Magic acid (FSO
3H·SbF
5) is a
superacid
In chemistry, a superacid (according to the classical definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superacid ...
consisting of a mixture, most commonly in a 1:1 molar ratio, of
fluorosulfuric acid
Fluorosulfuric acid (IUPAC name: sulfurofluoridic acid) is the inorganic compound with the chemical formula HSO3F. It is one of the strongest acids commercially available. It is a tetrahedral molecule and is closely related to sulfuric acid, H2SO4 ...
(HSO
3F) and
antimony pentafluoride
Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, formed when mixing liquid HF with liquid SbF5 in a 2:1 ratio. ...
(SbF
5). This conjugate
Brønsted–
Lewis
Lewis may refer to:
Names
* Lewis (given name), including a list of people with the given name
* Lewis (surname), including a list of people with the surname
Music
* Lewis (musician), Canadian singer
* "Lewis (Mistreated)", a song by Radiohead ...
superacid system was developed in the 1960s by the
George Olah
George Andrew Olah (born Oláh András György; May 22, 1927 – March 8, 2017) was a Hungarian-American chemist. His research involved the generation and reactivity of carbocations via superacids. For this research, Olah was awarded a Nobel Pr ...
lab at
Case Western Reserve University
Case Western Reserve University (CWRU) is a private research university in Cleveland, Ohio. Case Western Reserve was established in 1967, when Western Reserve University, founded in 1826 and named for its location in the Connecticut Western Reser ...
, and has been used to stabilize
carbocations
A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encount ...
and
hypercoordinated carbonium ion
In chemistry, a carbonium ion is any cation that has a pentavalent carbon atom. The name carbonium may also be used for the simplest member of the class, properly called methanium (), where the five valences are filled with hydrogen atoms.
The ...
s in liquid media. Magic acid and other superacids are also used to catalyze isomerization of
saturated hydrocarbon
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...
s, and have been shown to
protonate
In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid, ...
even weak bases, including
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
,
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
,
halogens
The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...
, and
molecular hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, ...
.
History
The term "superacid" was first used in 1927 when
James Bryant Conant found that
perchloric acid
Perchloric acid is a mineral acid with the formula H Cl O4. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxidizer when hot, but aqueous sol ...
could protonate
ketone
In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...
s and
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s to form salts in nonaqueous solution.
The term itself was coined by
R. J. Gillespie later, after Conant combined sulfuric acid with
fluorosulfuric acid
Fluorosulfuric acid (IUPAC name: sulfurofluoridic acid) is the inorganic compound with the chemical formula HSO3F. It is one of the strongest acids commercially available. It is a tetrahedral molecule and is closely related to sulfuric acid, H2SO4 ...
, and found the solution to be several million times more acidic than sulfuric acid alone. The magic acid system was developed in the 1960s by George Olah, and was to be used to study stable carbocations. Gillespie also used the acid system to generate electron-deficient inorganic cations. The name originated after a Christmas party in 1966, when a member of the Olah lab placed a
paraffin Paraffin may refer to:
Substances
* Paraffin wax, a white or colorless soft solid that is used as a lubricant and for other applications
* Liquid paraffin (drug), a very highly refined mineral oil used in cosmetics and for medical purposes
* Alkan ...
candle into the acid, and found that it dissolved quite rapidly. Examination of the solution with
1H-NMR showed a ''tert''-butyl cation, suggesting that the paraffin chain that forms the wax had been cleaved, then isomerized into the relatively stable tertiary carbocation. The name appeared in a paper published by the Olah lab.
Properties
Structure
Although a 1:1 molar ratio of HSO
3F and SbF
5 best generates carbonium ions, the effects of the system at other molar ratios have also been documented. When the ratio SbF
5:HSO
3F is less than 0.2, the following two equilibria, determined by
19F NMR spectroscopy, are the most prominent in solution:
:
![Magic acid structure](https://upload.wikimedia.org/wikipedia/commons/d/db/Magic_acid_structure.png)
(In both of these structures, the sulfur has tetrahedral coordination, not planar. The double bonds between sulfur and oxygen are more properly represented as single bonds, with
formal negative charges on the oxygen atoms and a formal plus two charge on the sulfur. The antimony atoms will also have a formal charge of minus one.)
In the above figure, Equilibrium I accounts for 80% of the NMR data, while Equilibrium II accounts for about 20%. As the ratio of the two compounds increases from 0.4–1.4, new NMR signals appear and increase in intensity with increasing concentrations of SbF
5. The resolution of the signals decreases as well, because of the increasing viscosity of the liquid system.
Strength
All proton-producing acids stronger than 100% sulfuric acid are considered superacids, and are characterized by low values of the
Hammett acidity function
The Hammett acidity function (''H''0) 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 fu ...
. For instance, sulfuric acid, H
2SO
4, has a Hammett acidity function, ''H''
0, of −12,
perchloric acid
Perchloric acid is a mineral acid with the formula H Cl O4. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxidizer when hot, but aqueous sol ...
, HClO
4, has a Hammett acidity function, of −13, and that of the 1:1 magic acid system, HSO
3F·SbF
5, is −23.
Fluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride, containing various cations and anions (the simplest being and ). This substance is a superacid that can be over a billion times stronger than 100% pure sulfuri ...
, the strongest known superacid, is believed to reach extrapolated ''H''
0 values down to −28.
Uses
Observations of stable carbocations
Magic acid has low nucleophilicity, allowing for increased stability of carbocations in solution. The "classical" trivalent carbocation can be observed in the acid medium, and has been found to be planar and sp
2-hybridized. Because the carbon atom is surrounded by only six valence electrons, it is highly electron deficient and electrophilic. It is easily described by
Lewis dot structures
Lewis structures, also known as Lewis dot formulas, Lewis dot structures, electron dot structures, or Lewis electron dot structures (LEDS), are diagrams that show the chemical bonding, bonding between atoms of a molecule, as well as the lone pairs ...
because it contains only two-electron, single bonds to adjacent carbon atoms. Many tertiary cycloalkyl cations can also be formed in superacidic solutions. One such example is the 1-methyl-1-cyclopentyl cation, which is formed from both the cyclopentane and cyclohexane precursor. In the case of the
cyclohexane
Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colorless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...
, the cyclopentyl cation is formed from isomerization of the secondary carbocation to the tertiary, more stable carbocation. Cyclopropylcarbenium ions, alkenyl cations, and arenium cations have also been observed.
:
As use of the Magic acid system became more widespread, however, higher-coordinate carbocations were observed. Penta-coordinate carbocations, also described as
nonclassical ion
Nonclassical carbocations are stabilized by charge delocalization from contributions of neighbouring or bonds, which can form bridged intermediates or transition states. Nonclassical ions have been extensively studied with the 2-norbornyl sy ...
s, cannot be depicted using only two-electron, two-center bonds, and require, instead, two-electron, three (or more) center bonding. In these ions, two electrons are delocalized over more than two atoms, rendering these bond centers so electron deficient that they enable saturated alkanes to participate in electrophilic reactions.
The discovery of hypercoordinated carbocations fueled the nonclassical ion controversy of the 1950s and 60s. Due to the slow timescale of
1H-NMR, the rapidly equilibrating positive charges on hydrogen atoms would likely go undetected. However,
IR spectroscopy
Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functiona ...
,
Raman spectroscopy
Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman sp ...
, and
13C NMR have been used to investigate bridged carbocation systems. One controversial cation, the norbornyl cation, has been observed in several media, Magic acid among them.
:
The bridging methylene carbon atom is pentacoordinated, with three two-electron, two-center bonds, and one two-electron, three-center bond with its remaining sp
3 orbital. Quantum mechanical calculations have also shown that the classical model is not an energy minimum.
Reactions with alkanes
Magic acid is capable of protonating
alkanes
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...
. For instance, methane reacts to form the ion at 140 °C and atmospheric pressure, though some hydrocarbon ions of greater molecular weights are also formed as byproducts. Hydrogen gas is another reaction byproduct.
In the presence of FSO
3D rather than FSO
3H, methane has been shown to interchange hydrogen atoms for deuterium atoms, and HD is released rather than H
2. This is evidence to suggest that in these reactions, methane is indeed a base, and can accept a proton from the acid medium to form . This ion is then deprotonated, explaining the hydrogen exchange, or loses a hydrogen molecule to form – the carbonium ion. This species is quite reactive, and can yield several new carbocations, shown below.
:
Larger alkanes, such as ethane, are also reactive in magic acid, and both exchange hydrogen atoms and condense to form larger carbocations, such as protonated neopentane. This ion is then cloven at higher temperatures, and reacts to release hydrogen gas and forms the t-amyl cation at lower temperatures.
:
It is on this note that George Olah suggests we no longer take as synonymous the names "alkane" and "paraffin." The word "paraffin" is derived from the Latin "parum affinis", meaning "lacking in affinity." He says, "It is, however, with some nostalgia that we make this recommendation, as ‘inert gases’ at least maintained their ‘nobility’ as their chemical reactivity became apparent, but referring to ‘noble hydrocarbons’ would seem to be inappropriate."
Catalysis with hydroperoxides
Magic acid catalyzes cleavage-rearrangement reactions of tertiary hydroperoxides and tertiary alcohols. The nature of the experiments used to determine the mechanism, namely the fact that they took place in superacid medium, allowed observation of the carbocation intermediates formed. It was determined that the mechanism depends on the amount of magic acid used. Near molar equivalency, only O–O cleavage is observed, but with increasing excess of magic acid, C–O cleavage competes with O–O cleavage. The excess acid likely deactivates the hydrogen peroxide formed in C–O heterolysis.
:
Magic acid also catalyzes electrophilic hydroxylation of aromatic compounds with hydrogen peroxide, resulting in high-yield preparation of monohydroxylated products. Phenols exist as completely protonated species in superacid solutions, and when produced in the reaction, are then deactivated toward further electrophilic attack. Protonated hydrogen peroxide is the active hydroxylating agent.
Catalysis with ozone
Oxygenation of alkanes can be catalyzed by a magic acid–SO
2ClF solution in the presence of
ozone
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
. The mechanism is similar to that of protolysis of alkanes, with an electrophilic insertion into the single σ bonds of the alkane. The hydrocarbon–ozone complex transition state has the form of a penta-coordinated ion.
:
Alcohols, ketones, and aldehydes are oxygenated by electrophilic insertion as well.
Safety
As with all strong acids, and especially superacids, proper personal protective equipment should be used. In addition to the obligatory gloves and goggles, the use of a faceshield and full-face respirator are also recommended. Predictably, magic acid is highly toxic upon ingestion and inhalation, causes severe skin and eye burns, and is toxic to aquatic life.
See also
*
Fluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride, containing various cations and anions (the simplest being and ). This substance is a superacid that can be over a billion times stronger than 100% pure sulfuri ...
, the strongest known superacid
References
{{DEFAULTSORT:Magic Acid
Superacids
Antimony(V) compounds
Sulfonic acids
Fluoro complexes