Superacid
Encyclopedia
According to the classical definition superacid is an acid
with an acidity greater than that of 100% pure sulfuric acid
, which has a Hammett acidity function
(H0) of −12. According to the modern definition, superacid is a medium, in which the chemical potential
of the proton
is higher than in pure sulfuric acid
. Commercially available superacids include trifluoromethanesulfonic acid
(CF3SO3H), also known as triflic acid, and 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
and a strong Brønsted acid. The strongest known superacid is fluoroantimonic acid
.
Unstable gas-phase heteroelement-hydride ions can be extremely acidic, e.g. the helium hydride ion. However, they are not considered as such because they exist only in extremely hot plasma
not in the liquid state.
in 1927 to describe acids that were stronger than conventional mineral acid
s. George A. Olah prepared the so-called magic acid
, so-named for its ability to attack hydrocarbons, by mixing antimony pentafluoride
(SbF5) and fluorosulfonic acid (FSO3H). The name was coined after a candle was placed in a sample of magic acid. The candle dissolved, showing the ability of the acid to protonate hydrocarbons, which under normal acidic conditions do not protonate to any extent.
It was shown 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:
, the strongest acid in the system, is 2×1016 times stronger than 100% sulfuric acid, and can produce solutions with a H0 down to –28. Fluoroantimonic acid is a combination of hydrofluoric acid
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
from superacids has been popularly said to become "naked," accounting for the system's extreme acidity. In the physical sense, the acidic "proton" is never completely free (as an unbound proton) in the acid, but rather hops from anion to anion in superacids via the Grotthuss mechanism
, just as happens to acidic "protons" in water. The extreme acidity of the acids is due to the ease with which this proton is transferred to substances that cannot normally be "protonated" (such as hydrocarbons), due to the very high stability of the conjugate-base anion (such as SbF6−) after it donates the proton.
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
with an acidity greater than that of 100% pure sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, 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. It was proposed by the physical organic chemist Louis Plack Hammett and is the best-known acidity function used to extend the measure of acidity beyond the...
(H0) of −12. According to the modern definition, superacid is a medium, in which the chemical potential
Chemical potential
Chemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...
of the proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
is higher than in pure sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
. Commercially available superacids include trifluoromethanesulfonic acid
Trifluoromethanesulfonic acid
Trifluoromethanesulfonic acid, also known as triflic acid, TFMS, TFSA, HOTf or TfOH, is a sulfonic acid with the chemical formula CF3SO3H. It is one of the strongest acids. Triflic acid is mainly used in research as a catalyst for esterification.-Properties:Triflic acid is a hygroscopic, colorless...
(CF3SO3H), also known as triflic acid, and 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
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
and a strong Brønsted acid. The strongest known superacid is fluoroantimonic acid
Fluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination forms the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2....
.
Unstable gas-phase heteroelement-hydride ions can be extremely acidic, e.g. the helium hydride ion. However, they are not considered as such because they exist only in extremely hot plasma
Plasma
Plasma may refer to:* Blood plasma, the yellow-colored liquid component of blood, in which blood cells are suspended* Plasma , an ionized state of matter similar to a gas...
not in the liquid state.
History
The term superacid was originally coined by James Bryant ConantJames Bryant Conant
James Bryant Conant was a chemist, educational administrator, and government official. As thePresident of Harvard University he reformed it as a research institution.-Biography :...
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 compounds. A mineral acid is not organic and all mineral acids release hydrogen ions when dissolved in water.-Characteristics:...
s. George A. Olah prepared the so-called magic acid
Magic acid
Magic acid , is a superacid consisting of a mixture, most commonly in a 1:1 molar ratio, of fluorosulfonic acid and antimony pentafluoride...
, so-named for its ability to attack hydrocarbons, by mixing antimony pentafluoride
Antimony pentafluoride
Antimony pentafluoride is the inorganic compound with the formula SbF5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, the strongest known acid...
(SbF5) and fluorosulfonic acid (FSO3H). The name was coined after a candle was placed in a sample of magic acid. The candle dissolved, showing the ability of the acid to protonate hydrocarbons, which under normal acidic conditions do not protonate to any extent.
It was shown that at 140 °C (284 °F), FSO3H–SbF5 will convert methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
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 valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...
, a reaction that begins with the protonation of methane:
- CH4 + H+ → CH5+
- CH5+ → CH3+ + H2
- CH3+ + 3 CH4 → (CH3)3C+ + 3H2
Superlatives and metaphors
Fluoroantimonic acidFluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination forms the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2....
, the strongest acid in the system, is 2×1016 times stronger than 100% sulfuric acid, and can produce solutions with a H0 down to –28. Fluoroantimonic acid is a combination of hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
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
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
and a weak base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
.
The proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
from superacids has been popularly said to become "naked," accounting for the system's extreme acidity. In the physical sense, the acidic "proton" is never completely free (as an unbound proton) in the acid, but rather hops from anion to anion in superacids via the Grotthuss mechanism
Grotthuss mechanism
The Grotthuss mechanism is the mechanism by which an 'excess' proton or protonic defect diffuses through the hydrogen bond network of water molecules or other hydrogen-bonded liquids through the formation or cleavage of covalent bonds....
, just as happens to acidic "protons" in water. The extreme acidity of the acids is due to the ease with which this proton is transferred to substances that cannot normally be "protonated" (such as hydrocarbons), due to the very high stability of the conjugate-base anion (such as SbF6−) after it donates the proton.