Dialkylimidazolium properties

We had no good way to predict if they would be liquid, but we were lucky that many were. The class of cations that were the most attractive candidates was that of the dialkylimidazolium salts, and our particular favorite was l-ethyl-3-methylimid-azolium [EMIM]. [EMIMJCl mixed with AICI3 made ionic liquids with melting temperatures below room temperature over a wide range of compositions [8]. We determined chemical and physical properties once again, and demonstrated some new battery concepts based on this well behaved new electrolyte. We and others also tried some organic reactions, such as Eriedel-Crafts chemistry, and found the ionic liquids to be excellent both as solvents and as catalysts [9]. It appeared to act like acetonitrile, except that is was totally ionic and nonvolatile. 

Many ionic liquids are based on N,N-dialkylimidazolium cations (BMI) which form salts that exist as liquids at, or below, room temperature. Their properties are also influenced by the nature of the anion e. g. BF T PFg. The C-2(H) in imidazole is fairly labile but the C-4(H) and the C-5(H) are less so. Under microwave-enhanced conditions it is therefore possible to introduce three deuterium atoms (Scheme 13.4). As hydrogen isotope exchange is a reversible reaction this means that the three deuterium atoms can be readily exchanged under microwave irradiation. For storage purpose it might be best to back-exchange the C-2(D) so that the 4,5-[2H2] isotopomer can be safely stored as the solid without any dangers of deuterium loss. The recently... 

The possibility of adjusting acidity/coordination properties opens up a wide range of possible interactions between the ionic liquid solvent and the dissolved transition metal complex. Depending on the acidity/coordination properties of the anion and on the reactivity of the cation (the possibility of carbene ligand formation from 1,3-dialkylimidazolium salts is of particular importance here [37]), the ionic liquid can be regarded as an innocent solvent, as a ligand precursor, as a co-catalyst or as the catalyst itself. 

The physical properties of ionic liquids have been extensively studied and some trends are beginning to emerge. In particular, ionic liquids based on 1,3-dialkylimidazolium cations have been investigated in detail, partly due the their wide use as solvents to conduct synthesis and catalysis. The attraction of the imidazolium cation in synthetic applications is because the two substituent groups can be varied to modify the properties of the solvent. For example, Table 4.1... 

Heretofore, ionic liquids incorporating the 1,3-dialkylimidazolium cation have been preferred as they interact weakly with the anions and are more thermally stable than the quaternary ammonium cations. Recently, the physical properties of 1,2,3,4-tetraalkylimidazolium- and 1,3-dialkylimidazolium-containing ionic liquids in combination with various hydrophobic and hydrophilic anions have been systematically investigated (36,41). The melting point, thermal stability, density, viscosity, and other physical properties have been correlated with alkyl chain length of the imidazolium cation and the nature of the anion. The anion mainly determines water miscibility and has the most dramatic effect on the properties. An increase in the alkyl chain length of the cations from butyl to octyl, for example, increases the hydrophobicity and viscosity of the ionic liquid, whereas densities and surface tension values decrease, as expected. 

Domarfska, U., Marciniak, A., and Bogel-Lukasik, R., Phase equilibria (SLE, LLE) of N, N-dialkylimidazolium hexafluorophosphate or chloride, in Ionic Liquids lllA Fundamentals, Progress, Challenges, and Opportunities Properties and Structure, R.D. Rogers and K.R. Seddon (Eds), Washington, D.C., 2005 ACS, NY, 2003. 

Ionic liquids (ILs), previously known as molten salts, were mainly used in electrochemistry studies due to their ionic nature. The most important step in the chemistry of the ILs occurred when Osteryoung described a mixture of 1-(1-butylpyridinium)-chloride and aluminium chloride which was liquid at room temperature30. Later on, Wilkes discovered other ionic liquids based on dialkylimidazolium salts that featured even more convenient physical and electrochemical properties than the butylpyridinium salts3i. 

Ionic liquids are quite simply liquids that are composed entirely of ions [96, 97]. They are generally salts of organic cations, e.g. tetraalkylammonium, alkylpyridi-nium, 1,3-dialkylimidazolium, tetraalkylphosphonium (Fig. 7.28). Room temperature ionic liquids exhibit certain properties which make them attractive media for performing green catalytic reactions. They have essentially no vapor pressure and are thermally robust with liquid ranges of e.g. 300 °C, compared to 100 °C for water. Polarity and hydrophilicity/hydrophobicity can be tuned by a suitable combination of cation and anion, which has earned them the accolade, designer solvents . 

Quek S K, LyapkaloHuynh 1 M, Huynh H V. Synthesis and properties of NdV -dialkylimidazolium bis(nonafluorobutane-l-sulfonyl)imides A new subfamily of ionic liquids. Tetrahedron. 2006. 62, 3137-3145. 

Dialkylimidazolium ionic liquids with BF4 and IT), anions are quite stable and while being polar (see below) are also non-nucleophilic. They can be prepared in a variety of ways, most commonly by metathesis reactions21-23 and also by a methylation reaction.24 These reactions are illustrated in Schemes 2 and 3, respectively. The chloride produced in the metathesis reactions (Scheme 2) is very difficult to remove completely and it effects the physical properties of the liquid and can also effect certain reactions carried out in the liquid. Figure 6 shows how the viscosity of l-butyl-3-methylimidazolium tetrafluoroborate varies with chloride concentration.25... 

Most ions constituting ionic liquids can be categorized according to their Lewis acid/base properties (i.e., their capability to accept or to donate an electron pair) nevertheless, some ions may be considered according to the Bronsted definition, i.e., on the basis of their ability to accept or donate a proton. Typical ionic liquids are those based on neutral or very weakly basic anions (BF4, PF, NOf, CHsSO, 4 f2N ) and neutral (tetraalkylammonium, dialkyl-pyrrolidium, trialkylsulfonium) or weakly acidic cations (1,3-dialkylimidazolium and 1,2,3-trialkylimidazolium) (Figure 4.1). 

Ionic liquids have recently appeared as clean alternatives to classical organic solvents for a wide variety of biochemical processes due to their unique properties [22-26]. Ionic liquids are organic salts which are liquid close to room temperature. They normally consist of an organic cation (e.g. dialkylimidazolium, tetraalkylam-momium) and a polyatomic inorganic cation (e.g. hexafluorophosphate, tetrafluoroborate, bis(trifluoromethylsulphonyl)imide) (Fig. 8.2). 

With respect to ILs as cosolvents, the interest was centred on two types of ILs aUcylammonium and 1,3-dialkylimidazolium-based ionic liquids. For the first type, we selected ethylammonium nitrate, considering that it is a protic ionic liquid (PIL) and that it can act as a potential acid catalyst. With respect to the second type of ILs, we selected those based on l-methyl-3-n-butylimidazolium cation, characterised by a slighter HBD acidity than that of the alkylammonium type (Figs. 13.2, 13.3). These ILs exhibit a wide spectrum of physicochemical properties. It was demonstrated that their water content, density, viscosity, surface tension, melting point and thermal stability are affected by the length of the alkyl chain and the nature of the anion. Several anions were incorporated in this class of ILs. 

In this chapter, we presented an approach related to the molecular-microscopic properties of binary mixtures composed of two types of ILs - alkylammonium and dialkylimidazolium-based ILs - with molecular solvents of different structures. 

Mancini PME, Fortunate G, Adam C, Vottero LR (2008) Solvent effects on chemical processes. New solvents designed on the basis of the molecular-microscopic properties of binary mixtures molecular solvent + 1,3-dialkylimidazolium ionic liquids. J Phys Org Chem... 

Not surprisingly, these materials are very popular and enjoy a plethora of applications in various domains of the physical sciences, and an impressive number of spedahzed reviews and books has appeared dealing with their synthesis, physicochemical properties and appHcations in synthesis, catalysis and separation processes [12-26]. This section does not intend to be comprehensive on the vast area of synthesis and appHcations of ILs rather it will attempt to provide a critical update of the basic principles and latest developments on the structure and properties of ILs (mainly those based on the 1,3-dialkylimidazolium cation), and their... 

Preparation and Some Physico-Chemical Properties of 1,3-Dialkylimidazolium ILs... 

As already pointed out, of the various known ILs, those derived from the combination of the 1,3-dialkylimidazolium cation with various anions are the most popular and investigated class (Scheme 3.5-9). This is most probably due to their facility of synthesis stability and the possibility of fine-tuning their physico-chemical properties by the simple choice of the N-alkyl substituents and/or anions (Table 3.5-3). 

Cations The types of cations used in synthesizing ionic liquids are tetraalkylam-monium, triaUcylsulfonium, tetra-alkylphosphonium, 1-3-dialkylimidazolium, A -alkylpyridinium, A -A -dialkylpyrrolidinium, iV-aUcylthiazolium, A -A -dial-kyltriazolium, iV-A -dialkyloxazolium and A -A -dialkylpyrazolium. The most commonly used cations are 1-3-dialkylimidazolium and A -alkylpyridinium, due to their important physicochemical properties (60). 

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