Superacid
In
History
The term superacid was originally coined by
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.
Origin of extreme acid strength
Traditionally, superacids are made from mixing a Brønsted acid with a Lewis acid. The function of the Lewis acid is to bind to and stabilize the anion that is formed upon dissociation of the Brønsted acid, thereby removing a proton acceptor from the solution and strengthening the proton donating ability of the solution. For example,
6) delocalizes charge effectively and holds onto its electron pairs tightly, making it an extremely poor nucleophile and base. The mixture owes its extraordinary acidity to the weakness of proton acceptors (and electron pair donors) (Brønsted or Lewis bases) in solution. Because of this, the protons in fluoroantimonic acid and other superacids are popularly described as "naked", being readily donated to substances not normally regarded as proton acceptors, like the C–H bonds of hydrocarbons. However, even for superacidic solutions, protons in the condensed phase are far from being unbound. For instance, in fluoroantimonic acid, they are bound to one or more molecules of hydrogen fluoride. Though hydrogen fluoride is normally regarded as an exceptionally weak proton acceptor (though a somewhat better one than the SbF6– anion), dissociation of its protonated form, the fluoronium ion H2F+ to HF and the truly naked H+ is still a highly endothermic process (ΔG° = +113 kcal/mol), and imagining the proton in the condensed phase as being "naked" or "unbound", like charged particles in a plasma, is highly inaccurate and misleading.[7]
More recently, carborane acids have been prepared as single component superacids that owe their strength to the extraordinary stability of the carboranate anion, a family of anions stabilized by three-dimensional aromaticity, as well as by electron-withdrawing group typically attached thereto.
In superacids, the proton is shuttled rapidly from proton acceptor to proton acceptor by tunneling through a hydrogen bond via the Grotthuss mechanism, just as in other hydrogen-bonded networks, like water or ammonia.[8]
Applications
In
Examples
The following are examples of superacids. Each is listed with its Hammett acidity function,[10] where a smaller value of H0 (in these cases, more negative) indicates a stronger acid.
- Helium hydride ion (HeH+, H0 = -63)
- Fluoroantimonic acid (HF:SbF5, H0 = -28)
- Magic acid (HSO3F:SbF5, H0 = −23)
- Triflidic acid (CH(CF3SO2)3, H0 = −18.6)
- Carborane acids (H(HCB11X11), H0 ≤ −18, indirectly determined and depends on substituents)
- Fluoroboric acid (HF:BF3, H0 = −16.6)
- Bistriflimidic acid (NH(CF3SO2)2, H0 = -15.8.[11] Estimated value calculated from pKa values in 1,2-dichloroethane in comparison to triflic acid)
- Fluorosulfuric acid (FSO3H, H0 = −15.1)
- Hydrogen fluoride (HF, H0 = −15.1)[12]
- Triflic acid (HOSO2CF3, H0 = −14.9)
- Oleum (SO3:H2SO4, H0 = −14.5)[13]
- Perchloric acid (HClO4, H0 = −13)
- Sulfuric acid (H2SO4, H0 = −11.9)
See also
References
- ^ .
- PMID 20715223.
- 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.
- ^ .
- PMID 15787527.
- S2CID 98483167.
- OCLC 29913262.
- ^ Schneider, Michael (2000). "Getting the Jump on Superacids". Pittsburgh Supercomputing Center. Archived from the original on 23 August 2018. Retrieved 20 November 2017.
- ISSN 0002-7863.
- ^ Fuller, Maurice (2022). Coordination Chemistry and its Application (PDF). Bibliotex. pp. 45, 46.
- ^ 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.
- ^ Olah, George (2009). SUPERACID CHEMISTRY (PDF). John Wiley & Sons, Inc. p. 47.