A superacid is an acid with an acidity greater than that of 100% pure sulfuric acid,. So, a superacid has a Hammett acidity function (H0) of −12. According to the modern definition, a superacid is a medium that has chemical potential of the proton higher than that of pure sulfuric acid.
Chemists use strong acids to break down other molecules. Some molecules are so strong (held together with strong chemical bonds) that they resist attack by common acids. Supercids can break down molecules that stand up to most other acids.
Commercially available superacids include trifluoromethanesulfonic acid (CF3SO3H), also known as triflic acid, and fluorosulfonic acid (FSO3H). Both acids are about a thousand times stronger (that is, have more negative H0 values) than sulfuric acid. The strongest superacids are prepared by the combination of two components, a strong Lewis acid and a strong Brønsted acid. The strongest known superacid is fluoroantimonic acid.
The term superacid was originally used by James Bryant Conant in 1927 to describe acids that were stronger than conventional mineral acids. George Andrew Olah prepared the so-called magic acid. It has that name because it is able to attack hydrocarbons. The name came after a candle was placed in a sample of magic acid. The candle dissolved, showing the ability of the acid to protonate hydrocarbons. (Under normal acidic conditions hydrocarbons do not protonate very much.) Mixing antimony pentafluoride (SbF5) and fluorosulfonic acid (FSO3H) makes magic acid.
- CH4 + H+ → CH5+
- CH5+ → CH3+ + H2
- CH3+ + 3 CH4 → (CH3)3C+ + 3H2
The strongest super acid system, fluoroantimonic acid, is a combination of hydrofluoric acid and SbF5. In this system, HF releases its proton (H+) at the same time that the antimony pentafluoride binds F−. The resulting anion (SbF6−) is both a weak nucleophile and a weak base. Fluoroantimonic acid is 2×1016 times stronger than 100% sulfuric acid, and can produce solutions with a H0 down to –28.
Superacids are mostly used to help create and maintain organic cations. The organic cations are useful as intermediate molecules in many chemical reactions. Chemists use superacids to make plastics and high-octane gasoline.
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- Hall NF, Conant JB (1927). "A Study of Superacid Solutions". Journal of the American Chemical Society 49: 3062–70. doi:10.1021/ja01411a010.
- Himmel D, Goll SK, Leito I, Krossing I (2010). "A Unified pH Scale for All Phases". Angew. Chem. Int. Ed. 49: 6885–6888. doi:10.1002/anie.201000252.
- George A. Olah, Schlosberg RH (1968). "Chemistry in Super Acids. I. Hydrogen Exchange and Polycondensation of Methane and Alkanes in FSO3H–SbF5 ("Magic Acid") Solution. Protonation of Alkanes and the Intermediacy of CH5+ and Related Hydrocarbon Ions. The High Chemical Reactivity of "Paraffins" in Ionic Solution Reactions". Journal of the American Chemical Society 90: 2726–7. doi:10.1021/ja01012a066.
- Olah, George A. (2005). "Crossing Conventional Boundaries in Half a Century of Research". Journal of Organic Chemistry 70 (7): 2413–2429. doi:10.1021/jo040285o. PMID 15787527.
- Herlem, Michel (1977). "Are reactions in superacid media due to protons or to powerful oxidising species such as SO3 or SbF5?". Pure & Applied Chemistry 49: 107–113. doi:10.1351/pac197749010107.
- Fluoroantimonic Acid