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For some, the word ‘superacid’ may conjure up images of otherworldly creatures and interstellar explorers from popular sci-fi movies such as ‘Alien’. Having powerful acid for blood was the main line of defense for the creatures in that particular film franchise, but could acids powerful enough to burn through spaceship hulls really exist in nature?
A superacid is just what it sounds like; an extremely powerful acid. Think of what ordinary acids are capable of—causing a nasty burn or an explosive reaction—and now multiply that effect by 100, 1,000 or even a million. At this level, superacids can seem fantastical as they react with and dissolve pretty much anything—from wax and rocks, to metal and even glass.
What are superacids?
As we know, common acids such as acetic acid found in vinegar can be defined by their pH value. The stronger the acid is and the higher its proton concentration, the lower the pH. However, pH values cannot define all acids. As superacids would have pH values that are far below the standard range and tend to have a violent reaction with water, they cannot be measured by pH. To create some form of standardization for these solutions, chemists created a new measurement, the Hammett acidity function (H0), to define superacids.
In an aqueous solution the predominant acid species is H30+, as measured by the pH scale. However, beyond the pH range, the effective hydrogen ion activity changes much more rapidly due to variability in the nature of the acid. In pure sulphuric acid, the predominant acid species is HSO+ instead of H30+, affecting the proton being measured and making the acid much stronger. The Hammett acidity function allows for this change of species, calculating the predominant acid species as a function of H30+. Pure sulfuric acid has a Hammett value of H = -12, which means that the acid species present (HSO+) has a protonating strength equivalent to H30+ at an ideal concentration of 1012 mol/L.
The Hammett acidity function uses sulphuric acid as its baseline, defining a superacid as a medium with acidity greater than 100% pure sulphuric acid, or where the chemical potential of protons is higher than that of pure sulphuric acid. Given that sulphuric acid is exceptionally corrosive, you can imagine that anything stronger would be extremely powerful. For example, the superacids ‘triflic acid’ and ‘fluorosulphuric acid’ are both about one thousand times stronger than sulphuric acid!
Fluoroantimonic acid—the strongest acid of them all?
Interestingly, most superacids are actually a combination of other acids. Let’s take a look at the strongest known superacid; fluoroantimonic acid, with a Hammett acidity function of H0 = -28.
Fluoroantimonic acid is made by combining hydrogen fluoride (HF) with antimony pentafluoride (SbF5), resulting in an acid that is 1016 times stronger than sulphuric acid. The hydrogen ion in HF is attached to fluorine by a very weak dipolar bond, which accounts for the extreme acidity of the superacid. As shown in the formula below, the free proton easily dissociates in the mixture and results in strong reactivity with other substances as it jumps between anions. This acid is so strong that it has to be stored in specially produced fluorine polymer coated containers (avid followers of the Alien movies believe that the acidic alien blood is a fluorine-based acid).
HF + SbF5 → H+ + SbF6-
Another powerful acid, dubbed ‘magic acid’, is a mix of antimony pentafluoride and fluorosulphonic acid. This acid is so potent that it even reacts with inert hydrocarbons found in wax candles. As wax is such a stable compound, it is an impressive accomplishment to initiate a reaction with it, let alone completely dissolve it. It is said that the researchers who discovered this phenomenon thought it was a magic trick, as it was thought impossible for any acid to dissolve a candle.
The acidic strength of many superacids comes from this ability to make protons available for reaction. In water-based acids, protons are hydrated, and so are stabilized by surrounding water molecules. However, in superacids, these protons are exposed, with nothing around to stabilize them, resulting in extreme reactivity and proton mobility.
What applications can superacids be applied to?
It is this highly reactive property of the protons in a superacid that provides such a strong utility. Given that superacids can form their own solvent or be used in organic solvents, they can be applied to many different reactions compared to water-based solvents. For example, acids do not usually react with hydrocarbons such as wax or petroleum oil, but superacids can. Superacids break hydrocarbons into positively charged hydrocarbon cations, which briefly exist as intermediates before being modified by continuing chemical reactions.
In addition, this strong proton donation characteristic of superacids provides a good basis for the study of carbocation intermediates. These molecules are extremely reactive and unstable, so studying them in any meaningful way is difficult. By mixing them with superacids, they become somewhat stable and can be examined more closely to help explain how one hydrocarbon can be transformed into a more useful one.
Ongoing hydrocarbon and carbocation research is supported by the chemicals industry, in which superacids are a common component. The petrochemical industry uses superacids as catalysts for primary thermal cracking to produce high-octane gas fractions, while the polymer industry uses superacids in the production of high-density polymers and as catalysts for an array of reactions including esterification, isomerization, alkylation, and polymerization. This wide range of applications remains active areas of research as we learn more about the value of superacids as powerful catalysts in a variety of chemical reactions.
What it all means…
Even though superacids are extremely dangerous and have the potential to be a toxic environmental hazard, these solutions test the limits of organic chemistry, allowing the advancement of carbonium ion research and contributing to chemical engineering.