Dielectric constant, ionic liquids, solvent

The rates for the Sn2 reactions of seven different anionic nucleophiles (Cl-, Br-, I-, OAc-, CN-, SCN- and trifluoromethylacetate-) with methyl / -nitrobenzenesulfonate have been determined in CH2CI2, MeOH, DMSO, and three ionic liquid solvents.122 The reactivity was not correlated with the dielectric constant for the solvents as predicted by the Hughes-Ingold rules and a different nucleophilic order was found in... 

Styrene and other monomers can be polymerized by potassium amide in liquid ammonia. The dielectric constant of the solvent is quite high 22) and this is one of the ionic systems in which the active centers behave kinetically as free ions. Initiation involves the dissociation of potassium amide followed by addition of amide ion to the first monomer unit ... 

Table 12.2 gives the properties of sub- and supercritical water. Subcritical water is the water that is in a state under a pressurized condition at temperatures above its boiling point under ambient pressure and below the critical point Tc = 374°C Pc = 22.1 MPa, pc = 320 kg/cm ). The dielectric constant of liquid water decreases with increasing temperature (Nanda et al., 2014b). At temperatures from 277 to 377°C, the dielectric constant becomes as low as those of polar organic solvents. The ionic product of water is maximized at temperatures between 227 and 372°C depending upon the pressure (Kruse and Dinjus, 2007). Thus, subcritical water acts as acid and/or base catalysts for reactions, such as hydrolysis of ether/ester bonds, and also as a solvent for the extraction of low molecular mass products (Brunner, 2009). 

Because of its low dielectric constant, Hquid hydrogen sulfide is a poor solvent for ionic salts, eg, NaCl, but it does dissolve appreciable quantities of anhydrous AlCl, ZnCl2, FeCl, PCl, SiCl, and SO2. Liquid hydrogen sulfide or hydrogen sulfide-containing gases under pressure dissolve sulfur. At equihbrium H2S pressure, the solubihty of sulfur in Hquid H2S at —45, 0, and 40°C is 0.261, 0.566, and 0.920 wt %, respectively (98). The equiHbria among H2S, H2S, and sulfur have been studied (99,100). 

Water, however, is a wonderful solvent for ionic-bonded substances such as salt. The secret to its success lies in the electric dipoles created by the polar covalent bonds between the hydrogen and oxygen atoms. In water, the polar bonds are asymmetric. The hydrogen side is positive the oxygen side is negative. One measure of the amount of charge separation in a molecule is its dielectric constant. Water has a dielectric constant that is considerably higher than that of any other common liquid. 

There is an extensive chemistry associated with the use of liquid ammonia as a nonaqueous solvent (see Chapter 10). Because it has a dielectric constant of 22 and a dipole moment of 1.46 D, ammonia dissolves many ionic and polar substances. However, reactions are frequently different than in water as a result of differences in solubility. For example, in water the following reaction takes place because of the insolubility of AgCl ... 

The properties of HF reflect the strong hydrogen bonding that persists even in the vapor state. As a result of its high polarity and dielectric constant, liquid HF dissolves many ionic compounds. Some of the chemistry of HF as a nonaqueous solvent has been presented in Chapter 10. Properties of the hydrogen halides are summarized in Table 15.9. 

Miscibility is an important consideration when selecting solvents for use in biphasic systems. Table 4.4 shows the miscibility of three ionic liquids with water and some organic solvents. [bmim][PFe] was found to be miscible with organic solvents whose dielectric constant is higher than 7, but was not soluble in less polar solvents or in water. Basic [bmim][AlCl4] was found to react with protic solvents, and the acidic form also reacted with acetone, tetrahydrofuran and toluene. 

Until very recently there was no information about how an MM pair potential should look, based upon calculations from the deeper BO level. In the simplest BO level model for an ionic solution the solvent molecules are represented as hard spheres with centered point dipoles and the ions as hard spheres with centered charges. Now there are two sets of calculations, (16,17) by very different approximation methods, for this model where all of the spheres are 3A in diameter, where the dipole moments are near 1 Debye, and where the ions are singly charged. The temperature is 25° and the solvent concentration is about 50M, corresponding to a liquid state. The dielectric constant of the model solvent is believed to be near 9 6. 

It is therefore apparent that dissociation constants may only be compared in the same solvent. Ammonia is a stronger donor than water, but liquid ammonia has a much lower dielectric constant than the latter. The acidity constant of hydrochloric acid in liquid ammonia is much lower than in water, in which it is completely ionized and completely dissociated, whereas the complete ionization in liquid ammonia is not followed by extensive ionic dissociation due to its low dielectric constant. On the other hand, the acidity constant of acetic acid is somewhat higher in liquid ammonia than in water since in the latter if Ion is much lower than in liquid ammonia, in which complete ionization is achieved. 

There are several physical properties of a solvent that are of importance in determining its behavior. Two of the most important from a pragmatic point of view are the melting and boiling points. These determine the liquid range and hence the potential range of chemical operations. More fundamental is the permittivity (dielectric constant). A high permittivity is necessary if solutions of ionic substances are 10 form readily. Coulombic attractions between ions are inversely proportional to the permittivity of the medium ... 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

The universal interaction is due to the collective influence of the solvent l a dielectric medium and depends on the dielectric constant D and the (Tractive index n of the solvent. Reasonably large environmental pertur-fions may be caused by van der Waals dipolar or ionic fields in solution, in solids or in gases at high pressures. In gases, liquids and perturbation must be averaged over the molecular... 

Selenium oxychloride is not the only inorganic solvent used for liquid lasers, several studies use as solvent phosphorus oxychloride, POCl3 (Blumenthal et al., 1968 Brecher and French, 1969 Brinkschulte et al., 1972). Although this solvent is a worse host for ionic salts due to its lower dielectric constant, it is much more attractive for liquid laser uses since it is far less toxic and corrosive than SeOCl2. The approach for the preparation of POCl3 solutions is slightly... 

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