superacids

Superacids are acids whose acidity exceeds that of pure sulfuric acid in the same solvent. In nonaqueous media, acidity is quantified by the Hammett acidity function H0; many superacids have H0 values well below −12, with some estimates for the strongest systems reaching into the −20s. The term was popularized by George Olah and colleagues to describe systems capable of protonating hydrocarbons and stabilizing carbocations that ordinary acids cannot.

Typical examples include fluoroantimonic acid, formed by mixing hydrogen fluoride (HF) with antimony pentafluoride (SbF5), which yields the very strong H[SbF6] species, and magic acid, a mixture of fluorosulfuric acid (HSO3F) with SbF5. These systems create extremely strong protonating environments and can convert hydrocarbons into highly reactive cations.

Carborane-based superacids, based on very weakly coordinating carborane anions, provide extraordinarily strong acidity while remaining relatively non-nucleophilic. They enable reactions that require extreme protonation and can stabilize unusual reaction pathways in organic substrates.

Handling and safety: Superacids are highly corrosive and reactive, and are used only under strictly controlled conditions. They are commonly operated in nonaqueous media such as liquid hydrogen fluoride or sulfur dioxide and require corrosion-resistant equipment and appropriate safety measures.

Applications and limitations: They are employed to protonate hydrocarbons, generate carbocation intermediates, catalyze alkylations and rearrangements, and unlock reactions inaccessible to conventional acids. Their extreme reactivity also limits substrate compatibility and complicates workup and disposal.