Acids and Bases in Nonaqueous Solutions

If you were to add sodium amide (NaNH2) to water in an attempt to carry out a reaction using the amide ion (NH2 ) as a very powerful base, the following reaction would take place immediately  

The amide ion would react with water to produce a solution containing hydroxide ion (a much weaker base) and ammonia. This example illustrates what is called the leveling effect of the solvent. Water, the solvent here, donates a proton to any base stronger than a hydroxide ion. Therefore, it is not possible to use a base stronger than hydroxide ion in aqueous solution. 

We can use bases stronger than hydroxide ion, however, if we choose solvents that are weaker acids than water. We can use amide ion (e.g., from NaNH2) in a solvent such as 

We can, for example, convert ethyne to its conjugate base, a carbanion, by treating it with sodium amide in liquid ammonia  

Most terminal alkynes (alkynes with a proton attached to a triply bonded carbon) have p a values of about 25 therefore, all react with sodium amide in liquid ammonia in the same way that ethyne does. The general reaction is 

---

The electrophilic aromatic substitution reaction may be advantageously modeled by isotopic exchange reactions. Shatenshtein and his co-workers77-79 studied hydrogen exchange catalyzed by acids and bases in nonaqueous solution, and their studies throw considerable light on both electrophilic substitution and protophilic (base-induced) replacement of hydrogen in the system (see Section V, A). 

The ionisation constants of many acidic organic compounds determined in water [110a] and in twelve of the most popular dipolar non-HBD solvents [110b] have been compiled, as have the methods of determination [111] and prediction [112] of p.Ka values. Particular attention has been paid to C—H acidic compounds [113]. Whereas the ionisation constants of Bronsted acids and bases for aqueous solutions are well known, the corresponding pAa values for nonaqueous solutions are comparatively scarce. 

The similarity of FIA and classical batch titrations is useful to recognize, because such recognition turns our attention to the wealth of chemistries exploited by classical titrations that are now accessible to FIA adaptation. Indeed, all traditional titrations, that is, acid-base, com-pleximetric, redox, and precipitation, can be performed in the FIA mode. Catalytic titrations and titrations in nonaqueous solutions, including Karl... 

Our initial definition of acids and bases in Section 3.3 involved the formation of hydronium ions (for acids) or hydroxide ions (for bases). This definition is attributed to Svante Arrhenius, but it can be expanded to include nonaqueous solutions, among other things. Formulated independently in 1923 by two chemists, Johannes Bronsted in Denmark and Thomas Lowry in England, the Bronsted-Lowry definition does just that. According to this definition, a Bronsted-Lowry acid is a proton (H+) donor, and a base is a proton acceptor. 

When chemists see a pattern in the reactions of certain substances, they formulate a definition of a class of substance that captures them all. The reactions of the substances we call acids and bases are an excellent illustration of this approach. The pattern in these reactions was first identified in aqueous solutions, and led to the Arrhenius definitions of acids and bases (Section J). However, chemists discovered that similar reactions take place in nonaqueous solutions and even in the absence of solvent. The original definitions had to be replaced by more general definitions that encompassed this new knowledge. 

Total ionic strengths of solutions in the cells were varied from about 0.005M to ca. 0.02Af. The concentrations of solutions in cell C were made so that the buffer ratio in Equation 15 always had a value between 0.4 and 0.6. The nonaqueous cosolvents used in this study were Reagent Grade or better, and they were tested to be sure that they were free from significant quantities of potentially interfering substances such as halide ions, acids, and bases. Densities of tetra-hydrofuran-water mixtures were determined pycnometrically at 15° C and at 35°C. 

Another obvious requirement of a nonaqueous solvent is chemical stability under a variety of conditions. Thus, methanol, especially after standing in the presence of air, may contain small amounts of formaldehyde which can react with groups on proteins and nucleic acids. Forma-mide, A, A-dimethylformamide, and related compounds, are slowly decomposed by acid or base in the solvent, and the possibility exists that such decomposition may be catalyzed to some extent by a protein dissolved in the solvent. Thus Rees and Singer (1956) found that the apparent osmotic pressure of a solution of insulin in lV,A -dimethylformamide continually increased over a period of a week at 25°C but reached equilibrium at 13.8°C, which might have been due to the slow decomposition of the solvent on the solution side of the osmotic membrane at the higher temperature. 

The purpose of this chapter is to consider the preparation and standardization of acids and bases and to review some of the important applications of acid-base titrations in aqueous and nonaqueous systems. For end points to be detected most precisely, the pH in the vicinity of the equivalence point should change sharply. For this reason a solution of strong acid or base is chosen as titrant whenever possible. 

The solubility of carbon dioxide in aqueous and non-aqueous solutions depends on its partial pressure (via Henry s law), on temperature (according to its enthalpy of solution) and on acid-base reactions within the solution. In aqueous solutions, the equilibria forming HCO3 and CO3 depend on pH and ionic strength the presence of metal ions which form insoluble carbonates may also be a factor. Some speculation is made about reactions in nonaqueous solutions, and how thermodynamic data may be applied to reduction of CO2 to formic acid, formaldehyde, or methanol by heterogenous catalysis, photoreduction, or electrochemical reduction. 

The selectivity of resins in the hydrogen ion or hydroxide ion form, however, depends on the strength of the acid or base formed between the functional group and the ion. The stronger the acid or base formed, the lower is the selectivity coefficient. It should be noted that these series are not followed in nonaqueous solutions, at high solute concentrations or at high temperature. 

From the Brpnsted-Lowry perspective, the only requirement for an acid-base reaction is that one species donates a proton and another species accepts it an acid-base reaction is a proton-transfer process. Acid-base reactions can occur between gases, in nonaqueous solutions, and in heterogeneous mixtures, as well as in aqueous solutions. 

In nonaqueous media such as alcohols, not only Arrhenius acids and bases but also Lewis acids and bases can be titrated. (Lewis acids and bases cannot be titrated in aqueous solutions.) Interpreting the curves obtained, however, is more complex than in aqueous solution. One factor that needs to be considered is the suppression or enhancement of the ionization of the acids or bases by the solvent another is the viscosity of the solvent (as viscosity increases, the ionic mobility decreases). In Lewis acid-base reactions, factors such as ion-pair formation, hydrogen bonding, and solute-solvent and solute-solute interactions must also be taken into account. 

Use of potentiometry for pH titration allows analyses to be carried out in colored or turbid solutions. Also, it solves the problem of selecting the correct indicator for a particular acid-base titration. The endpoint can be determined more accurately by using a first or second differential curve as described earlier. It also permits pH titrations in nonaqueous solvents for the determination of organic acids and bases as described subsequently. In addition, it can be readily automated for unattended operation. 

---