Ionic liquid solvents

Polar Solvents Ionic Liquids Nonpolar Solvents... 

Ionic liquids, which can be defined as salts that do not crystallize at room temperature [46], have been intensively investigated as environmentally friendly solvents because they have no vapor pressure and, in principle, can be reused more efficiently than conventional solvents. Ionic liquids have found wide application in organometallic catalysis as they facilitate the separation between the charged catalysts and the products. 

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. 

It has been shown that Diels-Alder reactions can be carried out successfully in a range of ionic liquids [12], As highly ordered H-bonding solvents, ionic liquids have the potential for dramatic effects as solvents for such reactions. The range of polarities which can be spanned by varying the cation or anion may be exploited and it has been shown that the endo exo ratio for the reaction between cyclopentadiene and methyl acrylate (Scheme 7.5) is dependent on the polarity of the ionic liquid used [13] (Table 7.2). When the reactions were carried out in a range of ionic liquids, the endo exo values were shown to correlate with the polarity as measured by the Ej scale. 

As solvents, ionic liquids can be uniquely tuned to a particular purpose by adjusting the anion/cation ratio. To decaffeinate coffee, for example, you could create an ionic liquid that would just dissolve caffeine and nothing else. Current research suggests that ionic liquids can be recovered from solution and reused. 

Ereemantle, M. Designer solvents—ionic liquids may boost clean technology development, Chem. Eng. News, 1998, 76(13), 32-37. 

In microwave-assisted synthesis, a homogeneous mixture is preferred to obtain a uniform heating pattern. For this reason, silica gel is used for solvent-free (open-vessel) reactions or, in sealed containers, dipolar solvents of the DMSO type. Welton (1999), in a review, recommends ionic liquids as novel alternatives to the dipolar solvents. Ionic liquids are environmentally friendly and recyclable. They have excellent dielectric properties and absorb microwave irradiation in a very effective manner. They exhibit a very low vapor pressure that is not seriously enhanced during microwave heating. This makes the process not so dangerous as compared to conventional dipolar solvents. The polar participants of organic ion-radical reactions are perfectly soluble in polar ionic liquids. 

Ionic liquids have drawn increasing interest as biotransformation media (21,270). In contrast to polar organic solvents, ionic liquids of higher polarity do not deactivate enzymes. Instead, in a number of investigations, high polarity in ionic liquids appeared to even lead to improved enzymatic performance (271). Furthermore, good solubility of polar substrates in ionic liquids can create a favorable situation for the biotransformations. 

Shvedene, N.V, Nemilova, M.Yu., Khachatryan, K.S., Mamonov, N.A., Shukhaev, A.V, Formanovsky, A.A., Pletnev, I.V, Extraction-voltammetric determination of catecholamines with the use of new-class solvents, ionic liquids, Moscow Univ. Chem. Bull., 45, 324-332, 2004. 

Hydrolytic enzymes such as lipases catalyze hydrolysis of esters in aqueous media, but when used in non-aqueous media such as organic solvents, ionic liquids and supercritical fluids, they catalyze reverse reactions the synthesis of esters. For example, lipases in natural environment catalyze the hydrolysis of fatty acid esters as shown in Figure 6(a). However, when they are used in organic solvents, they catalyze the esterification reaction (Figure 6(b)). 

Section 4.3 is devoted to electrodeposition in a special class of deep eutectic solvents/ionic liquids which are based on well-priced educts such as e.g. choline chloride. The impressive aspect of these liquids is their easy operation, even under air, as well as their large-scale commercial availability. One disadvantage has to be mentioned the choline chloride-based liquids especially are currently not yet... 

Abbott et al. [98-103] reported the synthesis and characterization of new moisture-stable, Lewis acidic ionic liquids made from metal chlorides and commercially available quaternary ammonium salts (see Chapter 2.3). They showed that mixtures of choline chloride (2-hydroxyethyltrimethylammonium chloride, [Me3NC2H40H]Cl and MCU (M=Zn, Sn) give conducting and viscous liquids at or around room temperature. These deep eutectic solvents/ionic liquids are easy to prepare, are water-and air-stable, and their low cost enables their use in large-scale applications. Furthermore, they reported [104] that a dark green, viscous liquid can be formed by mixing choline chloride with chromium(III) chloride hexahydrate and that the... 

The strategy best developed to date consists in the use of new types of solvents ionic liquids, fluorous phase chemistry, supercritical carbon dioxide, and biosolvents. Other technologies include the use of water as solvent, and the solventless approach which avoids the use of such substances. 

Extraction may also be performed with supercritical solvents. Ionic liquids have been described as designer solvents that is, their properties can be adjusted to suit the requirements of a particular process. Properties such as melting point, viscosity, density, and hydrophobicity can be modified by simple changes in the structure of the ions (Table 12.4). For example, the melting points of... 

Another special class of solvent, ionic liquids such as l-butyl-3-methylimidazolium hexaflurorophos-phate, was used for the polymerization of MMA with 1-24 (X = Br)/CuBr/L-9 (R = n-Pr), which proceeded even at 30 °C, reached 90% conversion within 5 h,... 

Garcia, S. Lourenco, N.M.T. Lousa, D. Sequeira, A.F. Mimoso, P. Cabral, J.M.S. Afonso, C.A.M. Barreiros, S. A comparative study of biocatalysis in non-conventional solvents ionic liquids, supercritical fluids and organic media. Green Chem. 2004, 6 (9), 466-470. 

Unlike typical organic solvents, ionic liquids tend not to give off vapors hence they are less hazardous and more convenient in the laboratory, and are less likely to pose air pollution problems. Products and chemical catalysts can be recovered or extracted easily from ionic liquids and then the liquid can be recycled and used over and over again. Reactions that occur in organic solvents have been the standard way to make countless products. Now, many of these old reactions are carried out in these new solvents. 

Innumerable production processes and applications rely on the proper selection of solvents. Organic solvents, although most widely used, are being slowly replaced with aqueous and supercritical solvents. Ionic liquids are still in the research stage and commercial applications are just beginning to appear. Solvent mixtures are used in many applications where a single solvent cannot do the job. 

Most widely used are N,N -dialkyhrnidazohum salts, since they are easily prepared. Ionic liquids have been used as solvents for numerous reactions. Their physical and chemical properties vary with the combination of cation and anion. This allows a degree of tuning of their properties. Since they are highly polar solvents, ionic liquids can dissolve many inorganic salts and transition metal complexes, and often form biphasic mixtures with non-polar organic solvents. Thus, organic products can be extracted from ionic liquids, while ionic transition metal catalysts are immobilized. Volatile products can be easily distilled off from ionic liquids, since the latter show no volatility [17]. 

Since 1980s, a large number of studies on the electrodeposition of metals have been reported. Historically, the water-sensitive chloroaluminate first-generation ILs are the most intensively studied. However, in future, the focus will rather be on air-and water-stable ionic liquids due to their variety and the less strict conditions under which they can be handled. Several review articles, books, and book chapters on the electrodeposition of metals and alloys from ILs have already been published [27-183], Ionic liquids can be classified as water/air sensitive (first-generation ionic liquids based on AIX3 (X=C1, Br)) and water/air stable (discrete anions based and eutectic solvents/ionic liquids such as ZnCl, urea, ethylene glycol, and choline chloride). A selection of published examples of the electrodeposition of metals and alloys from ionic liquids is listed in Table 5.3 the original work for each metal can be found in Refs. [23, 29-183], In this section, we mainly focus on the electrodeposition of active metals such as Al, Mg, and Ti in ionic liquids. 

---