Super Acidic Ionic Liquids

Super Acidic Ionic Liquids Choice of the Anion 

The most studied ionic liquids (ILs) in the literature are chloroaluminate-based ionic liquids. Chemical properties such as Lewis acidity depend on the nature of the anion, namely AI2CI7. Considering the acidity parameter as the key point to design a liquid-phase fluorination catalyst, a new range of acidic ILs has been set up [9]. These ILs have been synthesized starting from halogenated metallic acids MX, (with M = Ti, Nb, Ta, Sn, Sb and X = Cl and/or F), such as Ta(V)Cl Fj, Nb(V)Cl Fj, Ti(IV)Cl Fj, Sn(IV)Cl Fj, Sb(V)Cl Fj,. Among these compounds, anions derived from SbCl F, (x -i-y = 5,0 x 5 and 0 y 5) compounds turned out to 

The Lewis-acidic nature of this new type of ionic liquids has been obtained, as for chloroaluminate-based ILs, by adjusting the ratio between the halogenated acid and the cation precursor to above 1. 

A suitable cation must be chosen according to the targeted application. For liquid-phase fluorination catalysis with HF as the fluorinating agent, this cation needs to be stable in the presence of HF. 

Salts with carbenium ions and SbFg as counteranion have already been described, such as magic methyl CHj SbFg. However, these super-electrophilic species can be involved in electrophilic additions to double bonds and carbenium cations, and are therefore not suitable for fluorination reactions. 

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Figure 2.8 Acid strength of super acid ionic liquids compared to conventional super acids...

An example is the question of the pH-value of acidic solutes in ionic liquids. From a practical point of view, this issue is extremely difficult to resolve, as pH-electrodes cannot be used in ionic liquids. Furthermore, the pH-value is defined as the negative logarithm of the H30+ activity, but in a dry ionic liquid, water is not present to form this species with dissociated protons. Does this then mean that protons are naked , tending to exhibit super-acidic character The answer to this question lies of course in the solvation properties of the ionic liquid under investigation. 

Miran MS, Kinoshita H, Yasuda T, Susan MABH, Watanabe M (2012) Physicochemical properties determined by ApXa for protic ionic liquids based on an OTganic super-strong base with various Brpnsted acids. Phys Chem Chem Phys 14 5178-5186... 

The other major component in a liquid organic electrolyte is the lithium salt, which is the ionic source of the electrolyte. The salt is generally the lithium conjugate base of a super acid, which makes the anion anodicaUy stable and easily dissociates from Li in organic solvents. Compared with electrolyte solvents, the... 

Latent acidity and super acidity are two interesting phenomena in the field of acid base chemistry in ionic liquids. 

Superacidic ionic liquids are much safer and easier to handle compared to normal super acid systems and could therefore represent promising alternatives to the conventional... 

The potential super acid capability of excess aqueous HBF4 and HPFe in ionic liquid may facilitate formylation reactions of aromatics and were subsequently investigated. Toluene, being highly susceptible to CO formylation under acidic conditions, was therefor subjected to treatment with excess HPFe and HBF4 in BmimPFe and BmimBF4 respectively (Table 2.16, Runs 1 and 2). No formylation products were obtained which... 

It was now decided to revert to the use of HF/BF3 and evaluate its efficacy in ionic liquids. If this super acid could be used in catalytic quantities and remain in the ionic liquid during separation of the product, it could form the basis of a viable industrial process. Toluene, anisole and phenol were successfully formylated in bmimBF4 following HF/BF3 acidification (Table 2.17, Runs 1, 2 and 3). The total aldehyde yield of phenol (32%) was significantly higher compared to both toluene (5%) and anisole (7%) under similar conditions. Although a reaction time of 12hours was allowed, the total aldehyde yield of all three substrates remained inferior to earlier results obtained (Table 2.2). The difference between conversion and aldehyde product obtained in the case of... 

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