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Superacid



A superacid is an acid with an acidity greater than that of 100% sulfuric acid, which has a Hammett acidity function of -12. Commercially available superacids include trifluoromethanesulfonic acid (CF3SO3H), also known as triflic acid, and fluorosulfuric acid (FSO3H), both of which are about a thousand times stronger (i.e. 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.

Additional recommended knowledge

The term superacid was originally coined by James Bryant Conant in 1927 to describe acids that were stronger than conventional mineral acids.[1] George A. Olah was awarded the 1994 Nobel prize in chemistry for his investigations of superacids and their use in the direct observation of carbocations. Olah's "magic acid", so-named for its ability to attack hydrocarbons, is prepared by mixing antimony pentafluoride (SbF5) and fluorosulfuric acid. The name was coined after one of Professor Olah's post-doctoral associates placed a candle in a sample of magic acid. The candle was dissolved, showing the ability of the acid to protonate hydrocarbons (which are not basic).

The strongest super acid system, the so-called fluoroantimonic acid, is a combination of hydrogen fluoride and SbF5. In this system, HF releases its proton (H+) concomitant with the binding of F by the antimony pentafluoride. The resulting anion (SbF6) is both a weak nucleophile and a weak base. The proton effectively becomes "naked", which accounts for the system's extreme acidity. Fluoroantimonic acid is 2×1019 times stronger than 100% sulfuric acid,[2] and can produce solutions with a pH down to –25.[3]

Olah showed that at 140 °C (284 °F), FSO3H-SbF5 will convert methane into the tertiary-butyl carbocation, a reaction that begins with the protonation of methane:[4]

CH4 + H+ → CH5+
CH5+ → CH3+ + H2
CH3+ + 3 CH4 → (CH3)3C+ + 3/2 H2

Applications

Common uses of superacids include providing an environment to create and maintain organic cations which are useful as intermediate molecules in numerous reactions, such as involving plastics and high-octane gasoline production and study.[5]

See also

References

  1. ^ Hall NF, Conant JB (1927). "A Study of Superacid Solutions". Journal of the American Chemical Society 49: 3062–70.
  2. ^ Olah, George A. (2005). "Crossing Conventional Boundaries in Half a Century of Research". Journal of Organic Chemistry 70 (7): 2413–2429. doi:S0022-3263(04)00285-3 10.1021/jo040285o S0022-3263(04)00285-3.
  3. ^ 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. Retrieved on 2007-05-14.
  4. ^ 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.
  5. ^ http://www.newton.dep.anl.gov/askasci/chem03/chem03808.htm
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Superacid". A list of authors is available in Wikipedia.
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