Super acids

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As superacids in the chemistry acids stronger than concentrated (100 percent) denotes, sulfuric acid (H 2 SO 4 : pK s value = -3.0), respectively.

All super acids thus have a pK s value in negative territory. The Hammett acidity function is used to quantify the acid strength .

For example, fluorosulfonic acid (HSO 3 F) is several thousand times stronger than concentrated sulfuric acid. If it is combined with antimony pentafluoride, the even stronger magical acid is obtained . This mixture even reacts with alkanes . Another common super acid is fluoroantimonic acid (HSbF 6 : pK S = −17), which consists of antimony pentafluoride and anhydrous hydrogen fluoride. If these two acids are combined with one another, their acidity is increased by a factor of 10 3 . Organic acids can by certain groups pK s reach values <-11. An example of this is pentacyanocyclopentadiene . Per-halogenated carboranes H (CHB 11 Cl 11 ) have also been known as super acids since 2004 .

In an aqueous environment, it cannot be proven that a super acid is stronger than concentrated sulfuric acid, since a degree of protolysis greater than 1 is only possible in the absence of bases and therefore cannot be achieved in water. This effect is called the leveling effect of the water .

The importance of superacids in basic research lies in the fact that certain species such as carbocations , inorganic polycations and transition metals can be stabilized in low oxidation states in superacid media . The production of metal-xenon compounds, non-classical metal carbonyls and protonated fullerenes was also possible with the help of super acids. The decisive factor is the low nucleophilicity and the high steric demand of the acid residues.

Due to their extremely high acidity, superacids are sometimes involved in reactions that have long been considered impossible in chemistry: Among other things, they are able to protonate the extremely inert noble gases and generate carbon atoms with formally five bonds ( carbonium ions ) .

The name goes back to the chemist James Bryant Conant at Harvard (1927). They were especially used by George A. Olah for the preparation and stabilization of carbocations.

literature

  • George A. Olah, GK Surya Prakash, Jean Sommer, Arpad Molnar: Superacid Chemistry , 2nd edition, Wiley 2009, ISBN 9780471596684

See also

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  1. ^ NF Hall, JB Conant: A Study of Superacid Solutions. I. The use of chloranil in glacial acetic acid and the strength of certain weak bases. , in: J. Am. Chem. Soc. 1927 , 49 , 3047-3061; doi : 10.1021 / ja01411a010 .
  2. ↑ a b c C. Janiak, TM Klapötke , H.-J. Meyer, E. Riedel, Modern Inorganic Chemistry , 2nd edition, de Gruyter, Berlin, 2004 .
  3. ↑ JE Huheey, E. Keiter, RL Keiter, Inorganic Chemistry. Principles of structure and reactivity , 3rd edition, de Gruyter, Berlin, 2003 .
  4. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 .
  5. ^ RJ Gillespie, TE Peel, EA Robinson: Hammett acidity function for some super acid systems. I. Systems H2SO4-SO3, H2SO4-HSO3F, H2SO4-HSO3Cl, and H2SO4-HB (HSO4) 4 , in: J. Am. Chem. Soc. 1971 , 93 , 5083-5087; doi : 10.1021 / ja00749a021 .
  6. ↑ LP Hammett, AJ Deyrup: A series of simple basic indicators. I. The acidity functions of mixtures of sulfuric acid and perchloric acids with water. , in: J. Am. Chem. Soc. 1932 , 54 , 2721-2739; doi : 10.1021 / ja01346a015 .
  7. ↑ https://www.nature.com/news/2004/041115/full/news041115-5.html
  8. ↑ George A. Olah : My search for carbocations and their role in chemistry , Nobel Lecture in Chemistry, 1994 . ( PDF )
  9. ↑ a b G. A. Olah, GKS Prakash, J. Sommer, Superacids , John Wiley & Sons, New York, 1985 .
  10. ↑ a b T.A. O'Donnell, Superacids and Acidic Melts as Inorganic Reaction Media , VCH, Weinheim, 1993 .
  11. ↑ CG Barraclough, RW Cockman, TA O'Donnell, WSJ Schofield: Electronic spectra of titanium (II), vanadium (II), and chromium (II) in anhydrous hydrogen fluoride , in: Inorg. Chem. 1982 , 21 , 2519-2521; doi : 10.1021 / ic00136a088 .
  12. ^ A b I. C. Hwang, K. Seppelt: The Reduction of AuF3 in Super Acidic Solution , in: Z. anorg. allg. Chem. 2002 , 628 , 765-769; doi : 10.1002 / 1521-3749 (200205) 628: 4 <765 :: AID-ZAAC765> 3.0.CO; 2-E .
  13. ↑ T. Drews, S. Seidel, K. Seppelt: Gold Xenon Complexes , in: Angew. Chem. 2002 , 114 , 470-473; doi : 10.1002 / 1521-3757 (20020201) 114: 3 <470 :: AID-ANGE470> 3.0.CO; 2-U .
  14. ↑ S. Seidel, K. Seppelt: Xenon as a Complex Ligand: The Tetra Xenono Gold (II) Cation in AuXe42 + (Sb2F11-) 2 , in: Science 2000 , 290 , 117-118; doi : 10.1126 / science.290.5489.117 .
  15. ↑ CA Reed, KC Kim, RD Bolskar, LJ Mueller: Taming Superacids: Stabilization of the Fullerene Cations HC60 + and C60 · + , in: Science 2000 , 289 , 101-104; doi : 10.1126 / science.289.5476.101 .
  16. ↑ George Olah, My search for carbocations and their role in chemistry, Nobel Lecture 1994, p. 161, PDF