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The story of superacids.

The term superacid was first used by Conant and Hall in 1927. [1] These authors found that sulfuric acid and perchloric acid in solution in anhydrous acetic acid were able to protonate a variety of weak bases such as ketones and other carbonyl compounds that do not form salts in aqueous solutions of the same acids. They ascribed the high acidity of these solutions to the formation of the [CH.sub.3.CO.sub.2.H.sub.2.sup+] ion which is a stronger proton donor than [H.sub.3.O.sup.+].

[Mathematical Expressions Omitted]

They proposed to call these highly-acidic solutions 'superacidic solutions". However, as we shall see the term superacid has since been restricted to acidic media that are considerably more acidic than solutions of acids in acetic acid.

In 1932, Hammet and Deyrup [2] proposed a method for measuring the acidities of aqueous solutions of acids more concentrated than the dilute solutions to which conventional pH measurements are applicable. They proposed the acidity function [H.sub.o] defined by the equation

[H.sub.o] = [pK.sub.[BH.sup.+]] - log [[BH.sup.+]] / [B]

[H.sub.o] becomes equal to pH in very dilute aqueous solutions. Hammet and Deyrup determined the ionization ratio [BH.sup.+] / [B] for a series of successively weaker primary aromatic amines in the [H.sub.2.O] - [H.sub.2.SO.4] system and were able to measure [H.sub.o] from [H.sub.2.O] up to 100% [H.sub.2.SO.4]. According to more recent studies (4) - [H.sub.o] increases from ) for a 1M [H.sub.2.SO.sub.4] solution inwater to 12.1 for 100% [H.sub.2.SO.4]. We may say that 100% [H.sub.2.SO.4] is approximately [10.sup.12] times as acidic as a 1M solution of [H.sub.2.SO.4] which has [H.sub.o] [unkeyable] pH [unkeyable] ) so that sulfuric acid may truly be said to be a superacid. It has proved convenient to somewhat arbitrarily define a superacid as an acidic medium that has - [H.sub.o] equal to or greater than that of anhydrous sulfuric acid, that is, - [H.sub.o] [is greater than or equal to] 12. This definition does not however include the solutions in acetic acid that were called superacid solutions by Conant and Hall for which -[H.sub.0] is less than 12.

Some Superacid Systems

Sulfuric acid had in fact been studied by Hantzch [3] 20 years before the work of Conant and Hall on acetic acid solutions. Hantzch made freezing point depression measurements to study the ionization of solutes in 100% [H.sub.2.SO.sub.4]. He found that several substances that are not protonated in water, such as acetone and acetic acid, behave as strong bases in 100% [H.sub.2.SO.sub.4]. [CH.sub.3.CO.sub.2.H] + [H.sub.2.SO.sub.4] [right arrow] [CH.sub.3.CO.sub.2.H.sup.+.sub.2] + [HSO.sup.-.sub.4] [(CH.sub.3).sub.2.CO] + [H.sub.2.SO.sub.4] [right arrow] [(CH.sub.3).sub.3.COH.sup.+] + [HSO.sup.-.sub.4]

The behaviour of acetic acid reminds us that the terms acid and base are relative terms and that names of common acids and bases are based on their behaviour in water. Thus acetic acid is a weak acid in water but it behaves as a strong base in sulfuric acid.

Sulfuric acid undergoes self-ionization or autoprotolysis in the same manner as water and many other protonic solvents [4]: [Mathematical Expression Omitted] Just as the acidity of an aqueous solution is increased by increasing the concentration of [H.sub.3.O.sup.+], the acidity of [H.sub.2.SO.sub.4] can be further increased by a solute HA that behaves as an acid, thereby increasing the concentration of the [H.sub.3.SO.sup.+.sub.4] ion: [Mathematical Expression Omitted] There are only a few acids that are strong enough to ionize in this way. [4] These include disulfuric acid [H.sub.2.S.sub.2.O.sub.7], and flurosulfuric acid, [HSO.sub.3.F] [H.sub.2.S.sub.2.O.sub.7] + [H.sub.2.SO.sub.4] [right arrow] [H.sub.3.SO.sup.+.sub.4] + [HS.sub.2.O.sup.-.sub.7] [HSO.sub.3.F] + [H.sub.2.SO.sub.4] [right arrow] [H.sub.3.SO.sup.+.sub.4] + [SO.sub.3.F.sup.-] Gillespie and Peel [5] extended Hammett and Deyrup's acidity function measurements into these completely nonaqueous and still more highly acidic solutions. They showed that for [H.sub.2.SO.sub.4] - [HSO.sub.3.F] mixtures [-H.sub.0] increases from 12.1 to 15.1 for 100% [HSO.sub.3.F] so that [HSO.sub.3.F] is 10 [3] times more acidic than [H.sub.2.SO.sub.4].

Flurosulfuric acid has proved to be one of the most widely used superacid media. [5,6,7] In addition to its very high acidity, it has a conveniently large liquid range from -89 to +163.7 [degrees] C. It is readily purified by distillation and when pure, unlike HF, it does not attack glass. Moreover its acidity can be still further increased by the addition of a pentafluoride such as [SbF.sub.5] to values as high as [-H.sub.0] = 25 [5,6,7]. Antimony pentafluoride is a very strong Lewis acid, indeed we may say that it is a Lewis superacid. It coordinates strongly with the very weak base [SO.sub.3.F.sup.-] thus increasing the concentration of [H.sub.2.SO.sub.3.F.sup.+] [2HSO.sub.3.F] + [SbF.sub.5] [right arrow] [H.sub.2.SO.sub.3.F.sup.+] + [SbF.sub.5.(SO.sub.3.F).sup.-] Figure 1 shows the effect on the acidity of [HSO.sub.3.F] of adding acids as [AsF.sub.5] and [SbF.sub.5] and of bases such as [H.sub.2.SO.sub.4.] [H.sub.2.O] and [SO.sub.3.F.sup.-]. The plots here represent acid-base titrations analogous to pH titration curves in aqueous solution. The steep rise in acidity in the region of the pure solvent is a consequence of the very small autoprotolysis constant which is comparable to that of water. Very small additions of an acid such as [SbF.sub.5] or of a base such as [H.sub.2.SO.sub.4] or [SO.sub.3.F.sup.-] to the pure acid can cause very large changes in the acidity as measured by [H.sub.0]

Other superacids that have been extensively studied include HF [8,9] for which [-H.sub.0] = 15 and [CF.sub.3.SO.sub.3.H] (trifluoromethane sulfonic acid commonly known as triflic acid) for which [-H.sub.0] = 14.1. [10] The acidities of both these superacids can also be further increased by the addition of [SbF.sub.5].

Applications of Superacids in Organic Chemistry

One very early application of the superacid 100% [H.sub.2.SO.4] was the preparation of stable solutions of aryl carbenium ions such as the triphenyl carbenium ion [(C.sub.6.H.sub.5).sub.3.C.sup.+] from the corresponding alcohol. [11] [(CH.sub.6.H.sub.5).sub.3.COH] + [2H.sub.2.SO.sub.4] [right arrow] (CH.sub.6.H.sub.5).sub.3.C.sup.+] + [H.sub.3.O.sup.+] + [2HSO.sup.-.sub.4] More recently there has been considerable interest in the corresponding alkyl carbenium ions which were postulated as intermediates in some reactions but it was not until 1963 that Olah and his collaborators [12] were able to prepare stable solutions of these ions by the analogous reaction in [HSO.sub.3.F] - [SbF.sub.5]. Alkyl carbenium ions are much more electrophilic than the corresponding aryl ions and can only be obtained as stable species in a much more highly acidic and therefore much more weakly basic medium. [(CH.sub.3).sub.3.COH] + [2HSO.sub.3.F] [right arrow] [(CH.sub.3).sub.3.C.sup.+] + [H.sub.3.O.sup.+] + [2SO.sub.3.F.sup.-] The t-butylcarbenium ion can also be prepared by the reaction of t-butyl fluoride with an express of [SbF.sub.5] [(CH.sub.3).sub.3.CF] + [2SbF.sub.5] [right arrow] [(CH.sub.3).sub.3.C.sup.+] + [Sb.sub.2.F.sup.-.sub.11]

Since 1963 Olah and his coworkers have used superacid media, particularly [HSO.sub.3.F] - [SbF.sub.5], to obtain stable solutions of a wide variety of alkyl carbenium ions. [7] The [HSO.sub.3.F] - [SbF.sub.5] superacid is widely known as magic acid. Olah recounts that this name originated when one of his coworkers put a small piece of a Christmas candle left over from a lab party into [HSO.sub.3.F] - [SbF.sub.5] and found that it disolved readily and, to everyone's amazement gave a sharp proton nmr spectrum of the t-butyl cation. The long chain hydrocarbons of the candle had undergone extensive oxidation, cleavage and isomerization to give the stable tertiary ion. His coworkers were so impressed that they gave the name magic acid to the superacid [HSO.sub.3.F] - [SbF.sub.5]. [7] The industrial isomerization of saturated hydrocarbons is normally carried out with solid catalysts at temperatures above 100 [degrees] C. Superacids such as HF - [SbF.sub.5] and [HSO.sub.3.F] - [SbF.sub.5] isomerize saturated hycrocarbons via the formation of carbenium ions at or below room temperature, under which conditions the desired branched isomers are thermodynamically favoured.

Superacids such as [HSO.sub.3.F] and [HSO.sub.3.F] - [SbF.sub.5] have been used to study the protonation of many weak bases. The low freezing-point of fluorosulfuric acid enables the [sup.19.F] nmr spectrum to be obtained at a temperature low enough to slow up proton exchange with the solvent sufficiently that the spectrum of the protonated species can be observed and hence its structure deduced. For example, arenes such as benzene are portonated to give arenium ions [7]

[Mathematical Expression Omitted]

In fluorosulfuric acic at -80[degrees]C acetamide gives two-proton nmr signals indicating that protonation occurs on oxygen not nitrogen [7,13]

[Mathematical Expression Omitted]

The nmr spectrum of acetic acid in fluorosulfuric acid has two signals of equal area indicating that the conjugate acid of acetic acid has the structure [7,14]

[Mathematical Expression Omitted]

Many other applications of superacids in organic chemistry have been described by Olah [7].

Applications in Inorganic Chemistry

A very early application of the use of superacids in inorganic chemistry was the demonstration that the nitronium ion is formed in a solution of nitric acid in sulfuric acid [4]

[Mathematical Expression Omitted]

A more recent interesting application of superacids has been the preparation of stable solutions and crystalline salts of a variety of novel cations of non metallic elements that are too highly electrophilic to be obtained in aqueous media. For example, the elements of groups 6 [16] and 7 [17] can all be oxidized in superacids to give polyatomic species such as [Mathematical Expressions Omitted]. [15,16,17] In some cases, the superacid is also the oxidizing agent. In other cases oxidizing agent such as [S.sub.2.O.sub.6.F.sub.2] are used. For example, the diiodine cation, [Mathematical Expression Omitted] can be obtained by the reaction of iodine with [S.sub.2.O.sub.6.F.sub.2] in fluorosulfuric acid:

[Mathematical Expression Omitted]

The less electrophilic [Mathematical Expression Omitted] can be obtained in the less acidic and more basic sulfuric acid by the reaction

[Mathematical Expression Omitted]

[Mathematical Expression Omitted] is more electrophilic than [Mathematical Expression Omitted] and it can therefore only be obtained in solution at the highest acidities, for example in [HSO.sub.3.F] - [SbF.sub.5].

It has been known since the early 1800's that sulfur, selenium and tellurium dissolve in sulfuric acid or oleum ([H.sub.2.SO.sub.4] - [SO.sub.3]) to give highly-coloured solutions. However, it was not until 1968 that studies of the yellow solutions of selenium in oleum and [HSO.sub.3.F] showed that they contain the square planar [Mathematical Expression Omitted] ion. Subsequent work has shown that many other polyatomic cations of S, Se and Te can be obtained in superacids. These cations have novel cage and ring structures such as

[Mathematical Expression Omitted]

Superacids have proved to be very useful in many other areas of inorganic chemistry. For example, the superacids HF, HF - [SbF.sub.5], [HSO.sub.3F] - [SbF.sub.5] have been extensively used in the preparation of fluorocations of the noble gases and as solvents for the determination of their [19] F nmr spectra which have given valuable structural information. [18,19] For example

[Mathematical Expression Omitted]

References

[1] N.F. Hall, J.B. Conant, J. Am. Chem. Soc. 49, 3047 (1927)

[2] L.P. Hammett and A.J. Deyrup J. Am. Chem. Soc. 54, 2721 (1932)

[3] A. Hantzsch, Z. physikal chem. 61, 257 (1909); 65, 41 (1908)

[4] R.J. Gillespie and E.A. Robinson, Non-Aqueous Solvent Systems edited by T.C. Waddington, Academic Press 1965

[5] R.J. Gillespie and T.E. Peel, Adv. Phys. Org. Chem. 11, 1 (1971).

[6] R.J. Gillespie, Acc. Chem. Res. 1, 202 (1968)

[7] G.A. Olah, G.K.S. Prakash and J. Sommer, Superacids, John Wiley and Sons, 1985

[8] H.H. Hyman and J.J. Katz, Non-Aqueous Solvent Systems edited by T.C. Waddinton, Academic Press 1965

[9] R.J. Gillespie and J. Liang, J. Am. Chem. soc. 110, 6053 (1988)

[10] J. Grondin, R. Sagnes, and A. Commeyras, Bull. Soc. Chim. Fr., 1179 (1976)

[11] A. Hantzsch, Ber. dtsch. chem. Ges. 54b, 2573 (1921)

[12] G.A. Olah, W.S. tolgyesi, S.J. Kuhn, M.E. Moffet, I.J. Bastien, E.B. Baker, J. Am. Chem. Soc. 85, 1378 (1963)

[13] T. Birchall and R.J. Gillespie Can. J. Chem. 41, 2642 (1963)

[14] T. Birchall and R.J. Gillespie Can. J. Chem. 43, 1672 (1965).

[15] R.J. Gillespie and M.J. Morton, Quart. Rev. Chem. Soc., 25, 553 (1971)

[16] R.J. Gillespie and J. Passmore, Adv. Inorg. Radiochem., 17, 49 (1975)

[17] M. Burford, J. Passmore and J.C.P. Sanders, From Atoms to Molecules: Isoelectronic Analogies, Edited by J.F. Liebman and A. Greenberge. VCH Publishers (1989)

[18] R.J. Gillespie, and G.J. Schrobilgen, Inorg. Chem. 13, 2370 (1974)

[19] H. Selig and J. H. Holloway, Top. curr. Chem., 124, 33 (1984)
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Author:Gillespie, R.J.
Publication:Canadian Chemical News
Date:May 1, 1991
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