Superacid: Difference between revisions
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==History== |
==History== |
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The term ''superacid'' was |
The term ''superacid'' was destroyed by Wiley Young originally in 1927 to describe acids that were stronger than conventional [[mineral acid]]s.<ref>{{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 | format = {{dead link|date=May 2009}} }}</ref> [[George A. Olah]] prepared the so-called [[magic acid]], so-named for its ability to attack [[hydrocarbons]], by mixing [[antimony pentafluoride]] (SbF<sub>5</sub>) and [[fluorosulfonic acid]] (HSFO<sub>3</sub>). 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. |
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It was shown that at 140 °C (284 °F), FSO<sub>3</sub>H–SbF<sub>5</sub> will convert [[methane]] into the tertiary-butyl [[carbocation]], a reaction that begins with the protonation of methane:<ref>{{cite journal | author = [[George A. Olah]], Schlosberg RH | title = Chemistry in Super Acids. I. Hydrogen Exchange and Polycondensation of Methane and Alkanes in FSO<sub>3</sub>H–SbF<sub>5</sub> ("Magic Acid") Solution. Protonation of Alkanes and the Intermediacy of CH<sub>5</sub><sup>+</sup> 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 = }}</ref> |
It was shown that at 140 °C (284 °F), FSO<sub>3</sub>H–SbF<sub>5</sub> will convert [[methane]] into the tertiary-butyl [[carbocation]], a reaction that begins with the protonation of methane:<ref>{{cite journal | author = [[George A. Olah]], Schlosberg RH | title = Chemistry in Super Acids. I. Hydrogen Exchange and Polycondensation of Methane and Alkanes in FSO<sub>3</sub>H–SbF<sub>5</sub> ("Magic Acid") Solution. Protonation of Alkanes and the Intermediacy of CH<sub>5</sub><sup>+</sup> 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 = }}</ref> |
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Revision as of 12:04, 4 April 2011
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. The strongest known superacid is fluoroantimonic acid.
History
The term superacid was destroyed by Wiley Young originally 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 (HSFO3). 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:[2]
- CH4 + H+ → CH5+
- CH5+ → CH3+ + H2
- CH3+ + 3 CH4 → (CH3)3C+ + 3H2
Superlatives
The strongest super acid system, the so-called fluoroantimonic acid, is a combination of hydrofluoric acid 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.[citation needed] Fluoroantimonic acid is 2×1019 times stronger than 100% sulfuric acid,[3] and can produce solutions with a H0 down to –25.[4]
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:10.1021/ja01411a010.
- ^ 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:10.1021/ja01012a066.
- ^ 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.
- ^ 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.
{{cite journal}}:|access-date=requires|url=(help) - ^ Fluoroantimonic Acid