In chemistry, a superacid (according to the original definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (),
History
The term
superacid was originally coined by James Bryant Conant in 1927 to describe acids that were stronger than conventional
.
This definition was refined by
Ronald Gillespie in 1971, as any acid with an
H0 value lower than that of 100% sulfuric acid (−11.93).
George A. Olah prepared the so-called "
magic acid", so named for its ability to attack
hydrocarbons, by mixing antimony pentafluoride (SbF
5) and fluorosulfonic acid (FSO
3H).
[ 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 Protonation , 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+ →
- → + H2
- + 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 Octane rating gasoline.
Origin of extreme acid strength
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 the solution and strengthening the proton donating ability of the solution. For example, fluoroantimonic acid, nominally (), can produce solutions with a H0 lower than −28, giving it a protonating ability over a billion times greater than 100% sulfuric acid.
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 . Typical catalysts are sulfated oxides of titanium and zirconium or specially treated alumina or . The are used for alkylating benzene with ethene and propene as well as difficult , e.g. of chlorobenzene.[Michael Röper, Eugen Gehrer, Thomas Narbeshuber, Wolfgang Siegel "Acylation and Alkylation" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2000. ] In organic chemistry, superacids are used as a means of protonating alkanes to promote the use of carbocations in situ during reactions. The resulting carbocations are of much use in organic synthesis of numerous organic compounds, the high acidity of the superacids helps to stabilize the highly reactive and unstable carbocations for future reactions.
Examples
The following are examples of superacids. Each is listed with its Hammett acidity function,
-
Helium hydride ion (HeH+, H0 = −63)
-
Fluoroantimonic acid (HF:SbF5, H0 = −28)
-
Magic acid (HSO3F:SbF5, H0 = −23)
-
(H(HCB11X11), H0 ≤ −18, indirectly determined and depends on substituents)
-
Fluoroboric acid (HF:BF3, H0 = −16.6)
-
Bistriflimide (NH(CF3SO2)2, H0 = −15.8.
-
Fluorosulfuric acid (FSO3H, H0 = −15.1)
-
Hydrogen fluoride (HF, H0 = −15.1)
-
Triflic acid (HOSO2CF3, H0 = −14.9)
-
Oleum (SO3:H2SO4, H0 = −14.5)
-
Perchloric acid (HClO4, H0 = −13)
-
Sulfuric acid (H2SO4, H0 = −11.9)
See also
