Nonaqueous solvents, acid-base strength

If the data are available, one can predict the feasibility of any titration quite accurately. Unfortunately, the necessary data are not available for a great many bases. However, Streuli and others have shown that there is a relationship between the strength of a base in glacial acetic acid or any other nonaqueous solvent, and the strength of the same base in water. This means that if one has the necessary constants for just a few bases in both types of solvents and can predict how they will titrate in glacial acetic acid, then, knowing the constants... 

Hull and Conant in 1927 showed that weak organic bases (ketones and aldehydes) will form salts with perchloric acid in nonaqueous solvents. This results from the ability of perchlonc aad in nonaqueous systems to protonate these weak bases. These early investigators called such a system a superacid. Some authorities believe that any protic acid that is stronger than sulfunc aad (100%) should be typed as a superaad. Based upon this criterion, fluorosulfuric arid and trifluoro-methanesulfonic acid, among others, are so classified. Acidic oxides (silica and silica-aluminai have been used as solid acid catalysts for many years. Within the last few years, solid acid systems of considerably greater strength have been developed and can he classified as solid superacids. 

Some of the reasons for considering coulometric titrations in nonaqueous solvents are that many organic compounds are not soluble in water, metals can exist in oxidation states that are not found in water, and advantage can be taken of the acidity or basicity of the solvent to improve the basic or acidic strength of a base or acid, respectively. 

Our thinking in terms of acid strength is usually confined to aqueous solutions where we have considerable quantitative data, but in strongly basic or strongly acid nonaqueous solvents differences in acid or basic strength of weak acids or bases will be marked. As a specific example, let us consider the interaction of the methylbenzenes with the solvent anhydrous hydrogen fluoride. As this solvent has a high dielectric constant, the equilibrium constant for the reaction... 

As shown in eqn [3], in nonaqueous solvents activity coefficients must be taken into account to relate the color of an indicator and the pH. Moreover, acid-base indicators may be not only neutral (model I), but also positively or negatively charged. The ionic strength of the solution modifies activity coefficients in a different way for each kind of dissociation model, according to the ionic charge of chemical species. 

Reactions at the Silicon Nitride - Solution Interface - A study was conducted to determine the extent of aqueous reactions at the silicon nitride-water interface. It was demonstrated that up to 27 days are required to stabilize reactions at the interface as indicated by Ph and particle electrophoresis measurements. A semiautomatic titrator was also purchased and set up to utilize acid-base titrations to study the silicon nitride-solvent interface. A particular emphasis of this work will be on the nonaqueous potentiometric and conductometric titration to determine the strength of acid and base sites on the silicon nitride surface. 

Another method of analyzing a mixture of bases is to utilize the difference in the basicity of its components. As an example, let us take a mixture of aromatic and aliphatic amines. Since aliphatic amines are more strongly basic, one would expect to get a titration curve with two breaks, one for the aliphatic amine and one for the aromatic amine. However, you must not use glacial acetic acid for this titration because you will get a curve similar to curve B in Fig. 1. In other words, you get one potentiometric end point for the sum of the two. The reason is that glacial acetic acid reacts with aliphatic amines to form the acetate ion, which has about the same basic strength as the aromatic amine. Glacial acetic acid levels these two amines to the same strength. What you have to do is employ a nonaqueous solvent like acetonitrile and titrate with perchloric acid dissolved in dioxane. If you do this. 

The Bronsted-Lowry theory can be applied to acid-base reactions in nonaqueous solvents, where the relative strengths of acids and bases can differ from what they are in aqueous solutions. Indicate whether each of the following would be an acid, a base, or amphiprotic in pure liquid acetic acid, CH3COOH, as a solvent, (a) CHsCOO" (b) H2O (c) CH3COOH (d) HCIO4. [Hint Refer to Table 16.2.]... 

Solvatation, solvolysis and ionic dissociation phenomena, in both aqueous and nonaqueous solutions are subsumed by the Lewis definitions. In addition to the previous discussion of the dual polarity character of Lewis acids and bases, it should be noted that many of them are amphoteric, by definition. Donor number, DN, was developed in order to correlate the behavior of a solute in a variety of donor solvents with a given basicity or donicity. A relative measurement of the basicity of a solvent D is given by the enthalpy of its reaction with an arbitrarily chosen reference acid (SbCls in the Gutmann s scale). Latter Mayer introduced an acceptor number, AN, as the relative P NMR shift induced by triethylphosphine, and relative to acidic strength (AN=0 for hexane and 100 for SbCls). In 1989, Riddle and Fowkes modify these AN numbers, to express them, AN ", in correct enthalpic unit (kcaLmol). Table 10.2.3 gathers electron acceptor number AN and AN " and electron donor number DN for amphoteric solvents. 

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