Superacid

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Superacid

A superacid is an acid

Acid

An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
with an acidity greater than that of 100% sulfuric acid

Sulfuric acid

Sulfuric acid, hydrogen2sulfuroxygen4, is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry....
, which has a Hammett acidity function

Hammett acidity function

The Hammett acidity function is a measure of acidity that is used for very concentrated solutions of strong acids, including superacids. In such solutions, simple approximations such as the Henderson-Hasselbalch equation are no longer valid due to the variations of the activity coefficients in highly concentrated solutions....
(H₀) of -12. Commercially available superacids include trifluoromethanesulfonic acid

Trifluoromethanesulfonic acid

Trifluoromethanesulfonic acid, also known as triflic acid or TfOH, is a sulfonic acid with the chemical formula CF3SO3H ....
(CF₃SO₃H), also known as triflic acid, and fluorosulfuric acid

Fluorosulfuric acid

Fluorosulfuric acid is FSO3H; it is one of the strongest acids commercially available. It is also known by the alternative name, fluorosulfonic 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

Lewis acid

A Lewis acid is a chemical compound, A, that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB.Gilbert N....
and a strong Brønsted acid.

The term superacid was originally coined by James Bryant Conant

James Bryant Conant

James Bryant Conant was a chemist, educational administrator, and government official. He was born in Dorchester, Massachusetts in 1893 and graduated from the Roxbury Latin School in West Roxbury, Massachusetts in 1910....
in 1927 to describe acids that were stronger than conventional mineral acid

Mineral acid

A mineral acid is an acid derived from one or more inorganic chemistry compounds. A mineral acid does not contain any carbon atoms and all mineral acids release hydrogen ions when dissolved in water....
s.

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Encyclopedia

A superacid is an acid

Acid

Sulfuric acid

Hammett acidity function

Trifluoromethanesulfonic acid

Fluorosulfuric acid

Lewis acid

James Bryant Conant

Mineral acid

Nobel Prize in Chemistry

The Nobel Prize in Chemistry is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in chemistry, Nobel Prize in Physics, Nobel Prize in Literature, Nobel Peace Pri...
for his investigations of superacids and their use in the direct observation of carbocation

Carbocation

A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer Electron shell#Valence shell instead of the eight valence electrons that ensures maximum stability ....
s. Olah's magic acid, so-named for its ability to attack hydrocarbons, is prepared by mixing antimony pentafluoride

Antimony pentafluoride

Antimony pentafluoride is the chemical compound with the formula AntimonyFluorine5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, the strongest known acid....
(SbF₅) and fluorosulfuric acid

Fluorosulfuric acid

Fluorosulfuric acid is FSO3H; it is one of the strongest acids commercially available. It is also known by the alternative name, fluorosulfonic 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

Fluoroantimonic acid

Fluoroantimonic acid HSbF6 is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination affords the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2....
, is a combination of hydrogen fluoride

Hydrogen fluoride

Hydrogen fluoride is a chemical compound with the chemical formula HF. It is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers ....
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

Nucleophile

In chemistry, a nucleophile is a reagent that forms a chemical bond to its reaction partner by donating both bonding electrons. Because nucleophiles donate electrons, they are by definition Lewis bases ....
and a weak base

Base (chemistry)

In chemistry, a base is most commonly thought of as an aqueous substance that can accept protons. A base is also often referred to as an alkali if OH- ions are involved....
. The proton

Proton

The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H+....
effectively becomes "naked", which accounts for the system's extreme acidity. Fluoroantimonic acid is 2×10¹⁹ times stronger than 100% sulfuric acid, and can produce solutions with a H₀ down to –25.

Olah showed that at 140 °C (284 °F), FSO₃H–SbF₅ will convert methane

Methane

Methane is a chemical compound with the molecular formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees....
into the tertiary-butyl carbocation

Carbocation

A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer Electron shell#Valence shell instead of the eight valence electrons that ensures maximum stability ....
, a reaction that begins with the protonation of methane:

CH₄ + H⁺ → CH₅⁺

CH₅⁺ → CH₃⁺ + H₂

CH₃⁺ + 3 CH₄ → (CH₃)₃C⁺ + 3H₂

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.

Superacid

Applications

See also