Superacid
In chemistry, superacids are acids that are stronger than concentrated (100 percent) sulfuric acid (H2SO4: pKs value = -3.0).
All superacids thus have a pKs value in the negative range. The Hammett acidity function is 
used to quantify the acidity.
Fluorosulfonic acid (HSO3F), for example, is several thousand times stronger than concentrated sulfuric acid. If it is combined with antimony pentafluoride, you get the even stronger magic acid. This mixture even reacts with alkanes. Another common superacid is fluorine antimony acid (HSbF6: pKS = -17), which consists of antimony pentafluoride and anhydrous hydrogen fluoride. When these two acids are combined, their acidity is increased by a factor of 103. Organic acids can also reach pKs values < -11 through certain groups. An example of this is pentacyanocyclopentadiene. Since 2004, per-halogenated carboranes H(CHB11Cl11) are also known to be superacids.
In an aqueous environment, it cannot be demonstrated that a superacid is stronger than concentrated sulfuric acid, since a degree of protolysis greater than 1 is only possible in the absence of bases and thus cannot be achieved in water. This effect is called the levelling effect of 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 generation of metal-xenon compounds, non-classical metal carbonyls and protonated fullerenes was also successful with the help of superacids. The decisive factor here is the low nucleophilicity and the large steric demand of the acid residues.
Due to their extremely high acidity, superacids are sometimes involved in reactions that were long considered impossible in chemistry: Among other things, they are able to protonate the extremely inert noble gases and to generate carbon atoms with formally five bonds (carbonium ions).
The name goes back to the chemist James Bryant Conant at Harvard (1927). They were used especially by George A. Olah for the preparation and stabilization of carbocations.
See also
- Superbases
Questions and Answers
Q: What is a superacid?
A: A superacid is an acid that has a higher acidity than 100% pure sulfuric acid.
Q: What is the modern definition of a superacid?
A: According to the modern definition, a superacid has a chemical potential of the proton higher than that of pure sulfuric acid.
Q: What do chemists use strong acids for?
A: Chemists use strong acids to break down other molecules.
Q: Why are some molecules resistant to common acids?
A: Some molecules have strong chemical bonds that resist attack by common acids.
Q: What can superacids do?
A: Superacids can break down molecules that resist most other acids.
Q: What are some commercially available superacids?
A: Triflic acid and fluorosulfonic acid are commercially available superacids.
Q: What is the strongest known superacid?
A: Fluoroantimonic acid is the strongest known superacid, which is prepared by the combination of a strong Lewis acid and a strong Brønsted acid.
