Nonaqueous solvents dissociation constants

The attempt to use these salts originated from the hope that their dissociation constants would be high even in low dielectric media, and the organic nature of perfluorinated alkyls would always assist the solubility of the salts in nonaqueous solvents. Because of the requirement for electrochemical stability, lithium carboxylates (RF-C02Li, where Rp- = perfluorinated alkyls) are excluded from consideration, because their oxidation still occurs at - 3.5 V vs lithium, which is similar to the cases of their non-fluorinated counterparts. Obviously, the electron-withdrawing groups do not stabilize the carboxylate anions sufficiently to alter their oxidative stability. 

One major drawback of these sulfonate salts is their poor ion conductivity in nonaqueous solvents as compared with other salts. In fact, among all the salts listed in Table 3, LiTf affords the lowest conducting solution. This is believed to be caused by the combination of its low dissociation constant in low dielectric media and its moderate ion mobil-ityi29 3 compared with those of other salts. Serious ion pairing in LiTf-based electrolytes is expected, especially when solvents of low dielectric constant such as ethers are used. ... 

The higher dielectric constant of NMA, compared to water, might be expected to promote dissociation of acids but studies in other nonaqueous solvents and in mixed solvents indicate that the dielectric constant is seldom the predominant factor controlling acid dissociation processes199). Instead, whether a particular acid is stronger in one solvent or another, will likely be quite dependent on the relative solvation of the acid, of the proton and of the conjugate base in the two solvents. 

Regarding hydrochloric acid, in a concentration range of 30.10 4 to 300.10 4 mol/L, equivalent conductance assumes an extremely low and constant value of 0.03 S cm2/mol, as seen in Figure 3. This behavior certainly cannot be explained on the basis of simple dissociation phenomena. Thus we have interpreted these results on the basis of theoretical work by Caruso and co-workers (31) who consider the conductometric, potentiometric, and spectrophotometric behavior of weak acids and bases in nonaqueous solvents. In these solvents a weak acid, HA, besides undergoing simple ionic dissociation, also may undergo conjugation phenomena by the H+ and A" ions which lead to the formation of ionic complex species A(HA)/ or H(HA)/. Caruso shows that the... 

When the dielectric constant of the nonaqueous solvent goes below about 15, ions can associate not only in ion pairs but also in ion triplets. This comes about by one of the ions (e.g., M )ofan ion pairM -A Coulombically attracting a free ion A strongly enough to overcome the thermal forces of dissociation... 

In order to determine to what extent these speculations have validity, it is necessary to be able to evaluate more quantitatively the relative contributions of these interactions to the free energies of protein and nucleic acid molecules in water and nonaqueous solvents. For this purpose, a substantial body of quantitative data is required concerning the properties of suitable model compounds in a variety of solvents, including their solubilities, acid-base dissociation constants, and thermodynamics of hydrogen bond formation. The dearth of pertinent data on hydrogen bonds in solvents of interest is particularly frustrating to even a semiquantitative evaluation of the scheme presented in Fig. 7. 

We can write equilibrium constants for many types of chemical processes. Some of these equilibria are listed in Table 6.1. The equilibria may represent dissociation (acid/base, solubility), formation of products (complexes), reactions (redox), a distribution between two phases (water and nonaqueous solvent—solvent extraction adsorption from water onto a surface, as in chromatography, etc.). We will describe some of these equilibria below and in later chapters. 

The principles governing conductivity in nonaqueous solvents are the same as those for aqueous solutions, of course. The dependence of the conductivity on the viscosity of the solvent was discussed in Section 31.11. However, in solvents having low dielectric constants, there is a lessening of the degree of ionization of many substances. Electrolytes that are completely dissociated in water may be only partially dissociated in a low dielectric constant solvent. Hydrochloric acid is completely dissociated in water HCl is a strong acid. In ethyl alcohol, however, HCl is a half-strong acid, with a dissociation constant of about 1.5 X 10. ... 

Leffler EB, Spencer HM and Burger A, Dissociation constants of adrenergic amines,/ACS, 73,2611-2613 (1951). Cited in Chatten LG and Harris LE, Relationdiip between pKb(H20) of organic compounds and E1/2 values in several nonaqueous solvents. Anal. Chem., 34, 1495-1501 (1962). NB See Amphetamine for details. From... 

To a good approximation, there is an inverse relationship between the viscosity and the resulting current doubling the buffer viscosity will halve the current and ATRadiai- In nonaqueous solvents, electrical currents are generally lower than that for the same electrolyte dissolved in water, but predicting ATRadiai becomes complex as one needs to take into account variations in the mobilities of species, dissociation equilibria, dielectric constant, zeta potential, and thermal conductivity [18]. 

Kinetic parameters have been established for solvolysis of the pentacyanofer-rate(III) derivative [Fe(CN)5(N02)] . For aquation, which is acid-catalyzed at pH <5, A//= = 43kJmol-, =-80 J K" mol", and A =+2 cm" mol". Intrinsic and solvational contributions are presumably closely balanced in the case of A Rate constants for solvolysis of [Fe(CN)5(N02)] in water, methanol, dimethyl sulfoxide, and dimethylformamide correspond with the electron-donating abilities of the respective solvents. Activation volumes for the nonaqueous solvents, between +20 and +27 cm" mor reflect the dissociative nature of these solvoly-ses. " Rate constants for dissociation of the [Fe(CN)5(2,6-Me2pyrazine)]" anion in binary aqueous solvents containing methanol, acetone, or acetonitrile correlate well with acceptor numbers for the respective media, though with a very different... 

When an acid dissociates, as expressed by the relation HA A -i- H+, a major concern is the extent to which the solvent stabilizes the species involved, and in particular the ionic species A and H+, since these species are normally more strongly affected. This stabilization will depend on both the nature of the solute and the characteristics of the solvent, such as its dielectric constant and whether it is protic or nonprotic. Solvents that stabilize ions poorly will drive the equilibrium to the left, decreasing the acidity and increasing the pK. Because water is an exceptionally good solvent for ionic species, the pK s observed in almost all other solvents are ligher than those in water. A list of several properties of water and some common nonaqueous solvents is given in Table 8.1. 

An ideal electrolyte solute in lithium-ion cells completely dissolves and dissociate, in the nonaqueous media, and the solvated ions should be able to move in the media with high mobility, should be stable against oxidative decomposition at the positive electrode, should be inert to electrolyte solvents and other cell components, and should be nontoxic and remain stable against thermally induced reactions with electrolyte solvents and other cell components. LiPF6 is one of the most commonly used salts on commercial Li-ion cells. The success of LiPF6 was not achieved by any single outstanding property but, rather, by the combination of well-balanced properties, namely, conductivity, ionic mobility, dissociation constant, thermal stability, and electrochemical/chemical stability. 

A comprehensive review with 105 references was presented by Goddu and Hume in 1954 Reilley and Schweitzer discussed photometric titrations in a nonaqueous solvent Grunwald provided a method for calculating photometric endpoints. King and Hirt described their spectrotitrimeter, a device for determining dissociation constants by ultraviolet spectrophotometry. 

Detailed work by Davis and Hetzer (1951-1958) on acid-base systems in nonaqueous liquids such as benzene constitutes an attempt to clarify the position relative to the Bronsted-Lowry and the Lewis concepts of acidity. Even in this work, however, liquids such as benzene were described as inert because they do not give rise to significant quantities of free hydrogen ions. It was asserted that in such solvents the product of the interaction of a base with a Bronsted acid and the product of the interaction of that base with a Lewis acid are essentially of the same type, a highly polar addition compound. For organic bases in liquids such as benzene, these authors urged the adoption of association rather than dissociation constants A+B C. They referred to spectroscopic evidence showing that the anion is not completely detached from the proton of the cation. In the system... 

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