Superacid

A superacid is an acid with an acidity greater than that of 100% sulfuric acid, which has a Hammett acidity function ("H"₀) 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 "H"₀ 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 term "superacid" was originally coined by James Bryant Conant in 1927 to describe acids that were stronger than conventional mineral acids. [cite journal | author = Hall NF, Conant JB | title = A Study of Superacid Solutions | journal = Journal of the American Chemical Society | year = 1927 | volume = 49 | pages = 3062–70 | url = http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1927/49/i12/f-pdf/f_ja01411a010.pdf | doi = 10.1021/ja01411a010 ] 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, [cite journal | last = Olah | first = George A. | year = 2005 | title = Crossing Conventional Boundaries in Half a Century of Research | journal = Journal of Organic Chemistry | volume = 70 | issue = 7 | pages = 2413–2429 | doi = 10.1021/jo040285o S0022-3263(04)00285-3] and can produce solutions with a H₀ down to –25. [cite journal | last = Herlem | first = Michel | year = 1977 | title = Are reactions in superacid media due to protons or to powerful oxidising species such as SO₃ or SbF₅? | journal = Pure & Applied Chemistry | volume = 49 | pages = 107–113 | accessdate=2007-05-14 | doi = 10.1351/pac197749010107 ]

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: [cite journal | author = George A. Olah, Schlosberg RH | title = 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 = Journal of the American Chemical Society | year = 1968 | volume = 90 | pages = 2726–7 | doi = 10.1021/ja01012a066 | url = ] :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. [ [http://www.newton.dep.anl.gov/askasci/chem03/chem03808.htm Fluoroantimonic Acid ] ]

ee also

*Superbase

References