Bronsted-Lewis superacid

Conjugate Bronsted-Lewis superacids Superacidic proton donor acids comprised of a combination of Bronsted and Lewis acids. 

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... 

According to [19], the strong acidity (or superacidity ) of SZ materials is originated from the presence of both Lewis and Bronsted sites, the strength of the latter being enhanced by the neighborhood of the former. For this reason, the optimum of catalytic performances is found in the samples with a Bronsted/Lewis ratio (B/L) near to 1 [19]. SZl sample approximates this value. 

Scheme 12 summarizes a y-radiolysis performed on a mixture of benzene-CF4, and ethylene. Ionization of CF4 yields CFj, which has been called an ionic Lewis superacid.Electrophilic attack on benzene-(i6 gives the conjugate acid of perdeuterated trifluorotoluene, CF3C6DJ, as the first step of Scheme 12. Trifluorotoluene is more basic than ethylene hence, the ion-molecule reaction of CF3C6DJ with ethylene cannot give a Bronsted acid-base reaction. The only thermochemically accessible pathway to ethyl trifluorotoluenes is via ion-neutral complexes containing the ethyl cation...

Superacids Acid systems that encompass both Bronsted and Lewis superacids as well as their conjugate combinations. 

Superelectrophiles Electrophiles that are further activated by Bronsted or Lewis superacid complexation. 

Thus, the classification of Lewis superacids as those stronger than anhydrous aluminum trichloride is only arbitrary. Just as in the case of Gillespie s classification of BrOnsted superacids, it is important to recognize that acids stronger than conventional Lewis acid halides exit, with increasingly unique properties. 

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. 

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) ... 

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... 

The addition of water causes the breakage of the coordination bonds to yield Bronsted acid sites strengthening Lewis acid sites, as shown in Scheme 17.4, for example. Many research groups report the simultaneous existence of Bronsted and Lewis acid sites or the reversible transformation between Bronsted and Lewis acidity upon hydration or dehydration [61, 106, 152]. Fraenkel suggests that in order to be an effective superacid, sulfated zirconia should contain a critical amount of moisture [155bj. Several workers propose that the strong acidity requires the presence of both Lewis and Br0nsted sites. 

---