(chemistry) An acidic medium that has a proton-donating ability equal to or greater than 100% sulfuric acid. A solution of acetic or phosphoric acid.
Superacid
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(¦sü·pər′as·əd)
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- Acid dissociation constant
- Acid-base extraction
- Acid-base reaction
- Dissociation constant
- Acidity function
- Buffer solutions
- pH
- Proton affinity
- Self-ionization of water
- Acids:
- Lewis acids
- Mineral acids
- Organic acids
- Strong acids
- Superacids
- Weak acids
- Bases:
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. Commercially available superacids include trifluoromethanesulfonic acid (CF3SO3H), also known as triflic acid, and fluorosulfonic 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.
Contents |
History
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 fluorosulfonic acid (HSFO3). 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 under normal acidic conditions do not protonate to any extent.
Magic acid hydrolyzes explosively to sulfuric acid and fluoroantimonic acid. The reaction of magic acid with water is as follows:
HSFO3.SbF5 + H2O → H2SO4 + HSbF6
Superlatives
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 H0 down to –25.[3]
George Andrew 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+ + 3H2
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
- ^ Hall NF, Conant JB (1927). "A Study of Superacid Solutions" ([dead link]). Journal of the American Chemical Society 49: 3062–70. doi:. http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1927/49/i12/f-pdf/f_ja01411a010.pdf.
- ^ Olah, George A. (2005). "Crossing Conventional Boundaries in Half a Century of Research". Journal of Organic Chemistry 70 (7): 2413–2429. doi:.
- ^ 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:.
- ^ 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:.
- ^ Fluoroantimonic Acid
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