Carborane acids H(CXB11Y5Z6) (X, Y, Z = H, Alk, F, Cl, Br, CF3) are a class of superacids,[1] some of which are estimated to be at least one million times stronger than 100% sulfuric acid in terms of their Hammett acidity function values (H0 ≤ –18) and possess computed pKa values well below –20, establishing them as some of the strongest known Brønsted acids.[2][3][4] The most well studied example is the highly chlorinated derivative H(CHB11Cl11). The acidity of H(CHB11Cl11) was found to vastly exceed that of triflic acid, CF3SO3H, and bistriflimide, (CF3SO2)2NH, compounds previously regarded as the strongest isolable acids.

Their high acidities stem from the extensive delocalization of their conjugate bases, carboranate anions (CXB11Y5Z6–), which are usually further stabilized by electronegative groups like Cl, F, and CF3. Due to the lack of oxidizing properties and the exceptionally low nucleophilicity and high stability of their conjugate bases, they are the only superacids known to protonate C60 fullerene without decomposing it.[5][6] Additionally, they form stable, isolable salts with protonated benzene, C6H7+, the parent compound of the Wheland intermediates encountered in electrophilic aromatic substitution reactions.

The fluorinated carborane acid, H(CHB11F11), is even stronger than chlorinated carborane acid. It is able to protonate butane to form tert-butyl cation at room temperature and is the only known acid to protonate carbon dioxide to give the bridged cation, [H(CO2)2]+, making it possibly the strongest known acid. In particular, CO2 does not undergo observable protonation when treated with the mixed superacids HF-SbF5 or HSO3F-SbF5.

As a class, the carborane acids form the most acidic group of well-defined, isolable substances known, far more acidic than previously known single-component strong acids like triflic acid or perchloric acid. In certain cases, like the nearly perhalogenated derivatives mentioned above, their acidities rival (and possibly exceed) those of the traditional mixed Lewis-Brønsted superacids like magic acid and fluoroantimonic acid. (However, a head-to-head comparison has not been possible thus far, due to the lack of a measure of acidity that is suitable for both classes of acids: pKa values are ill-defined for the chemically complex mixed acids while H0 values cannot be measured for the very high melting carborane acids).