Superacid

According to the classical definition, a superacid is an acid with an acidity greater than that of 100% pure sulfuric acid,[1] which has a Hammett acidity function (H0) of −12. According to the modern definition, superacid is a medium in which the chemical potential of the proton is higher than in pure sulfuric acid.[2] Commercially available superacids include trifluoromethanesulfonic acid (CF3SO3H), also known as triflic acid, and fluorosulfuric acid (HSO3F), 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. The strongest known superacid is fluoroantimonic acid.

Unstable gas-phase heteroelement-hydride ions can be extremely acidic, e.g. the helium hydride ion. However, they are not considered as such because they exist only in extremely hot plasma and not in the liquid state.

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 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. The candle dissolved, showing the ability of the acid to protonate hydrocarbons, which under normal acidic conditions do not protonate to any extent.

It was shown 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:[3]

CH4 + H+ → CH+
5
CH+
5 → CH+
3 + H2
CH+
3 + 3 CH4 → (CH3)3C+ + 3H2

Nomenclature and Mechanism

Fluoroantimonic acid (H
2FSbF
6), the strongest acid in the system, is 1016 times stronger than 100% sulfuric acid,[4] and can produce solutions with a H0 down to –28.[5] Fluoroantimonic acid is a combination of hydrogen fluoride (HF) and antimony pentafluoride(SbF5). In this system, HF releases its proton (H+) concomitant with the binding of F− by the antimony pentafluoride. The resulting anion (SbF−
6) is both a weak nucleophile and a weak base.

The proton from superacids has been popularly said to become "naked," accounting for the system's extreme acidity. In the physical sense, the acidic "proton" is never completely free (as an unbound proton) in the acid, but rather hops from anion to anion in superacids via the Grotthuss mechanism, just as happens to acidic "protons" in water.[6] The extreme acidity of the acids is due to the ease with which this proton is transferred to substances that cannot normally be "protonated" (such as hydrocarbons), due to the very high stability of the conjugate-base anion (such as SbF−
6) after it donates the proton.

Applications

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 high-octane gasoline.[7]

Superacids are one of the few solvents that can dissolve carbon nanotubes.[8]

See also

References

  1. ↑ 1.0 1.1 Hall NF, Conant JB (1927). "A Study of Superacid Solutions". Journal of the American Chemical Society 49 (12): 3062–70. doi:10.1021/ja01411a010.
  2. ↑ Himmel D, Goll SK, Leito I, Krossing I (2010). "A Unified pH Scale for All Phases". Angew. Chem. Int. Ed. 49 (38): 6885–6888. doi:10.1002/anie.201000252.
  3. ↑ 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 (10): 2726–7. doi:10.1021/ja01012a066.
  4. ↑ 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.
  5. ↑ 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.
  6. ↑ Computer modeling of proton-hopping in superacids.
  7. ↑ Fluoroantimonic Acid
  8. ↑ Proceedings of the 7th Aachen-Dresden International Textile Conference, November 28–29, 2013, Aachen, Germany.