Superacid

A superacid is an acid with an acidity greater than that of 100% sulfuric acid, which has a Hammett acidity function of -12. Commercially available superacids include trifluoromethanesulfonic acid (CF₃SO₃H), also known as triflic acid, and fluorosulfuric acid (FSO₃H), both of which are about a thousand times stronger (i.e. have more negative H₀ values) than sulfuric acid. The strongest superacids are prepared by the combination of two components, a strong Lewis acid and a strong Brønsted acid.

Additional recommended knowledge

Essential Laboratory Skills Guide

Daily Visual Balance Check

Safe Weighing Range Ensures Accurate Results

The term superacid was originally coined by James Bryant Conant in 1927 to describe acids that were stronger than conventional mineral acids.^[1] George A. Olah was awarded the 1994 Nobel prize in chemistry for his investigations of superacids and their use in the direct observation of carbocations. Olah's "magic acid", so-named for its ability to attack hydrocarbons, is prepared by mixing antimony pentafluoride (SbF₅) and fluorosulfuric acid. The name was coined after one of Professor Olah's post-doctoral associates placed a candle in a sample of magic acid. The candle was dissolved, showing the ability of the acid to protonate hydrocarbons (which are not basic).

The strongest super acid system, the so-called fluoroantimonic acid, is a combination of hydrogen fluoride and SbF₅. In this system, HF releases its proton (H⁺) concomitant with the binding of F⁻ by the antimony pentafluoride. The resulting anion (SbF₆⁻) is both a weak nucleophile and a weak base. The proton effectively becomes "naked", which accounts for the system's extreme acidity. Fluoroantimonic acid is 2×10¹⁹ times stronger than 100% sulfuric acid,^[2] and can produce solutions with a pH down to –25.^[3]

Olah showed that at 140 °C (284 °F), FSO₃H-SbF₅ will convert methane into the tertiary-butyl carbocation, a reaction that begins with the protonation of methane:^[4]

CH₄ + H⁺ → CH₅⁺

CH₅⁺ → CH₃⁺ + H₂

CH₃⁺ + 3 CH₄ → (CH₃)₃C⁺ + 3/2 H₂

Applications

Common uses of superacids include providing an environment to create and maintain organic cations which are useful as intermediate molecules in numerous reactions, such as involving plastics and high-octane gasoline production and study.^[5]

See also

• Superbase

References