Acidity constant , 196 Amphiprotic solvents

If the dielectric constant of an amphiprotic solvent is small, protolytic reactions are complicated by the formation of ion pairs. Acetic acid is often given as an example (denoted here as AcOH, with a relative dielectric constant of 6.2). In this solvent, a dissolved strong acid, perchloric acid, is completely dissociated but the ions produced partly form ion pairs, so that the concentration of solvated protons AcOH2+ and perchlorate anions is smaller than would correspond to a strong acid (their concentrations correspond to an acid with a pK A of about 4.85). A weak acid in acetic acid medium, for example HC1, is even less dissociated than would correspond to its dissociation constant in the absence of ion-pair formation. The equilibrium... 

Amphiprotic solvents have both acidic and basic properties in terms of the Bransted acid-base concept. If we denote an amphiprotic solvent by SH, it donates a proton by SH S +H+ and accepts a proton by SH + H+ = SH2. Overall, the autoprotolysis (autoionization) occurs by 2SH = SH2 + ST The extent of autoprotolysis is expressed by the autoprotolysis constant, Kjh = aSH2 aS, the values of which are also included in Table 1.5 as pKSH values (for more details, see Table 6.6). 

The characteristics of acid-base reactions in dipolar aprotic solvents, compared to those in dipolar amphiprotic solvents, are the easy occurrence of homo- and heteroconjugation reactions [2, 3, 5]. However, before discussing the homo- and heteroconjugations, we first discuss the solvent effects on the acid dissociation constants in dipolar aprotic solvents. 

Autoprotolysis constant — The ion-product calculated from the ion activities of the conjugate acidic and basic species of an -> amphiprotic solvent (SH). The chemical equation of such self-ionization reactions can be schematized as 2HS H2S+ + S , where H2S+ is the conjugate cation, S the conjugate anion. The autoprotolysis constant can be formulated as JCauto = [H2S+] ... 

From the practical point of view, nonaqueous solvents can be described broadly as acidic, basic, or neutral. For carrying out titrations, properties such as dielectric constant, melting point, boiling point, and (for amphiprotic solvents. Section 4-3) autoprotolysis constant are important (Table 6-1). 

Barbosa J, Bosch E, Cortina JL, and Roses M (1992) Ionic equilibria in amphiprotic solvents of low dielectric constants. Titration curves and buffer capacity of bases in anhydrous acetic acid. Analytica Chimica Acta 256 211-220. 

The pH concept is most commonly used for dilute aqueous media however, a similar formalism can be extended to other systems. The extent of the pH scale, which in aqueous media can be described as 14 units, depends on the autoprotolysis constant of the amphiprotic solvent, so that the equivalent range, e.g., in methanol, equals 16.7 units, in sulfuric acid 2.9 units, and in acetic acid 14.5 units. In such solvents, as in water, the pH of neutrality corresponds to the middle of this range. Such reasoning cannot be extended to protophilic (e.g., pyridine, ethers), and aprotic (e.g., hydrocarbons) solvents, for which the logan+ scale is from one or both sides, respectively, unlimited. 

A prediction of the probable behavior of a solute in an amphiprotic solvent can be made by comparing the acid constant of the solute when dissolved in water with the acid constant of the solvent when dissolved in water. For example, in water perchloric acid is about 10 times as strong as sulfuric acid. Its great proton-donating power probably applies also when sulfuric acid is the solvent. 

Values of the autoprotolysis constants of sulfuric acid and some other amphiprotic solvents, as determined by measuring the electric conductivity of the solvent and some of its solutions, are given in Table 12-1. 

The constants denoted K are known as conventional acidity and basicity constants. In these, S is absorbed into the rational constant - a procedure justified on account of the small extent of ionization of amphiprotic solvents. 

Glacial acetic acid, though like water in being classed as amphiprotic, represents a solvent type distinctly different from water in that it is a much weaker base. This weak basicity makes it a useful solvent for the titration of weakly basic substances. As mentioned in Section 4-3, the autoprotolysis constant has a p T h value of 14.45 for the reaction... 

Table 1 Ionization Constants of Neutral and Cation Acids of type HA and HB respectively, in Water, Amphiprotic, and Dipolar Aprotic Solvents...

The extent of the ionization step depends on the relative strength of the conjugate acid-conjugate base pairs. The amphiprotic properties of the solvent have an essential effect on the equilibrium constant of this reaction step. The extent of the dissociation step is influenced by the polarity of the solvent, increasing with the dielectric constant of the solvent. In water, all products of acid-base reactions of moderate to low concentrations are essentially completely dissociated into solvated ions (Pecsok et al., 1976). The dissociation step is suppressed by addition or substitution with cosolvents of lower polarity, e.g., alcohols in aqueous formulations. The ion-pair aggregates may have absorption spectra different from the dissociated species. Thus, the amphiprotic properties and polarity (expressed as the dielectric constant) of the solvent are essential for the acid-base equilibrium of the drug and thus the absorption spectrum of the compound. This subject is further discussed in Section 14.2.3. 

Table 1 Ionization constants of neutral and cation acids of type HA and HB+, respectively, in water, amphiprotic, and dipolar aprotic solvents.

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