, and fluorosulfuric acid (HSO₃F), both of which are about a thousand times stronger (i.e. have more negative H₀ values) than sulfuric acid. Strong superacids are prepared by the combination of a strong Lewis acid and a strong Brønsted acid. The strongest superacid is fluoroantimonic acid.

The term superacid was originally coined by James Bryant Conant in 1927 to describe acids that were stronger than conventional mineral acids.George A. Olah prepared the so-called magic acid, so-named for its ability to attack hydrocarbons, by mixing antimony pentafluoride (SbF₅) and fluorosulfonic acid (FSO₃H). The name was coined after a candle was placed in a sample of magic acid. The candle dissolved, showing the ability of the acid to protonate hydrocarbons, which under normal acidic conditions do not protonate to any extent.

At 140 °C (284 °F), FSO₃H–SbF₅ protonates methane to give the tertiary-butyl carbocation, a reaction that begins with the protonation of methane:

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