Imidazolium substituent alkyl chain length

Similar to the imidazolium ILs, very low solubility of the ammonium ILs was observed in alkanes. Eor example, the solubility of butyl(2-hydroxyethyl) dimethylammonium bromide, [(Ci)2C4HOC2N]Br, exhibited a simple eutectic system with immiscibility in the liquid phase in the IL mole fraction range from XjL = 0.02 to 0.70 [53] the other ammonium salt, (benzyl)dimethyl-alkylammonium nitrate, [Be(Ci)2C N] [NOJ showed very small solubility in the liquid phase in hexadecane and it was slightly better in hexane (immiscibility from XjL = 10 to 0.90) [99] for the ammonium salt with an alkane substituent only, didecyldimethylammonium nitrate, [(Cio)2(Q)2N][N03], the solubility in the liquid phase was better in hexane (immiscibility from XjL = 10 to 0.50) than in hexadecane, where the immiscibility was observed in the whole IL mole frachon [100]. In all systems with imidazolium and ammonium salts, an increase in the alkyl chain length of the alkane (solvent) resulted in a decrease of solubility. 

Lin et al. have studied variations in the chemical shifts for a variety of methyl-imidazolium salts with different alkyl chain substituents and Br, BF , and BF" anions [8]. They observed a high sensitivity in chemical shift (depending, among other factors, predominantly on the alkyl chain length) for the 2-proton in the imidazolium ring. The effect was most pronounced for the bromide salts. Additionally, they observed unexpected H/D exchange for the 2-proton. Today, these results can be interpreted as H-bonding effects, and of course the acidity of H2 comes to no surprise ([3,9] and references therein). 

Chemical and physical properties of ILs depend on the combination of cations and anions, and their nature and possible substituents. In ILs, forces between the ions are predominantly Coulomb forces. Examples for bulky organic and often nonsymmetrical cations are imidazolium, pyridinium, pyr-rolidinium, piperidinium, ammonium, and phosphonium, which all can carry different alkyl chains (Figure 29.1). The modification of the cation by A-alkylation plays an important role in the design of ILs as well. In general, an increasing chain length and hence an increasing size of the cation... 

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