Solvent effects, ionic liquids

Ionic liquid Organic liquid Tunability of the solubility characteristics of the ionic liquids Solvent effect Ionic liquid costs Disposal of spent ionic liquids... 

Table 4.3 Microwave heating effects of doping organic solvents with ionic liquids (IL) A and B (data from [63]). a ...

Water is particularly suitable for use in biphasic catalysis. It readily separates from organic solvents because of its polarity, density and because of the hydrophobic effect. Water will form biphasic systems with fluorous solvents, some ionic liquids, many volatile organic solvents, and also with scCC>2 [18],... 

The commonest solvent for TPAP in organic oxidations is CH Clj (DCM), normally in conjunction with 4 A powdered molecular sieves (PMS) to remove water formed during the oxidation. Addition of CH3CN, as in many Ru-catalysed oxidations, makes reactions with TPAP/NMO more effective [59], and occasionally CH3CN is used as the only solvent [159]. Ionic liquids, e.g. [emim](PF )/PMS [479] and [bmim](BF )/PMS [480] have been used with TPAP/NMO. It has also been used in supercritical CO [457]. 

Najdanovic-Visak, V. et al.. Pressure, isotope, and water co-solvent effects in liquid-liquid equilibria of (ionic liquid + alcohol) systems, /. Phys. Chem. B, 107, 12797, 2003. 

The sieving effect of the carbon host was also demonstrated by measuring the capacitance values of an AC in a series of solvent-free ionic liquids (ILs) of increasing cation size [17], Since ions are not solvated in pure ILs, it was easy to interpret the electrochemical properties by comparing the nanoporous characteristics of carbon and the size of cations calculated by molecular modeling. It was found that the overall porosity of the carbon is noticeably underused, due to pores smaller than the effective size of the cations. The results with ILs confirm that the optimal pore size depends on the kind of electrolyte, i.e., the dimensions of pores and ions must match each other. 

Ania CO, Pernak J, Stefaniak F, Raymundo-Pinero E, Beguin F. Solvent-free ionic liquids as in situ probes for assessing the effect of ion size on the performance of electrical double layer capacitors. Carbon 2006 44 3113-3148. 

Water has several attractive features as a solvent and, as we have said elsewhere, the best solvent is no solvent, but if one has to use a solvent then let it be water. Water is the most abundant molecule on the planet and is, hence, readily available and inexpensive. It is nonflammable and incombustible and odorless and colorless (making contamination easy to spot). It has a high thermal conductivity, heat capacity and heat of evaporation, which means that exothermic reactions can be controlled effectively. It readily separates from organic solvents owing to its polarity, density and because of the hydrophobic effect [12], which makes it eminently suitable for biphasic catalysis. Indeed, water forms biphasic systems with many organic solvents, with fluorous solvents, some ionic liquids and with scC02 [13]. 

The aim of the present chapter is to give a brief overview of protease-catalyzed synthesis of sugar esters in hydrophilic solvents and to present some of the most recent investigations on the effect of these solvents on activity and stability of proteases. Consequently, the perspectives in solvent engineering are outlined with focus on hydrophilic solvents and ionic liquids. 

L-proline-derived sulfonamides 15b and 15c are also effective catalysts in the Mannich reaction of cyclic ketones with N-PMP-protected cx-imino ethylglyoxylate in both classical organic solvents and ionic liquids as the reaction media. ... 

SOLVENT EFFECTS IN CONVENTIONAL SOLVENTS AND IONIC LIQUIDS... 

The n values were high for all of the ionic liquids investigated (0.97-1.28) when compared to molecular solvents. The n values result from measuring the ability of the solvent to induce a dipole in a variety of solute species, and they will incorporate the Coulombic interactions from the ions as well as dipole-dipole and polarizability effects. This explains the consistently high values for all of the salts in the studies. The values for quaternary ammonium salts are lower than those for the monoalkylammonium salts. This probably arises from the ability of the charge center on the cation to approach the solute more closely for the monoalkylammonium salts. The values for the imidazolium salts are lower still, probably reflecting the delocalization of the charge in the cation. 

An alternative avenue for the exploration of the polarity of a solvent is by investigation of its effect on a chemical reaction. Since the purpose of this book is to review the potential application of ionic liquids in synthesis, this could be the most productive way of discussing ionic liquid polarity. Again, the field is in its infancy, but some interesting results are beginning to appear. 

To date, most studies of ionic liquids have used a small set of ionic liquids and have been based on the idea that, if the response of a particular probe molecule or reaction is like that in some known molecular solvent, then it can be said that the polarities of the ionic liquid and the molecular solvent are the same. This may not necessarily be the case. Only systematic investigations will show whether this is tme, and only when a wide range of ionic liquids with a wide range of different solvent polarity probes have been studied will we be able to make any truly general statements about the polarity of ionic liquids. Indeed, in our attempts to understand the nature of solvent effects in ionic liquids, we will probably have to refine our notion of polarity itself However, it is possible to draw some tentative general conclusions. 

The physical properties of ionic liquids can often be considerably improved through the judicious addition of co-solvents [55-58]. Flowever, surprisingly, this approach has been relatively underutilized. Flussey and co-workers investigated the effect of co-solvents on the physical properties of [EMIM]C1/A1C13 ionic liquids [55, 56]. They found significant increases in ionic conductivity upon the addition of a variety of co-solvents. Figure 3.6-5 displays representative data from this work. The magnitude of the conductivity increase depends both on the type and amount of the co-solvent [55, 56]. 

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