Superacid [1] (or superacid [2] ) is a substance or mixture of substances whose acidity parameter exceeds the acidity of 100% sulfuric acid . In this case, the acidity parameter is taken as the ability of the acid to protonate an arbitrary base and in fact coincides with the function of acidity . For 100% sulfuric acid, the acidity parameter is H 0 = −11.93 . The presence of water reduces the acidity of superacids, since in the presence of water the strength of an acid is limited by the acidity of the hydroxonium ion H 3 O + .
Content
History
The term superacid (used in English-language literature) was introduced by James Conant in 1927 to classify more strong acids than ordinary mineral acids . In 1994, George Ola was awarded the Nobel Prize [3] for his research on superacids and their use in direct observation of carbocations .
Magic acid got its name for its exceptional ability to dissolve wax. It is a mixture of antimony pentafluoride , which is a Lewis acid , and fluorosulfonic acid — Brensted acid . One of the strongest superacid systems is hexafluoro - antimony acid - a mixture of hydrofluoric acid and antimony pentafluoride . In this system, hydrofluoric acid releases a proton (H + ), and the conjugate base (F - ) is isolated by coordination coordination with antimony pentafluoride . This forms a large octahedral anion (SbF 6 - ), which is a very weak nucleophile and a very weak base . Having become "free", the proton causes the system to be extremely acid. Fluoro-antimony acid is 2⋅10 19 times stronger than 100% sulfuric acid .
Chemical Properties
To characterize the acidity of substances in non-aqueous media, the Hammett’s acidity function H 0 is used . Fluids are known for which H 0 is more negative than for concentrated aqueous solutions of very strong acids, such as HNO 3 , H 2 SO 4 . These fluids are called superacids. Examples: 100% H 2 SO 4 ( H 0 = −12 ), anhydrous fluorosulfonic acid HSO 3 F ( H 0 = −15 ), a mixture of HF and SbF 5 ( H 0 = −28 ), 7% solution SbF 5 in HSO 3 F ( H 0 = −19.4 ). Equimolar mixture of HSO 3 F and SbF 5 is called “ magic acid ”. Superacidity is due to the exceptional weakness of the interaction with the proton of the corresponding anions (HSO 4 - , SbF 6 - , etc.). In the medium of super acids, substances that usually do not exhibit basic properties, in particular hydrocarbons, are protonated. This phenomenon is used in practice, mainly in organic synthesis ( Friedel-Crafts alkylation , petroleum hydrogenation, etc.).
Proton superacids
Simple superacids include trifluoromethanesulfonic (CF 3 SO 3 H), chloric , chlorosulfonic , pentafluoroethanesulfonic , fluorosulfonic (FSO 3 H) acids. In many cases, superacid is not a single compound, but a mixture of compounds combined to achieve high acidity.
Carboranic acid is one of the strongest acids that can be stored in bottles. It is a million times stronger than concentrated sulfuric acid. Established at the University of California ( USA ) with the participation of employees of the Institute of Catalysis SB RAS ( Novosibirsk ). The strongest superacidity of all at the moment is fluorinated carboranic acid .
The strongest known acid among all systems is helium hydride ), but this system cannot be stored in any appreciable quantity.
Lewis superacids
Lewis superacids include most metal pentafluorides and boron trifluoride .
Notes
- ↑ Superacids // Big Russian Encyclopedia : [in 35 t.] / Ch. ed. Yu. S. Osipov . - M .: The Great Russian Encyclopedia, 2004—2017.
- ↑ Superacids // Polytechnical Dictionary. - 3rd. - Moscow: Soviet Encyclopedia, 1989. - p. 470. - 656 p. - ISBN 5-85270-003-7 .
- ↑ The Nobel Prize in Chemistry 1994
Links
- A. Olah. J. Org. Chem., 70, 2005, 2413-2429
- Superacids (STEINGARZ VD, 1999), CHEMISTRY pdf file