Ionic Lewis superacid

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...

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 pH-neutral ionic liquids are highly polar and noncoordinating. These liquids have potential applications as solvents for metallic and organometallic reagents in two-phase reactions, and as replacements for polar, aprotic solvents like dimethyl-foimamide. The Lewis-acidic and superacidic ionic liquids are being investigated for use as catalytic solvents. 

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... 

Ionic liquids can also exhibit superacidity (Brpnsted superacids are acids that are more acidic than pure H2SO4). The dissolution of gaseous HCl in acidic [emim][Cl]/AlCl3 (55mol% of Lewis acid) leads to a superacidic system that has similar properties to those of liquid HF and can be used for the protonation of arenes. The Brpnsted acidity of products dissolved in water-stable ionic liquids can be increased HNTf2 and TfOH display higher chemical activity in [bmim][NTf2] and [bmim][BF4] than in water. ... 

Ionic polymerizations catalyzed by immobilized complexes, in particular Lewis acids on polymer supports, so-called solid superacids, are now extensively developed. A variety of cationic catalyst (protonic and Lewis acids), their combinations (complexes) and complexes with different supports are summarized in a review [111] (Table 12-9). 

Sodium borohydride in excess of triflic acid at low temperature provides" a highly efficient reductive superacid system, which was found effective for the reductive superacid isomerization of unsaturated polycyclic precursers to cage hydrocarbons in high yields (equation 25). The key to the success of the ionic hydrogenation system appears to be due to the in situ formation of boron tris(triflate), a highly superacidic Lewis acid. 

Room-temperature ionic liquids have recently attracted a great deal of industrial interest (Carmichael, 2000 Guterman, 1999 see also the brief overview by Earle et al. (2003) and the extensive and detailed review by Olivier-Bourbigou et al. (2010). They are versatile as solvents or nonsolvents for organic substances, and some exhibit strongly temperature-dependent water solubility. They are nonvolatile. Some, notably those containing haloaluminate anions, have widely variable Lewis and Brpnsted acidity (into the superacid range). Others, such as those with tetrafluoroborate or hexafluorophosphate... 

Both titania (anatase more than rutile) and, even more, zirconia (tetragonal more than monoclinic), when sulfated or covered with tungsten oxide become very active for some hydrocarbon conversion reactions such as -butane skeletal isomerization [263]. For this reason, a discussion began on whether these materials have to be considered superacidic. Spectroscopic studies showed that the sulfate ions [264] as well as the tungstate ions [265,266] on ionic oxides in dry conditions, are tetracoordinated with one short S=0 and W=0 bond (mono-oxo structure) as shown in Scheme 9.3(11). Polymeric forms of tungstate species could also be present [267]. However, in the presence of water the situation changes very much. According to the Lewis acidity of wolframyl species, it is believed that it can react with water and be converted in a hydrated form, as shown in Scheme 9.3. Residual... 

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