Hydrogen-bonded solvents of low dielectric constant

Because of ion-pairing, medium effects are not uniform for transfer of acid-base equilibria from water or methanol to hydrogen-bonded solvents of low dielectric constant. Nor are acidity functions, as Hammett clearly recognised, applicable to such solvents. ... 

In the absence of charges and dipoles, short-range effects such as hydrogen bonding and hydrophobic hydration become important. The complex solute-solvent interactions in such cases, however, are not well understood. In addition there is the possibility of ion or ion-pair catalysis. Especially in cases with solvents of low dielectric constant, it has been observed that k3 rather than log k3 becomes linear in cs... 

Cd, Sn, and In, of medium CO selectivity, do not strongly adsorb CO2 . CO2 will be mostly freely present in aqueous electrolyte owing to the hydrogen bond stabilization by water molecules high dielectric constant of water molecule will also contribute to the stabihzation of CO2 . CO2 " stabihzed in the electrolyte will be further reduced to HCOO . However, CO2 is not sufficiently stabilized in nonaqueous electrolyte due to lack of hydrogen bond formation and low dielectric constant of the solvents. Thus CO2 adsorbed on Cd, Sn, and In may be relatively stabler than CO2 dissolved in the electrolyte. These metals yield CO in nonaqueous media in the same manner as Au. Ag, and Zn in Fig. 11(2). The CO selectivity mentioned above will be closely connected to the stability of adsorbed CO2 on the electrode. 

Liquid ammonia is the most extensively studied solvent and resembles water to a large extent. In both ammonia and water there is a fairly large degree of association due to hydrogen bonding. However, its low dielectric constant (22 at 240 K) resulting in a decreased solubility of certain ionic substances such as phosphates, sulphates and its restricted liquid range 195 K to 240 K makes it a less important solvent than water. In certain aspects particularly for non polar molecules it is a better solvent than water. 

Most neutral acids are much weaker acids in aprotic solvents than in water. This is due largely to their smaller dielectric constants, which increases the energy required for charge separation in the dissociation process. Table 7.9 summarizes the pATa values for several acids in five different solvents.62 There is a tendency in solvents with low dielectric constants, which cannot stabilize the anion of weak Brpnsted acids (HA) by hydrogen bonding, for the anion (A-) to hydrogen bond with the undissociated acid to yield the species AHA-... 

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