Superacids Bronsted

The easiest way to create a Bronsted acidic ionic liquid is to dissolve a strong Bronsted acid in an ionic liquid. Already in 1989, Smith and coworkers described that mixtures of HC1 and acidic chloroaluminate ionic liquids result in the formation of superacidic Bronsted acids (more acidic than 100% sulfuric acid). This is due to the reaction of HCl with the acidic anions (e.g. [AI2CI7]-) of the melt forming a proton with extremely low solvation and therefore very high acidity [46],... 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

The alkoxysiladioxanes described by Davis were shown to undergo selective axial addition of allyltrimethylsilane [49]. Moderate selectivity (7-13 1) was observed when the reactions were promoted by Lewis acids (TMSOTf, SnCl4), while higher selectivity was realized when a Bronsted superacid was used (Eq.23). 

Ionic liquids containing chloroaluminate ions are strong Lewis, Franklin and Bronsted acids. Protons present in [emim][AlCl4] have been shown to be superacidic with Hammett acidities up to —18. Such highly acidic ionic liquids are, nonetheless, easily handled and offer potential as non-volatile replacements for hazardous acids such as HF in several acid-catalyzed reactions. 

In Ref. 6, the Bronsted superacidity of HCl in liquid chloroaluminate IL ([C2Cilm]Cl/AlCl3) was studied by the protonahon of arens during which the degree of protonahon was measured by using absorption spectroscopy. The arens were stable in the liquid chloroaluminate for many hours and their protonated forms (arenium ions) were stable for 1 h or more. 

Smith, G.R, Dworkin, A.S., Ragni, R.M., Zingg, S.R, Bronsted superacidity of HCl in liquid chloroaluminate, A1C13—l-ethyl-3-methyl-lH-imidazolium chloride, J. Am. Chem. Soc., Ill, 525-529,1989. 

Like the trialkyloxonium superelectrophiles, the salts of trimethyl sul-fonium (CH3)sS+, selenonium (CEL Se"1", and telluronium (CE Te4" ions have also been shown by Laali et al. to undergo superelectrophilic activation.41 These onium salts methylate toluene in FSChH-SbFj, but with the weaker Bronsted superacid CF3SO3H (triflic acid, //q —14.1), no methylation takes place (eq 12). 

Carbon-Halogen Vicinal-Dieations Trihalomethyl cations are shown to have enhanced reactivities in superacid solution, while poly-halomethanes in the presence of excess AlBr3 or AICI3 exhibit the properties of aprotic superacids.79 The trihalomethyl cations CX3+ (178, X=C1, Br, I) have been characterized by NMR and IR spectroscopy. The stability of these species is attributed to substantial resonance-stabilization by back-donation from the nonbonded electron pairs of the halogen atoms.22 Trihalomethyl cations are capable of hydride abstraction from alkanes and alkyl groups when the reactions are carried out in the presence of Bronsted or Lewis superacids (eq 46-48).80... 

The type of superacid sites on SO /metal oxides evacuated at 773 K is only a Lewis type according to the IR absorption bands of adsorbed pyridine [31]. Mortcrra ct al. [32] have shown that pyridine adsorbed on Lewis acid sites dominated the spectra of samples evacuated at 673 K and that the addition of water at 300 K significantly increased the amount of Bronsted acidity. Nascimcnto ct al. [8] report that both Bronsted and Lewis acid sites exist on SO4 /Zr02 treated at 723 K and the ratio of Bronsted to Lewis sites changes with the change of sulfur content. Recently. Lunsford ct al. revealed by use of 31P MAS NMR spectra of adsorbed trimcthylphosphinc that three types of Lewis... 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

This representation is over-simplified, each of the ions being further solvated in each acid. Autoprotolysis constants have been reported as 3 x 10 13 mol2 kg-2 (0°C) for HF[6], 3.8 x 10 8 (25°C) for HS03F[7] and 7.9 x 1(T7 (25°C) for CF3S03H[8]. Protonic media are made more acidic by addition of an entity which increases the proton concentration. Superacids are themselves so very weakly basic that very few, if any, compounds can act as Bronsted acids to donate protons to the solvent directly. Lewis acids combine with X- to shift the autoprotolysis equilibria to increase the proton concentration. Superacids are rendered basic by direct addition of the X species, the base of the system, (e.g. from an alkali metal compound MX) or by addition of compounds which accept protons from the medium, increasing the concentration of the base X. ... 

In the absence of effective Bronsted acids, enhancement of acidity in superacidic media is achieved by use of Lewis acids. For example in the HF system, SbF5 acts as a Lewis acid by accepting F to form SbFg, as represented in highly idealised form in Eqn. (6) ... 

The converse of lack of Bronsted acid activity towards the three highly acidic fluoro-acids is that there is a vast range of compounds which act as Bronsted bases — i.e. they are protonated by the superacids. As indicated in the Introduction, carboxylic and many other oxo-acids are protonated and/or solvolysed. Compounds having coordinately unsaturated O, S, N or P atoms, e.g. alcohols, aldehydes, esters, ketones, amines, amides, etc, are Bronsted bases in the superacids. H20 is a case requiring special emphasis because it is a likely impurity in the solvent, especially for HF and CF3SO3H. Protonation of H20 causes enhancement of the concentration of the conjugate base of the solvent, so that H() values for the neat acid can be much less than that for the pure solvent. 

As with the other superacids, the basicity of HSO3F is most easily adjusted quantitatively by direct addition of the conjugate base S03F in the form of the alkali metal cation or ammonium salts. Less directly, the basicity is increased by proton acceptors, e.g. H20 and a vast range of Bronsted bases. Such an increase in basicity is frequently an unwanted result of protonation of solutes or of reaction products but must be taken into account in considering the final level of acidity in any reaction mixture. 

Common superacids in use are the Bronsted acid FSO3H and the Lewis acid SbFj dissolved in SO ClF or mixtures of SOjClF and SO Fj. To be able to study liquid ionic solutions at very low temperatures (ca —160°C), e.g. by nmr spectroscopy, freons like CHCljF may be added to the solution to keep the viscosity at a tolerable level. Superacids can be up to a billion times stronger acids than sulphuric acid. Carbocations are generated in the reactions of e.g. alcohols and olefins with FSO3H and of chlorides with e.g. SbFj or by hydride abstraction. The superacid chemistry has been treated in a number of reviews (e.g. Olah, 1979). A general survey of the chemistry of superacids is given by Olah et al. (1979b). Other reviews have appeared... 

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