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Concept
Superacide
Résumé
In chemistry, a superacid (according to the original definition) 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, a 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 (), also known as triflic acid, and fluorosulfuric acid (), both of which are about a thousand times stronger (i.e. have more negative H0 values) than sulfuric acid. Most strong superacids are prepared by the combination of a strong Lewis acid and a strong Brønsted acid. A strong superacid of this kind is fluoroantimonic acid. Another group of superacids, the carborane acid group, contains some of the strongest known acids. Finally, when treated with anhydrous acid, zeolites (microporous aluminosilicate minerals) will contain superacidic sites within their pores. These materials are used on massive scale by the petrochemical industry in the upgrading of hydrocarbons to make fuels.
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 (−11.93). 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 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 (284°F), FSO3H–SbF5 protonates methane to give the tertiary-butyl carbocation, a reaction that begins with the protonation of methane:
CH4 + H+ → CH5+
CH5+ → CH3+ + H2
CH3+ + 3 CH4 → (CH3)3C+ + 3H2
Common uses of superacids include providing an environment to create, maintain, and characterize carbocations.
Source officielle
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