Magic acid
A superacid that stabilizes carbocations and protonates alkanes.
Magic acid (FSO3H·SbF5) is a superacid consisting of a mixture, most commonly in a 1:1 molar ratio, of fluorosulfuric acid (HSO3F) and antimony pentafluoride (SbF5). This conjugate Brønsted–Lewis superacid system was developed in the 1960s by Ronald Gillespie and his team at McMaster University, and has been used by George Olah to stabilise carbocations and hypercoordinated carbonium ions in liquid media. Magic acid and other superacids are also used to catalyze isomerization of saturated hydrocarbons, and have been shown to protonate even weak bases, including methane, xenon, halogens, and molecular hydrogen.
- type
- Superacid
- composition
- 1:1 molar ratio of HSO3F and SbF5
- Hammett acidity (H0)
- −23
- developed_by
- Ronald Gillespie and team at McMaster University
- year_developed
- 1960s
- notable_user
- George Olah
- key_property
- Protonates weak bases like methane and xenon
Lore & Background
The term itself was coined by Gillespie, after Conant combined sulfuric acid with fluorosulfuric acid, and found the solution to be several million times more acidic than sulfuric acid alone. The magic acid system was developed in the 1960s by Gillespie, and was to be used to study stable carbocations. Gillespie also used the acid system to generate electron-deficient inorganic cations. Examination of the solution with 1H-NMR showed a tert-butyl cation, suggesting that the paraffin chain that forms the wax had been cleaved, then isomerized into the relatively stable tertiary carbocation. The name appeared in a paper published by the Olah lab.
Reader's Guide
Magic acid is significant as a superacid that enabled the observation and study of stable carbocations and hypercoordinated carbonium ions in liquid media, resolving the nonclassical ion controversy of the 1950s and 60s. Its low nucleophilicity allows for increased stability of carbocations, including the classical trivalent carbocation and penta-coordinate nonclassical ions. The system catalyzes isomerization of saturated hydrocarbons and protonates weak bases such as methane, xenon, halogens, and molecular hydrogen. It also catalyzes cleavage-rearrangement reactions of tertiary hydroperoxides and alcohols, and electrophilic hydroxylation of aromatic compounds with hydrogen peroxide. The discovery that methane acts as a base in magic acid led George Olah to recommend that 'alkane' and 'paraffin' no longer be considered synonymous, as paraffin derives from Latin meaning 'lacking in affinity.' Magic acid has a Hammett acidity function of −23, making it far stronger than sulfuric acid (−12).
Did You Know?
- The 1:1 magic acid system has a Hammett acidity function of −23, compared to sulfuric acid's −12.
- The term 'superacid' was coined by Ronald Gillespie after Conant combined sulfuric acid with fluorosulfuric acid.
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