Acid-Base Equilibria in Nonaqueous Solvents

Acid-base reactions in solvents other than water are of both theoretical and practical significance, and their fundamental chemistry is becoming increasingly understood. It should be realized at the outset that solvents play an active rather than a passive role in acid-base reactions and that water as a solvent, though of unique importance, is highly atypical. The important considerations are general dielectric-constant efiects, acidic behavior and basic behavior of solvents, and specific interactions of solvent with solute. 

The dielectric constant is a property of major concern in understanding acid-base behavior in various solvents. When the dielectric constant of a solvent is low, ion association and homoconjugation can take place, resulting in modification of otherwise simple proton transfer reactions. 

In this chapter several general topics important to understanding acid-base systems are considered and then illustrated by acid-base reactions in three typical solvent types. Finally, pH measurements in solvent mixtures and the Hammett acidity function are discussed. 

A study of the acid-base properties of solutes in nonaqueous solvents must include consideration of hydrogen ion activities and in particular a comparison of their activities in different solvents. Attempting to transpose interpretations and methods of approach from aqueous to nonaqueous systems may lead to diflSculty. The usual standard state (Section 2-2) for a nonvolatile solute is arbitrarily defined in terms of a reference condition with activity equal to concentration at infinite dilution. Comparisons of activities are unsatisfactory when applied to different solvents, because different standard states are then necessarily involved. For such comparisons it would be gratifying if the standard state could be defined solely with reference to the properties of the pure solute, as it is for volatile nonelectrolytes (Section 2-7). Unfortunately, for ionic solutes a different standard state is defined for every solvent and every temperature. 

To compare activities of solutes in different solvents, a single reference state for the solute must be chosen. Although from some points of view it is awkward, water is a logical choice for a single reference solvent in which the behavior of solutes in other solvents can be compared. To make comparisons of solute activities among solvents, it is convenient to consider separately the effect of dilution within a given solvent and the difference in the usual reference states of a solute at infinite dilution in different solvents. The activity coefficient yt of a species i in a solvent may be considered the product of two terms 

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S8 CHAPTER 4 ACID-BASE EQUILIBRIA IN NONAQUEOUS SOLVENTS... 

Kolthoff, I.M., Chantooni, M.K., Jr, Acid-base equilibria and titrations in nonaqueous solvents. A. General introduction to acid-base equilibria in nonaqueous organic solvents C. Dipolar aprotic solvents, in Ref. 1, pp. 239-302, 349-384. 

Kolthoff, I. M. and M. K. Chantooni, General introduction to acid-base equilibria in nonaqueous organic solvents, in Treatise on Analytical Chemistry (I. M. Kolthoff and R J. Elving, eds.), John Wiley Sons, New York, 1979, pp. 239-301. 

The commercial preparation of ammonia is accomplished in huge quantities by the Haber process [Equation (16.12)], discussed in Section 16.4 under Nitrogen Fixation. In the laboratory the most common preparation is the treatment of ammonium salts with strong bases, as represented in Equation (16.13).The self-ionization of liquid ammonia (analogous to that of liquid water) is shown in Equation (16.14). NH3(/) is often used as a nonaqueous solvent. As shown in Equation (16.15), NH3 acts as a weak base in water and serves as a prototype for a number of nitrogen-containing bases such as methylamine, pyridine, and aniline, which you may recall from studying acid-base equilibria in earlier courses. 

Steigman, J. Acid-base equilibria and titrations in nonaqueous solvents. D. Inert solvents, in Ref. 1, pp. 385-423. 

In nonaqueous solvents, such treatment with dry hydrohahc acids is the only way to cleave the /r-oxo dimer nonoxidatively. However, the /x-oxo dimers of water-soluble porphyrins are readily cleaved by Tewis bases such as hydroxide, imidazole, histidine, and pyridine. Both the equilibria and kinetics of such reactions have been reported. In addition, /x-oxo dimers of water-insoluble Fe porphyrins in dichloromethane can be oxidatively cleaved to yield PFe p2 and (probably) an Fe species. Studies of the picosecond decay of the excited state of (TPPFe)20 in benzene following a 532- or 355-nm 25-ps pulse suggest that the intermediate state is a photodissociated pair, (TPP -)Fe -l-TPPFe — (0 ), and a small amount of disproportionation reaction products, TPPFe -f TPPFe = O. ... 

Their unique relation to water systems favors the inclusion of acid-base reactions in deuterium oxide with aqueous acid-base equilibria, even though some aspects of the chemistry suggest inclusion with nonaqueous solvents. In studies such as those of deuterium isotope effects, it is desirable to be able to measure pD as an index of acidity in heavy water. Glass electrodes respond in a nemstian way to changes in deuterium ion concentration, and therefore the usual combination of glass and calomel electrodes can form the basis of an operational definition of pD ... 

Water is highly unusual in the extent of its interactions with solutes, but even minimal solvent-solute interactions can play a major role in the nature of chemical reactions. To calculate pH during acid-base titrations in a nonaqueous solvent, we must consider not only the equilibria discussed in Chapter 3 but also reactions discussed in Sections 4-2, 4-3, and 4-4. 

Dielectric constants cannot explain, quantitatively, most physicochemical properties and laws of solutions, and we shall soon see that they can become unimportant. The molecules of more polar solvents, which tend to cluster around the ions and dipole ions, produce a preferential or selective solvation that is reflected in measurements of such properties as solubility, acid—base equilibria, and reaction rates. Nonelectrostatic effects, such as the basicity of some solvents, their hydrogen-bonding, and the internal cohesion and the viscosity of mixtures, probably interfere with the electrostatic effects and thus reduce their actual influence. On the other hand, mixtures of water and nonaqueous solvents are enormously complicated systems, and their effective microscopic properties may be vastly different from their macroscopic properties, varying with the solute because of selective attraction of one of the solvents for the solute. 

The most general view of acids and bases was advanced by G. N. Lewis. In this model, acids are substances which have an affinity for lone electron pairs, and bases are substances which possess lone electron pairs. Water and ammonia are the most common substances which possess lone electron pairs, and therefore behave as bases in the Lewis scheme. The reaction of silver ion, Ag with cyanide ion, CN , and boron trifluoride, BF3 (an electron-deficient compound), with ammonia, NH3, are two examples of Lewis acid-base reactions. The Lewis acid-base concept is most useful in chemical reactions in nonaqueous solvents. We will not find it useful in our study of ionic equilibria in water. 

A potentiometric method for determination of ionization constants for weak acids and bases in mixed solvents and for determination of solubility product constants in mixed solvents is described. The method utilizes glass electrodes, is rapid and convenient, and gives results in agreement with corresponding values from the literature. After describing the experimental details of the method, we present results of its application to three types of ionization equilibria. These results include a study of the thermodynamics of ionization of acetic acid, benzoic acid, phenol, water, and silver chloride in aqueous mixtures of acetone, tetrahydrofuran, and ethanol. The solvent compositions in these studies were varied from 0 to ca. 70 mass % nonaqueous component, and measurements were made at several temperatures between 10° and 40°C. 

Reaction of CO2 with bases - either as solvent or solute - is by far the most significant effect on CO2 solubility in aqueous media. The equilibria are well-known for aqueous solutions (1, 2, 3, 7), but little data has been systematically compiled on acid-base reactions of CO2 in nonaqueous solutions (see Introduction). 

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. 

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