Solvent polarity, ionic liquids

Solvatochromic Determination of Ionic Liquid Solvent Polarity. 293... 

Ionic liquids, being polar and ionic in character, couple to the MW irradiation very efficiently and therefore are ideal microwave absorbing candidates for expediting chemical reactions. An efficient preparation of the 1,3-dialkylimidazolium halides via microwave heating has been described by Varma et al. that reduces the reaction time from several hours to minutes and avoids the use of a large excess of alkyl ha-lides/organic solvents as the reaction medium (Scheme 6.56) [26-28]. 

Ionic liquids may be used in a similar fashion, but in contrast to the extremely nonpolar fluorous solvents, ionic liquids are polar. They are completely nonvolatile and so cannot be lost to the atmosphere. A range of ionic compounds that are liquid at room temperature and their use in synthetic chemistry are described in Chapter 4. 

Ionic liquids have polarities comparable to lower alcohols, but, unlike alcohols and other polar solvents, they are essentially non-coordinating (occasionally... 

Ionic liquids have polarities comparable to lower alcohols, which are amongst the most widely used solvents in which to conduct homogeneously catalysed reactions. In contrast to lower alcohols, however, many ionic liquids are non-nucleophilic, which can have a pronounced effect on a catalysed reaction. For example, solvated catalytic intermediates are unlikely to form, and this feature could lead to different reaction mechanisms, and hence to different yields and selectivities. The non-nucleophilic environment presented by many ionic liquids is also less likely to deactivate a catalyst and can lead to increased turnover numbers, which is essential in biphasic processes where catalyst recycling and reuse is required. 

As such, polar organic solvents like dichloromethane and propan-2-one are miscible with ionic liquids, solvents of low polarity show partial miscibility and non-polar solvents such as hexane are immiscible. The polarity of ionic liquids can be modified by incorporation of various groups onto the cation and, for example, perfluorinated chains have the effect of considerably lowering polarity such that the resulting ionic liquid is miscible with organic solvents of low polarity and immiscible with polar organic solvents. 

The polarity of some l-alkyl-3-methylimidazolium ionic liquids has been probed using the solvatochomic dye Nile Red (Figure 1(d)), and found to be comparable to that of lower alcohols.31 As such, polar organic solvents like dichloromethane and diethylether are miscible with ionic liquids, solvents of low polarity show partial miscibility, and nonpolar solvents are essentially immiscible. Extractants such as crown ethers can be used to extract cations such as Na+, Cs+, and Sr2+ from ionic liquids.32... 

Other properties of ionic liquids (such as viscosity, degree of order and the cohesive pressure) must be considered carefully, however, when these salts are employed as solvents. The importance of these latter parameters, moreover, increases when the solutes (reagents and transition states) are not able to give competitive interactions with the components of the ionic liquid. In this situation, all the properties which determine the ionic liquid s polarity become unimportant, and the systems. 

Full internal selectivity can be achieved in ionic liquids using bidentate ligands. The cationic pathway has been suggested to be favoured by the polar ionic liquid solvent, and no isomerization of the products to the corresponding carbonyl compounds was observed in Figure 3.35. This method adds nicely to the previously reported routes, as full internal... 

Stoppa A, Hunger J, Buchner R (2009) Conductivities of binary mixtures of ionic liquids with polar solvents. J Chem Eng Data 54 472-479... 

Stoppa, A. Hunger, J. Buchner, R. (2009). Conductivities of Binary Mixtures of Ionic Liquids with Polar Solvents. J.Chem.Eng.Data, 54, 2 (February 2009) 472-479. 

Ionic liquids are similar to dipolar, aprotic solvents and short-chain alcohols in their solvent characteristics. These vary with anion (from very ionic Cl to more covalent [BETI] ). IFs become more lipophilic with increasing alkyl substitution, resulting in increasing solubility of hydrocarbons and non-polar organics. 

Polar Solvents Ionic Liquids Nonpolar Solvents... 

Table 3.5-1 Solvent polarity measurements for some ionic liquids.

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. 

One of the earliest solvent polarity scales is Person s D scale. This scale is based on the endojexo ratio of the Diels-Alder reaction between cyclopentadiene and methyl acrylate (Figure 3.5-2, O = logio endo/exo). This reaction has been conducted in a number of ionic liquids, giving values in the 0.46-0.83 range [26]. 

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. 

Not all ionic liquids are the same, different combinations of anions and cations produce solvents with different polarities. No ionic liquids have shown themselves to be super-polar regardless of the method used to assess their polarities, ionic liquids come within the range of molecular solvents. Most general measures of overall polarity place ionic liquids in the range of the short- to medium-chain alcohols. 

Ionic liquids with wealdy coordinating, inert anions (such as [(CF3S02)2N] , [BFJ , or [PFg] under anhydrous conditions) and inert cations (cations that do not coordinate to the catalyst themselves, nor form species that coordinate to the catalyst under the reaction conditions used) can be looked on as innocent solvents in transition metal catalysis. In these cases, the role of the ionic liquid is solely to provide a more or less polar, more or less weakly coordinating medium for the transition metal catalyst, but which additionally offers special solubility for feedstock and products. 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

---