The term superacid was originally coined by James Bryant Conant in 1927 to describe acids that were stronger than conventional mineral acids. This definition was refined by Ronald Gillespie in 1971, as any acid with an H0 value lower than that of 100% sulfuric acid. George A. Olah prepared the so-called magic acid, so-named for its ability to attack hydrocarbons, by mixing antimony pentafluoride and fluorosulfonic acid. The name was coined after a candle was placed in a sample of magic acid after a Christmas party. The candle dissolved, showing the ability of the acid to protonate alkanes, which under normal acidic conditions do not protonate to any extent. At 140 °C, FSO3H–SbF5 protonates methane to give the tertiary-butyl carbocation, a reaction that begins with the protonation of methane: Common uses of superacids include providing an environment to create, maintain, and characterize carbocations. Carbocations are intermediates in numerous useful reactions such as those forming plastics and in the production of high-octanegasoline.
Traditionally, superacids are made from mixing a Brønsted acid with a Lewis acid. The function of the Lewis acid is to bind to and stabilize the anion that is formed upon dissociation of the Brønsted acid, thereby removing a proton acceptor from solution and strengthening the proton donating ability of the solution. For example, fluoroantimonic acid, nominally, can produce solutions with a H0 lower than –21, giving it a protonating ability over a billion times greater than 100% sulfuric acid. Fluoroantimonic acid is made by dissolving antimony pentafluoride in anhydrous hydrogen fluoride. In this mixture, HF releases its proton concomitant with the binding of F− by the antimony pentafluoride. The resulting anion delocalizes charge effectively and holds onto its electron pairs tightly, making it an extremely poornucleophile and base. The mixture owes its extraordinary acidity to the weakness of proton acceptors in solution. Because of this, the proton in fluoroantimonic acid and other superacids are popularly described as "naked", being readily donated to substances not normally regarded as proton acceptors, like the C–H bonds of hydrocarbons However, even for superacidic solutions, protons in the condensed phase are far from being unbound. For instance, in fluoroantimonic acid, they are bound to one or more molecules of hydrogen fluoride. Though hydrogen fluoride is normally regarded as an exceptionally weak proton acceptor, dissociation of its protonated form, the fluoronium ion H2F+ to HF and the truly naked H+ is still a highly endothermic process, and imagining the proton in the condensed phase as being "naked" or "unbound", like charged particles in a plasma, is highly inaccurate and misleading. More recently, carborane acids have been prepared as single component superacids that owe their strength to the extraordinary stability of the carboranate anion, a family of anions stabilized by three-dimensional aromaticity, as well as by electron-withdrawing group typically attached thereto. In superacids, the proton is shuttled rapidly from proton acceptor to proton acceptor by tunneling through a hydrogen bond via the Grotthuss mechanism, just as in other hydrogen-bonded networks, like water or ammonia.
Applications
In petrochemistry, superacidic media are used as catalysts, especially for alkylations. Typical catalysts are sulfated oxides of titanium and zirconium or specially treated alumina or zeolites. The solid acids are used for alkylating benzene with ethene and propene as well as difficult acylations, e.g. of chlorobenzene.
Examples
The following values show the Hammett acidity function for several superacids, the strongest being fluoroantimonic acid. Increased acidity is indicated by smaller values of H0.
