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, a 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. Most strong superacids are prepared by the combination of a strong Lewis acid and a strong Brønsted acid. The strongest superacid of this kind is fluoroantimonic acid.

History

The term superacid was originally coined by James Bryant Conant in 1927 to describe acids that were stronger than conventional mineral acids.[1] 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, or roughly speaking, −12).[3] 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).[4] 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:[4]

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

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.

Nomenclature and mechanism

Fluoroantimonic acid, nominally (H
2FSbF
6), is 1016 times stronger than 100% sulfuric acid,[5] and can produce solutions with a H0 lower than –21.[6] 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 poor 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) but is bound to the anion, albeit weakly. The proton hops from anion to anion in superacids via the Grotthuss mechanism, just as happens to acidic "protons" in water.[7] 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

In petrochemistry, superacidic media are used as catalysts, especially for alkylations. Typical catalysts are sulfated oxides of titanium and zirconium or specially treated alumina or zeolites. The solid acids are used for alkylating benzene with ethene and propene as well as difficult acylations, e.g. of chlorobenzene.[8]

Examples

The following values show the Hammett acidity function for several superacids, the strongest being fluoroantimonic acid.[9] Increased acidity is indicated by smaller (in this case, more negative) values of H0.

See also

References

  1. 1 2 Hall NF, Conant JB (1927). "A Study of Superacid Solutions". Journal of the American Chemical Society. 49 (12): 3062&ndash, 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&ndash, 6888. doi:10.1002/anie.201000252.
  3. ↑ Gillespie, R. J.; Peel, T. E.; Robinson, E. A. (1971-10-01). "Hammett acidity function for some super acid systems. I. Systems H2SO4-SO3, H2SO4-HSO3F, H2SO4-HSO3Cl, and H2SO4-HB(HSO4)4". Journal of the American Chemical Society. 93 (20): 5083–5087. doi:10.1021/ja00749a021. ISSN 0002-7863. The work of Jorgenson and Hartter formed the basis for the present work, the object of which was to extend the range of acidity function measurements into the super acid region, i.e., into the region of acidities greater than that of 100% H2SO4.
  4. 1 2 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&ndash, 7. doi:10.1021/ja01012a066.
  5. ↑ 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.
  6. ↑ Herlem, Michel (1977). "Are reactions in superacid media due to protons or to powerful oxidising species such as SO3 or SbF5?". Pure and Applied Chemistry. 49: 107–113. doi:10.1351/pac197749010107.
  7. ↑ Schneider, Michael (2000). "Getting the Jump on Superacids". Pittsburgh Supercomputing Center. Retrieved 20 November 2017.
  8. ↑ Michael Röper, Eugen Gehrer, Thomas Narbeshuber, Wolfgang Siegel "Acylation and Alkylation" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2000. doi:10.1002/14356007.a01_185
  9. ↑ Gillespie, R. J.; Peel, T. E. (1973-08-01). "Hammett acidity function for some superacid systems. II. Systems sulfuric acid-[fsa], potassium fluorosulfate-[fsa], [fsa]-sulfur trioxide, [fsa]-arsenic pentafluoride, [sfa]-antimony pentafluoride and [fsa]-antimony pentafluoride-sulfur trioxide". Journal of the American Chemical Society. 95 (16): 5173–5178. doi:10.1021/ja00797a013. ISSN 0002-7863.
  10. ↑ Liang, Joan-Nan Jack (1976). The Hammett Acidity Function for Hydrofluoric Acid and some related Superacid Systems (Ph.D. Thesis, advisor: R. J. Gillespie) (PDF). Hamilton, Ontario: McMaster University. p. 109.
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