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Protonation of alkanes with superacids

Superacids are acids stronger that 100% sulphuric acid. A superacid for protonation of alkanes can be made from hydrofluoric acid, HF, a strong Brønsted acid.

 

    
 

 

However, its acidity may be dramatically increased by adding the Lewis acid antimonpentafluoride (SbF5) which binds F- strongly and displaces the equilibrium strongly far to the right.

 

 
    
 

 

Superacids can be more that 1018 times stronger that 100% sulphuric acid. The solvents used for such acids are compounds with extremely low basicity e.g. SO2, SO2C1F and SO2F2.
    Olah and the Dutch chemist H. Hogeveen showed independently at the end of the 1960s that superacids such as HF:SbF5 may cause protons to bind to alkanes. Methane, for example, gives CH5+. This discovery opened up a whole new chemistry for building up, breaking down and isomerizing molecules.

 

 
    
 

 

Protonation of methane followed by elimination of H2.

 

 
    
 

 

The methyl cation so obtained then attacks a new molecule of methane and protonated ethane is formed.

 

 
    
 

 

Elimination of H+ yields ethane. Similarly, higher hydrocarbons may be built.

 

 



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