The root cause of the super acidity of fluoroantimonic acid lies in its unique molecular structure, which is formed by the combination of hydrofluoric acid and antimony pentafluoride in a molar ratio of 1:1. This combination produces a synergistic effect. Its Hammett acidity function is as high as -28, while the value of pure sulfuric acid is only -12. This means that the acidity of fluoroantimonic acid is 10¹⁶ times that of sulfuric acid. This number is large enough to concentrate the acidity of a swimming pool of sulfuric acid into a drop of water. Antimony pentafluoride, as a strong Lewis acid, can effectively capture fluoride ions, thereby releasing the proton activity in the system to the extreme and bringing its protonation capacity to an almost absolute level.
The quantitative manifestation of this extremely strong acidity is its ability to protonate substances that were traditionally considered completely inert. For instance, at a temperature of -40°C, fluoroantimonic acid can protonate alkanes such as isobutane and n-pentane to form tert-butyl carbocations, and its reaction rate is 10⁵ times faster than that of traditional acid catalysts. According to the research of Professor George Ola, the Nobel Prize winner in Chemistry in 1994, even methane, the most stable alkane, can be activated in a superacidic environment, opening up a brand-new path for the chemical transformation of hydrocarbons, with the reaction efficiency increased by more than 99%.
The reaction of fluoroantimonic acid with substances is usually explosive and highly exothermic. When a drop of acid comes into contact with cellulose such as wood, it releases more than 200 kilojoules of energy per kilogram within milliseconds, almost instantly completing the dehydration process. It is extremely dangerous and must be stored in containers made of inert materials such as Teflon, as it reacts immediately upon contact with glass, with an erosion rate as high as 1 millimeter per second. Laboratory operations need to be carried out in a low-temperature environment below -20°C to keep the reaction activity within the safety threshold.
Despite its extremely destructive nature, fluoroantimonic acid has crucial applications in industrial catalysis, especially in the alkylation process of petroleum reforming. It can increase the conversion efficiency of hydrocarbon compounds to nearly 95%, which is much higher than the approximately 70% level of traditional aluminum chloride catalysts. However, the cost of processing a high-purity fluoroantimonic acid can exceed $5,000, which includes a complete set of heavy metal protection, environmental control and waste neutralization systems. The cost of waste liquid treatment accounts for more than 40% of the total operating cost. This extreme acidity characteristic makes it an irreplaceable tool for exploring carbocation chemistry and synthesizing new materials.