The ability of a chemical substance to donate a proton, or hydrogen ion, defines its acidity. Common household items like vinegar and lemon juice represent the mild end of the acidic spectrum. However, some chemical substances possess an acidity so extreme they defy ordinary chemistry. These ultra-strong compounds have a proton-donating power far beyond concentrated industrial acids, requiring scientists to develop new methods to measure and describe them.
Quantifying Acid Strength
The most familiar method for measuring acidity is the \(\text{pH}\) scale, which uses a logarithmic system to represent the concentration of hydrogen ions in an aqueous solution. This scale ranges from 0 to 14 and functions well for dilute solutions in water. However, the \(\text{pH}\) scale becomes ineffective for highly concentrated acids or those dissolved in non-aqueous solvents. Once an acid exceeds the strength of 100% sulfuric acid, the leveling effect of water masks the true strength, rendering the \(\text{pH}\) value meaningless.
To accurately measure extreme acidity, chemists rely on the Hammett acidity function, denoted as \(H_0\). This function extends the concept of \(\text{pH}\) to highly concentrated or non-aqueous media, quantifying acids far stronger than those measured on the traditional scale. The \(H_0\) function is determined by observing the protonation of a series of very weak organic bases, called indicators. The value for 100% pure sulfuric acid, a common benchmark, is approximately \(-12\) on the \(H_0\) scale.
The Hammett function allows for negative values, where a more negative \(H_0\) number indicates greater acidity. An \(H_0\) value of \(-15\), for instance, indicates an acid that is ten times stronger than an acid with an \(H_0\) of \(-14\), due to the logarithmic nature of the measurement. This specialized scale provides the scientific foundation necessary to compare and categorize the most potent acids.
Defining Superacids
A superacid is formally defined as any acidic medium that is stronger than 100% pure sulfuric acid (\(\text{H}_2\text{SO}_4\)). This means any substance with an \(H_0\) value more negative than \(-12\) is classified as a superacid. These substances are characterized by their exceptional ability to protonate chemical species traditionally considered extremely poor bases. They can even protonate saturated hydrocarbons, which normally do not react with conventional acids.
The strongest superacids are typically created by combining a strong Brønsted acid and a powerful Lewis acid. The Brønsted acid, such as hydrogen fluoride (\(\text{HF}\)), functions as the primary proton donor. The Lewis acid, such as antimony pentafluoride (\(\text{SbF}_5\)), acts as an ion acceptor rather than donating a proton itself.
The Lewis acid aggressively binds to the conjugate base of the Brønsted acid, forming an extremely stable, non-nucleophilic anion. For instance, \(\text{SbF}_5\) prevents the fluoride ion (\(\text{F}^{-}\)) from accepting a proton back when reacting with \(\text{HF}\). By stabilizing the resulting anion, the proton is left highly active and “free” to attack the weakest bases. This cooperative effect creates an acidic environment dramatically stronger than either component alone.
The World’s Strongest Acids
For decades, the title of the world’s most acidic substance has been held by Fluoroantimonic acid, a mixture of hydrogen fluoride (\(\text{HF}\)) and antimony pentafluoride (\(\text{SbF}_5\)). The strength of this mixture is dependent on the ratio of its components, but the most potent combinations have a Hammett acidity function (\(H_0\)) value that can reach as low as \(-28\). This measurement indicates that Fluoroantimonic acid is approximately \(10^{16}\) to \(10^{19}\) times stronger than 100% sulfuric acid, an almost unimaginable difference in proton-donating power.
Its immense strength results from the formation of the \(\text{SbF}_6^{-}\) or \(\text{Sb}_2\text{F}_{11}^{-}\) anion. Antimony pentafluoride sequesters the fluoride ion from \(\text{HF}\), creating an exceptionally stable and extremely weak conjugate base. Because this base is inert and reluctant to accept a proton back, the proton remains highly active. This allows Fluoroantimonic acid to protonate even methane, which is normally unreactive toward acids.
A newer class of compounds, Carborane acids, challenges Fluoroantimonic acid’s status. These are considered the strongest pure Brønsted acids that can be isolated as a single compound. While their \(H_0\) values are difficult to measure directly, estimates suggest they are comparable to, or may exceed, the strength of mixed superacids.
The advantage of Carborane acids lies in their unique structure, which features a large, stable, three-dimensional \(\text{B}_{11}\text{C}\) cage. The resulting carboranate anion is chemically inert, non-oxidizing, and features extremely low nucleophilicity. This “gentle” nature allows Carborane acids to protonate sensitive molecules without causing decomposition, a major limitation of mixed superacids containing oxidizing Lewis acids like \(\text{SbF}_5\). The stability of the carboranate anion allows chemists to isolate and study highly reactive, unstable cations, such as protonated benzene.
Practical Uses and Safety Considerations
Despite their extreme nature, superacids have important applications in industrial catalysis and chemical research. Their unique ability to protonate alkanes and other weak bases makes them invaluable for transforming hydrocarbons in the petrochemical industry. Superacids catalyze reactions like alkylation and isomerization, necessary steps in producing high-octane gasoline and other valuable products.
In research, superacids generate and stabilize highly reactive, positively charged molecules known as carbocations. These carbocations are short-lived intermediates in many organic reactions, and superacids allow scientists to isolate and study them to understand reaction mechanisms. The non-oxidizing environment provided by Carborane acids has opened new pathways for synthesizing complex organic compounds.
Handling these substances requires extreme caution due to their corrosive power. Fluoroantimonic acid, for instance, will dissolve glass and most common protective equipment, including standard laboratory gloves. It is highly reactive with water, which can cause an explosive release of heat and toxic fumes. Storage of the most potent superacids necessitates specialized containers made of highly resistant fluorinated polymers, such as polytetrafluoroethylene (\(\text{PTFE}\)), commonly known as Teflon.