The term superacids was introduced into the chemical literature in 1927 by James Conant and Norris Hall to describe solutions of sulfuric or perchloric acid in glacial acetic acid. They found that these nonaqueous-superacid solutions reacted with weak bases which did not react with either sulfuric or perchloric acid in water. Conant and Hall's initial report received little attention; however, interest in superacids increased dramatically in the 1960s. Since then, the reactivity of superacids with very weak bases has been studied extensively, particularly with regard to the protonation of saturated hydrocarbons in acid-catalyzed organic reactions. In 1972, Ronald Gillespie defined superacids as acids which are stronger than 100% sulfuric acid. Gillespie's definition has become the accepted definition for Brönsted superacids. Lewis superacids are generally considered to be those which are stronger than anhydrous aluminum trichloride (AlCl3). In 1987, George Olah received the Nobel Prize in chemistry for his pioneering research in the activation of carbon-carbon and carbon-hydrogen bonds by superacids.
There are several classes of superacids: Brönsted, Lewis, conjugate Brönsted, conjugate Brönsted-Lewis, and solid superacids. The Brönsted superacids include perchloric acid (HClO4), halosulfuric acids (e.g. fluorosulfuric acid (FSO3H)), and perfluoroalkane sulfonic acids (e.g. trifluoromethanesulfonic acid (CF3SO3H)). Brönsted superacids behave no differently than typical Brönsted acids, except that they are stronger than the arbitrarily-chosen standard of sulfuric acid, and are thereby categorized as superacids. The Lewis superacids include antimony pentafluoride (SbF5), arsenic pentafluoride (AsF5), tantalum pentafluoride (TaF5), and niobium pentafluoride (NbF5). Like the Brönsted superacids, Lewis superacids are simply very strong Lewis acids.
Conjugate superacids involve the mixture of two strong acids to create an exceptionally acidic medium. Conant and Hall's sulfuric acid:acetic acid mixture provides an example of how conjugate Brönsted superacids function. Sulfuric acid is a strong acid. In aqueous solution, the acid-base reaction between sulfuric acid and water goes to completion:
H2SO4 + H2O HSO4- + H3O+
In glacial acetic acid, on the other hand, no water is present, so the strongest base available is acetic acid. Therefore, the acid-base equilibrium between sulfuric acid and acetic acid lies far to the right:
H2SO4 + CH3CO2H [lrhar2] HSO4- + CH3C(OH)2+
CH3C(OH)2+ is a much stronger acid than H3O+, so some weak bases which do not react with H3O+ will react with CH3C(OH)2+. Thus, the key to conjugate Brönsted superacidity is that no water is present to react as a base, so it is possible to generate stronger acids than just H3O+.
Conjugate Brönsted-Lewis superacids behave in a similar manner. The most common conjugate Brönsted-Lewis superacid is a mixture of fluorosulfuric acid and antimony pentafluoride, known as "Magic Acid." Antimony pentafluoride, a strong Lewis acid, stabilizes fluorosulfate, the deprotonated form of fluorosulfuric acid. Therefore, the presence of antimony pentafluoride shifts the fluorosulfuric acid autoprotolysis equilibrium to the right:
2HSO3F + SbF5 [lrhar2] H2SO3F+ + SbF5 (SO3F)-
H2SO3F+ is an extremely strong acid, and will readily protonate even exceedingly weak bases which are added to the Magic Acid. Thus, conjugate Brönsted-Lewis superacids create very strong Brönsted acids in nonaqueous media by promoting the autoprotolysis of the Brönsted acid through stabilization of the dissociated form of the acid.
Solid superacids are composed of solid media treated with either Brönsted or Lewis acids. The solids used include natural clays and minerals, metal oxides and sulfides, metal salts, and mixed metal oxides. Typical solid Brönsted superacids include titanium dioxide:sulfuric acid (TiO2:H2SO4) and zirconium dioxide:sulfuric acid (ZrO2:H2SO4) mixtures. The most common solid Lewis superacids involve the incorporation of antimony pentafluoride into metal oxides, such as silicon dioxide (SbF5:SiO2), aluminum oxide (SbF5:Al2O3), or titanium dioxide (SbF5:TiO2). Solid superacids are typically not as strong as liquid phase-conjugate superacids. However, they are advantageous for organic catalysis, due to their ease of separation from liquid phase reaction mixtures. Therefore, a great deal of research has been devoted to the discovery and characterization of solid superacids since the early 1970s.
To date, the most significant application of superacids has been as catalysts for organic reactions such as carbon-carbon and carbon-hydrogen bond protolysis, alkane isomerization, and alkane alkylation. These reactions, some of which are run on a large scale in the petroleum industry, are initiated by the protonation of saturated hydrocarbons. Saturated hydrocarbons are very weak bases, so high temperatures are required to initiate these reactions with conventional acid or noble metal catalysts. Due to their extreme acidity, however, superacids are capable of protonating saturated hydrocarbons at much lower temperatures. Thus, the discovery of superacids has opened up a new realm of organic chemistry and may also significantly impact the petroleum industry.
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