1.3- Dialkyl imidazolium cations

Ionic liquids are a class of solvents and they are the subject of keen research interest in chemistry (Freemantle, 1998). Hydrophobic ionic liquids with low melting points (from -30°C to ambient temperature) have been synthesized and investigated, based on 1,3-dialkyl imidazolium cations and hydrophobic anions. Other imidazolium molten salts with hydrophilic anions and thus water-soluble are also of interest. NMR and elemental analysis have characterized the molten salts. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. The influence of the alkyl substituents in 1,2, 3, and 4(5)-positions on the imidazolium cation on these properties has been scrutinized. Viscosities as low as 35 cP (for l-ethyl-3-methylimi-dazolium bis((trifluoromethyl)sulfonyl)amide (bis(triflyl)amide) and trifluoroacetate) and conductivities as high as 9.6 mS/cm were obtained. Photophysical probe studies were carried out to establish more precisely the solvent properties of l-ethyl-3-methyl-imidazolium bis((trifluoromethyl)sulfonyl)amide. The hydrophobic molten salts are promising solvents for electrochemical, photovoltaic, and synthetic applications (Bon-hote et al., 1996). 

Matsumi et al. (2009) published novel ILs bearing an ortfto-carborane anion with different 1,3-dialkylated imidazolium cations (l-ethyl-3-methyl, l-butyl-3-methyl and l-butyl-3-ethyl residues) (Figure 29.13). They are prepared easily from the carbene obtained from imidazolium halides. The ionic conductivity of the [EM1M]+ salt, 2.9 x 10" S cm at 51°C, is quite low compared to other ILs. The melting point of this ortfto-carborane salt (30°C) is lower than the melting points for [EMIM]+ salts of the carboranes [1-C3H7CB H ] (45°C) and [1-C4H9CB H ] (40°C) described by Larsen et al. (2000). 

Suarez P A Z, Selbach V M, Dullius J E L, et al. Enlarged electrochemical window in dialkyl-imidazolium cation based room-temperature air and water-stable molten salts. Electrochim. Acta. 1997. 42, 2533-2535. 

The most common salts in use are those with alkylammonium, alkylphosphonium, N-alkylpyridinium and N,N -dialkyl imidazolium cation (Scheme 1). 

RTILs consist of large, unsymmetrical ions, such as 1,3-dialkyl-imidazolium, 1-alkylpyridinium, 1-alkylpyrazolium, tetralkylammonium or tetralkyl-phosphonium cations and tetrachloroaluminate, tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonate or bis((trifluoromethyl) sulfonyl)imide anions. 

This method is not only limited to halide-based ionic liquids. Synthesis of ionic liquids containing a tetrafluoroborate anion and imidazolium cation has been performed successfully [16] by use of a modified domestic microwave oven with pulsed irradiation (5 x 30 s). Approximately 90% yields of the desired ionic liquids were usually obtained compared with 36% after the same reaction time using conventional heating. Microwave irradiation has been used in the synthesis of l-ethyl-3-methylimidazolium benzoate and dialkyl imidazolium tetrachloroaluminate ionic liquids [17, 18]. These reactions were again performed in a domestic microwave oven using pulsed irradiation. 

Ionic liquids are molten salts, which are liquids at or close to room temperature. While the cations are generally large organic compounds such as A, iV -dialkyl-imidazolium, A -alkyl-pyridinium, and iV,iV-alkylpyrolinium, the anions are smaller, usually AlCU , HF2, BF4, PFs , SbFs , CFsSOs , etc. (Figures 30 and and some of them are... 

Cationic nanoclays were treated with various novel modifiers based on ionic liquids of different structure and/or molecular weight. Initial experiments with short chain dialkyl imidazolium and monoalkyl pyridinium based modifiers were followed by treatments with phosphonium based ionic liquids having longer chain cations. The modified clays, characterized by spectroscopy, thermal analysis, microscopy and X-ray diffraction, were melt compounded with polypropylene. The effects of the ionic hquid structure and chain length on extent of clay dispersion, intercalation and thermal stabihty were investigated. 

ILs viscosity iuCTeased when the chain lengA of the side chain increases, the side chain is composed of fluoride or an imidazole ring in position 2 methylation, or there is increased cation symmetry. Generally, when the cation is composed of the thermally stable imidazolium ion, the anion determines the thermal stability of the entire IL, increasing in the order TfjN >BF >PF ">halogen ions. Minami et al. [18] studied thermo-oxidative stability and volatility of ILs and found that ILs have greater thermal stability than conventional synthetic lubricants. It was also found that the stability of an ILs depends on the structures of both the anion and the cation. For example, the stability of 1, 3-dialkyl-imidazoUum is more stable than quaternary ammonium. 

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