Nitrobenzene, relative permittivity

In contrast, dipolar aprotic solvents possess large relative permittivities (sr > 15), sizeable dipole moments p > 8.3 10 ° Cm = 2.5 D), and average C.f values of 0.3 to 0.5. These solvents do not act as hydrogen-bond donors since their C—H bonds are not sufficiently polarized. However, they are usually good EPD solvents and hence cation sol-vators due to the presence of lone electron pairs. Among the most important dipolar aprotic solvents are acetone, acetonitrile [75], benzonitrile, A,A-dimethylacetamide [76, 77], A,A-dimethylformamide [76-78], dimethylsulfone [79], dimethyl sulfoxide [80-84], hex-amethylphosphoric triamide [85], 1-methylpyrrolidin-2-one [86], nitrobenzene, nitro-methane [87], cyclic carbonates such as propylene carbonate (4-methyl-l,3-dioxol-2-one) [88], sulfolane (tetrahydrothiophene-1,1-dioxide) [89, 90, 90a], 1,1,3,3-tetramethylurea [91, 91a] and tetrasubstituted cyclic ureas such as 3,4,5,6-tetrahydro-l,3-dimethyl-pyr-imidin-2-(l//)-one (dimethyl propylene urea, DMPU) [133]. The latter is a suitable substitute for the carcinogenic hexamethylphosphoric triamide cf. Table A-14) [134]. 

Figure 1. (a) Relative permittivity, k, as a function of temperature, showing melting and freezing along with the hysteresis, (b) Shift in the melting temperature ATm as a function of l/H for nitrobenzene in CPG. The linear behavior is consistent with the Gibbs-Thomson equation. 

Many examples exist of interfaces formed between two immiscible liquids. A well-known one is the interface between a long-chain hydrocarbon, for example, dodecane, and water, which is commonly known as the oil water interface. Dodecane and water are immiscible because the hydrocarbon phase is nonpolar. Liquid liquid interfaces are also formed between water and organic liquids with polar groups such as octanol and heptanoic acid, which also have rather long hydrocarbon chains. The polar liquid nitrobenzene, which has a relative permittivity of 35, is also immiscible with water. Another well-known system is the mercury polar liquid interface. This has been studied extensively, especially for aqueous electrolyte solutions. However, the mercury polar liquid interface is also an example of a metal solution interface which was considered in the previous section. The discussion here is limited to liquids with relative dielectric permittivities falling in the range 1-200, and systems which have poor conductivities as pure liquids. 

Subsequently, the water-nitrobenzene interface will be described and experimentally studied. The reason for using nitrobenzene, which is not a physiologically occurring environment, is its high relative permittivity (e = 34.8 at 25 °C), which makes it very convenient for performing many studies. When the experimental techniques are perfected, in particular, when high resistance of less polar solvents can be overcome, the conclusions and experience can be explained to naturally occurring lipophilic environments with lower relative permittivity. 

However, if a reference substance, which does not dissociate in a solvent of low donicity but high relative permittivity (e.g., nitrobenzene or nitromethane), is titrated with a donor solvent, the polarization ensured by coordination of the donor solvent is continued in dissociation in the system with high relative permittivity (see p. 39) conductometric monitoring of this process is suitable for characterization of the donor strength of the titrant solvent. Thus, if trimethyltin(I V) iodide is titrated with various donor solvents in nitrobenzene solution, at any given D (CH3)3SnI ratio the conductivity of the solution depends on the donicity of the solvent (Fig. 4.2). With increasing donicity the conductivity increases [Gu 68]. 

From these results one can conclude that for the formation of the monomeric solvates a solvent of high donicity and relative permittivity is necessary (e.g., ethanol). High relative permittivity (nitrobenzene) or high donicity (pyridine) alone... 

Liszi [Li 75] studied the phenomenon of dipole association in nitrobenzene-carbon tetrachloride, nitrobenzene-n-heptane and nitrobenzene-benzene mixtures, i.e., mixtures in which the second component is an apolar solvent. With a knowledge of the densities, he used the dielectrometrically (at a constant frequency of 3 MHz) determined relative permittivities of the solutions to calculate the molar... 

Most BF3 affinities in Table 3.3 are primary values measured in dichloromethane. This solvent was chosen because it can dissolve most BF3 complexes, while being rather inert (low hydrogen-bond acidity, Lewis basicity and relative permittivity). Nitrobenzene also exhibits good solvent properties towards BF3 complexes, and some measurements were run on representative bases, in both C6H5NO2 and CH2CI2. A very good linear relationship was observed [5] between the enthalpy changes of reaction 3.4 in dichloromethane and nitrobenzene ... 

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