Basic solvents, levelling effect

Comparison of reactions 4.9, 4.10, 4.12, 4.13 and 4.15 leads to another important conclusion, viz., in an amphiprotic solvent its own solvonium cation represents the strongest acid possible, and its own anion the strongest base. Even when a very strong foreign acid or base is dissolved, excessive proton donation to and proton abstraction from the solvent molecule yield the respective acid or base this phenomenon is generally known as the levelling effect, which in an amphiprotic solvent takes place on both the acid and the basic... 

In Fig. 4.1b one sees the occurrence of the levelling effect on the basic side only in protogenic solvents (dotted lines above). 

The ionization of the acid depends not only on the basicity of the solvent, but also on its dielectric constant and its ion-solvating ability. The dependence of the acidity and basicity constants of a compound on the basicity and acidity, respectively, of the solvents, leads to a distinction between leveling and differentiating solvents. When the solvent is a stronger base than water, its leveling effect will apply also to weaker acids. Similarly, strong bases will also have equal basicities in sufficiently acidic solvents. All bases stronger than the HO ion are adjusted to the basicity of this ion in water. 

Fig. 3-3. Schematic description of the levelling effect of water on (a) acids and (b) bases in aqueous solution [2]. Relative orders of acidity and basicity are not invariable to changes of solvent and of the conjugated acid or base, respectively.

Aprotic Solvents.—The colorimetric method of studying dissociation constants has found a special application in aprotic solvents such as benzene these solvents exhibit neither acidic nor basic properties, and so they do not have the levelling effects observed with acids in proto-... 

Inert solvents, with neither acidic nor basic properties, allow a wider range of acid-base behavior. For example, hydrocarbon solvents do not limit acid or base strength because they do not form solvent acid or base species. In such solvents, the acid or base strengths of the solutes determine the reactivity and there is no leveling effect. Balancing the possible acid-base effects of a solvent with requirements for solubility, safety, and availability is one of the challenges for experimental chemists. 

Nearly all organic solvents stabilize anions worse than water. For this reason, the pKa values of neutral acids are larger in organic solvents (it is obvious that strongly basic solvents hke amines may level out this effect). In lower alcohols, for example, the y>Ka values increase by several units. In acetonitrile, which has generally even less cation stabilization ability in addition, the yiKa values may increase by 16 units. The effect of many solvents on weak bases (expressed by the pKa of the corresponding cation add) is much lower. pKa values change only by few units. An exception is acetonitrile, due to the reason mentioned earlier. 

The possibility of relating solvent effects to polymorphic crystallization at the molecular level may be realized when the individual crystal structures of the polymorphs are known. An analysis of this kind was carried out for the antiinflammatory drug piroxicam [57] which was fovmd to crystallize as the a-poly-morph from proton donor and basic solvents, but as the -polymorph from non-polar solvents. On the assumption that crystallization of the drug requires... 

Non-aqueous solvents that are good proton acceptors (e.g. NH3) encourage adds to ionize in them. Thus, in a basic solvent, aU acids are strong. The solvent is said to exhibit a levelling effect on the acid, since the strength of the dissolved acid carmot exceed that of the protonated solvent. For example, in aqueous solution, no addic spedes can exist that is a stronger add than [H30]. In an addic solvent (e.g. MeCOzH, H2SO4), ionization of bases is fadUtated most acids are relatively weak under these conditions, and some even ionize as bases. 

On the other hand, although for many years acetic acid has been the most popular solvent for titration of bases, acetonitrile - which is a considerably weaker base and much weaker acid than water - shows a greater pH jump in the titration curves of bases. Acetonitrile solvation of anions is less than that of cations and, from an analytical viewpoint, conjugation is negligible for bases. This fact makes acetonitrile very suitable for the determination of very weak bases and mixtures of bases. In this medium many workers have used a solution of perchloric acid in anhydrous acetic acid as a titrant. However, for the differentiating titration of two or more bases of different strengths, it is necessary to avoid the leveling effect of acetic acid, whose presence decreases the potential break in acetonitrile, especially in the basic side. A solution of perchloric acid monohydrate in nitromethane is stable and very suitable for titrations in acetonitrile. 

Figure 12.11 Osmotic pressure, (a) The levels of the pure solvent (left) and of the solution (right) are equal at the start, p) During osmosis, the level on the solution side rises as a result of the net flow of solvent from left to right. The osmotic pressure is equal to the hydrostatic pressure exerted by the column of fluid in the right tube at equilibrium. Basically, the same effect occurs when the pure solvent is replaced by a more dilute solution than that on the right.

I would now like to consider the titration of acidic compounds in nonaqueous solutions. If you wish to titrate an acid in nonaqueous solution, you should choose a solvent that is not acidic and a titrant that is as strong a base as possible. The paper that really aroused people s imagination and created a lot of interest was the one published by Moss, Elliot, and Hall in 1948, in which they introduced ethylenediamine as a solvent. This compound certainly doesn t have any acidic properties and these authors showed that you can titrate phenol, which is normally too weak to titrate as an acid. In recent years, however, the trend has been away from the use of strongly basic solvents because they have a leveling effect on many bases and they are somewhat unpleasant to handle. Solvents now in use are pyridine, which is an inert solvent and a very weak base, acetonitrile, and acetone. Acetone and certain other ketones are surprisingly good. Recently we have done some work with tertiary butyl alcohol, an excellent solvent for certain cases. Sodium or potassium hydroxide can be used as tltrants, but these are not particularly... 

When an acid is dissolved in a solvent, the initial reaction between the acid and the solvent depends primarily upon two factors the strength of the acid (its tendency to accept an electron pair), and the basic strength of the solvent (its tendency to donate an electron pair). In a given solvent, the strength of the acid can be measured, within the limits of the leveling effect of Hantzsch (to be discussed later), by means of the equilibrium constant of the reaction with the solvent. For example, if glacial acetic acid, a typical covalent liquid which conducts an electric current poorly, reacts with water according to the equation... 

The leveling effect is one phenomenon that must be taken into account when comparing relative acid or base strengths. Since water levels downward the strengths of the strong acids like hydrogen chloride, it is necessary to use a less basic solvent such as acetic acid or even an inactive one like benzene. In acetic acid the reaction... 

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