superacid

A superacid is an acid whose acidity exceeds that of pure sulfuric acid (100% H2SO4). In nonaqueous media, superacidity is often described using acid-strength scales or by the ability of the medium to stabilize the conjugate base; superacids enable protonation and stabilization of species that ordinary acids cannot handle. The concept is relative and depends on the solvent and the paired Lewis acidity.

Common examples include fluorosulfuric acid (HSO3F), which itself is extremely strong in many solvents; fluoroantimonic acid (a mixture or associated species of HF and SbF5, often cited as one of the strongest known acids); and magic acid, a mixture of fluorosulfuric acid and antimony pentafluoride (SbF5) that creates a highly acidic medium capable of generating carbocations in hydrocarbons.

Carborane-based acids, such as H(CHB11Cl11), and related salts with very weakly coordinating anions, are among the strongest stable superacids. These acids provide extremely high proton-donating power while minimizing anion coordination, allowing unusual reactivity.

Applications and limitations: Superacids are used to generate and stabilize highly reactive intermediates, enable challenging alkylations or isomerizations, and facilitate reactions in hydrocarbons that are difficult with conventional acids. They require specialized handling due to extreme corrosivity and potential release of hazardous reagents; reactions are typically conducted in carefully designed apparatus and inert or controlled environments.

In summary, superacids are acids stronger than sulfuric acid, enabling unique chemistry through stabilization of destabilized conjugate bases and highly reactive intermediates.