1- Methylimidazole, ionic liquid

A typical example of a volatile impurity that can be found as one of the main impurities in low-quality ionic liquids with alkylmethylimidazolium cations is the methylimidazole starting material. Because of its high boiling point (198 °C) and its strong interaction with the ionic liquid, this compound is very difficult to remove from an ionic liquid even at elevated temperature and high vacuum. It is therefore important to make sure, by use of appropriate allcylation conditions, that no unreacted methylimidazole is left in the final product. 

Traces of bases such as methylimidazole in the final ionic liquid product can play an unfavorable role in some common applications of ionic liquids (such as bipha-sic catalysis). Many electrophilic catalyst complexes will coordinate the base in an irreversible manner and be deactivated. 

A number of different methods to monitor the amount of methylimidazole left in a final ionic liquid are known. NMR spectroscopy is used by most academic groups, but may have a detection limit of about 1 mol%. The photometric analysis described by Holbrey, Seddon, and Wareing has the advantage of being a relatively quick method that can be performed with standard laboratory equipment [13]. This makes it particularly suitable for monitoring of the methylimidazole content during commercial ionic liquid synthesis. The method is based on the formation and colorimetric analysis of the intensely colored complex of l-methylimidazole with cop-per(II) chloride. 

The synthetic routes used to prepare ionic liquids vary depending upon the ionic liquid being made. Ionic liquids with metal halide anions are, at least in principle, very simple to prepare. Scheme 4.1 illustrates the synthesis of imidazolium-based ionic liquids with a chloroaluminate anion, commencing with methylimidazole [6],... 

In the BASF BASIL process that utilizes A-methylimidazole to scavenge HCl byproduct, the acidic ionic liquid A-methylimidazolium chloride [HMIMJCl was formed, with a melting point of 75°C (13,102). Recently, the group of Bronsted acidic ionic liquids with the same cation was extended to include other anions, such as BFF, TfO , and TsO . The melting point of the salt is between 30 and 109°C. Strong hydrogen bonding in the tosylate salt was characterized by IR spectroscopy. 

The primary advantage in the first step of the method described here (using 1-chlorobutane diluted in MeCN) is that it eliminates long reaction periods and allows the use of secondary alkyl halides without competitive elimination reactions. For example, the reaction of sec-butyl bromide with N-methylimidazole using the classical method (in neat alkyl halide) produces, along with the desired product, 20-30% of butenes and 1-methylimidazole hydrobromide. In the second step, the use of water as solvent allows the anion metathesis reaction to be quantitative in a very short time and allows the easy purification of the ionic liquids. Moreover, employing the potassium salt avoids the use of corrosive and difficult to handle hexafluorophosphoric add and the expensive silver tetrafluoroborate. ... 

Holbrey, J. D., Turner, M. B., Reichert, W. M., and Rogers, D. R., New ionic liquids containing an appended hydroxyl functionality from the atomefficient, one-pot reaction of 1-methylimidazole and acid with propylene oxide, Green Chem., 5, 731-736, 2003. 

In their test system, the researchers used the ionic liquid l-butyl-3-methylimidazol-ium hexafluorophosphate (bmim)(PF6), which is stable in the presence of oxygen and water, with naphthalene as a low-volatility model solute. Spectroscopic analysis revealed quantitative recovery of the solute in the supercritical CO2 extract with no contamination from the ionic liquid. They found that CO2 is highly soluble in (bmim)(PF6) reaching a mole fraction of 0.6 at 8 MPa, yet the two phases are not completely miscible. The phase behavior of the ionic liquid-C02 system resembles that of a cross-linked polymer-solvent system (Moerkerke et al., 1998), even though... 

The transition states (27) calculated at the B3LYP/6-31+-l-G and B3LYP/6-31G levels of theory for the alkylation of 1-methylimidazole (26) by alkyl bromides and chlorides, forming imidazolium halide ionic liquids (28),110 had a hydrogen bond between the incipient halide ion and the hydrogen on C(2) of the substrate. 

BASF s original acid scavenger, triethylamine, created viscosity and work-up problems because of the need to maintain anhydrous conditions. BASF s use of 1-methylimidazole instead of triethylamine, at their reaction temperature of 80°C, led to the formation of two liquid phases, an upper diethoxyphenylphosphine phase, and a lower methylimidazolium chloride phase (fortuitously, this ionic liquid melts at 75°C ). Moreover, methylimidazolium chloride proved to be a nucleophilic catalyst. The processing revolution generated by these discoveries enabled creation of a high-productivity continuous process for diethoxyphenylphosphine manufacture and created a whole new business in acid scavenging technology. 

One interesting application of an ionic liquid does not involve its use as a solvent. The company BASF used N-methylimidazole to scavenge the HC1 formed in the reaction of dichlorophenyl phosphine with ethanol (Fig. 7.33). This results in the formation of N-methylimidazoliumchloride (Hmim-Cl), which has a... 

Sulfonic acid functionalised ionic liquids may also be prepared via a zwitterionic intermediate from a Michael-type addition, as shown in Scheme 2.5.[84 85] In the first step 1-methylimidazole reacts with the sulfonic acid precusor 1,3-propane sultone to form a zwitterionic intermediate. Protonation with Bronsted acids affords ionic liquids with high purity that have proven to be highly efficient reaction media in, for example, esterification reactions.[86] Task-specific ionic liquids may also be prepared using semi-combinatorial methods.1871... 

One large scale application of an ionic liquid is as a base to remove HCl during the preparation of alkoxyphenylphosphines. BASF use V-methylimidazole which forms the ionic liquid [Hmim]Cl and forms a second liquid layer that may easily be removed from the system and recycled via phase separation. Although the ionic liquid is not employed as a solvent, the process makes use of biphasic technology to give a more efficient process which avoids an inconvenient filtration step. 

When the temperature is changed, the Nyquist plot also changes, as shown in Figure 6.4 , due to the change in conductivity. Figure 6.4 shows the Nyquist plots for the ionic liquid prepared by the neutralization of l-benzyl-2-methylimidazole and HTFSI. The temperature dependence of the ionic conductivity is generally depicted by an Arrhenius plot (Figure 6Ab). 

Synthesis of 1,3-dialkylimidazolium ionic liquids does not require very much effort. One can start from commercially available 1-methylimidazole for a one-step or imidazole for a two-step synthesis (Scheme 3). 

In a preliminary study, the effect of ionic liquid-inherent impurities on the peak area A (and hence the vapour pressure) of a solute was investigated using the example of toluene in [C6mim]Cl, to which either water or 1-methylimidazole were deliberately added to simulate the most likely impurities occurring in ionic liquids. 

Fig. 3 Effect of the content of the impurities water (filled circles) and 1-methylimidazole (open circles) on the resulting peak areas (and hence vapour pressure) of 1 pL toluene in [C6mim]Cl (ionic liquid + impurity = 7.872 mmol) [74]...

Ionic liquids Table 1 displays the source, water and 1-methylimidazole content of the ionic liquids investigated, determined by Karl-Fischer titration and HPLC, respectively [180], Own syntheses were carried out according to literature methods [29, 181]. 

Jonathan and Mikami [49] reported a straightforward synthesis of new chiral ionic liquids bearing an imidazolium core, an easy and efficient method (6). Commercially available ethyl lactate was converted into its triflate derivative which upon reaction with 1-methylimidazole gave the triflate salt as a solid in excellent yield. Then anion metathesis was performed to obtain different anions with imidazole cation ... 

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