Superacidic

Superacidic refers to acid systems or species that are stronger than pure sulfuric acid (H2SO4) in proton-donating ability. In practice, strength is often described using the Hammett acidity function, H0, where lower values indicate stronger acidity. By this standard, certain Brønsted acid systems exceed the acidity of H2SO4 and can protonate hydrocarbons and stabilize highly reactive cations that would not form under ordinary conditions.

Classic examples include fluoroantimonic acid, HSbF6, formed by mixing hydrogen fluoride with antimony pentafluoride, which is frequently cited as one of the strongest known superacids. Another well-known family is magic acids, mixtures such as fluorosulfonic acid with SbF5 (FSO3H–SbF5). These systems are typically extremely moisture-sensitive and require strictly anhydrous, controlled environments.

More recently, isolated superacids based on weakly coordinating anions, such as carborane-derived acids, have been developed. These acids, including protonated carborane species, exhibit very low H0 values and can protonate alkanes and other weak bases under controlled conditions.

Applications are primarily in fundamental research and specialized synthesis, where superacids enable reactions like hydrocarbon rearrangements, alkane activation, and the formation of highly reactive carbocations. They are not generally used in standard laboratory chemistry due to corrosivity, handling hazards, and the need for inert, dry conditions.