Superacid

Acids and bases
pH Reaction Titration Extraction Dissociation constant (acid) Acid strength Acidity function Buffer solutions Proton affinity Self-ionization of water Amphoterism
Acid types
Brønsted Lewis Mineral Organic Strong Superacids Weak
Base types
Brønsted Lewis Organic Strong Superbases Non-nucleophilic Weak

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. A particularly strong superacid is fluoroantimonic acid.

History

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:

CH₄ + H⁺ → CH+
5

CH+
5 → CH+
3 + H₂

CH+
3 + 3 CH₄ → (CH₃)₃C⁺ + 3H₂

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-octane gasoline.

Nomenclature and mechanism

Fluoroantimonic acid, nominally (H
2FSbF
6), is 10¹⁶ times stronger than 100% sulfuric acid, and can produce solutions with a H₀ down to –28. Fluoroantimonic acid is a combination of hydrogen fluoride (HF) and antimony pentafluoride (SbF₅). In this system, HF releases its proton (H⁺) concomitant with the binding of F⁻ by the antimony pentafluoride. The resulting anion (SbF−
6) is both a weak nucleophile and a weak base.

The proton from superacids has been popularly said to become "naked", accounting for the system's extreme acidity. In the physical sense, the acidic "proton" is never completely free (as an unbound proton) but is bound to the anion, albeit weakly. The proton hops from anion to anion in superacids via the Grotthuss mechanism, just as happens to acidic "protons" in water. The extreme acidity of the acids is due to the ease with which this proton is transferred to substances that cannot normally be "protonated" (such as hydrocarbons), due to the very high stability of the conjugate-base anion (such as SbF−
6) after it donates the proton.

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 ethylene and propylene as well as difficult acylations, e.g. of chlorobenzene.

See also

• Superbase