Acidic solvents, differentiating

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. 

Consequently there appears to be a sound empirical basis for the use of the OTs scale of solvent ionizing power. Its use should be restricted to sulphonates, however, because of the differential effects of electrophilic solvation in acidic solvents (see Section 4). The importance of these effects can be seen by comparing the Y and Iqxs values for carboxylic acids (Table 5) it appears that, relative to 80% ethanol/water, a carboxylic acid ionizes a tosylate about ten times more rapidly than a chloride. 

Other acids are differentiated [59] HBr > H2SO4 > HNO3 > HCl and picric acid. When the solvent is a stronger base than water, its levelling effect will apply also to weaker acids. Thus, in liquid ammonia, even the carboxylic acids are practically fully ionized. 

Zhong, S., Haghjoo, K., Kettner, C., and Jordan, E, Proton magnetic resonance studies of the active center histidine of chymotrypsin complexes to peptideboronic acids Solvent accessibility to the and A sites can differentiate slow-binding and rapidly reversible inhibitors, J. Am. Chem. Soc. 117, 7048-7055 (1995). 

Substitution generates HBr whereas the addition does not. If the reaction is performed in an organic solvent, bubbles of HBr can be observed, and HBr gas escaping into moist air will generate a cloud. If the reaction is performed in water, then adding moist litmus paper to test for acid will differentiate the results of the two compounds. 

Acidic solvents also differentiate some acids levelled by water. In pure sulphuric acid the strongest acid is the solvent cation H3SO4+ and HCIO4 is a weak acid with p equal to about 4. The solvent molecule H2SO4 is a still weaker acid. 

Acetonitrile, an aprotic solvent, is a very weak base and an exceptionally weak acid. Little levelling of acids and virtually no levelling of bases occurs in this solvent. Differentiation is the rule. Perchloric acid appears to be strong in acetonitrile other adds are differentiated ... 

Free peptides can be examined by paper electrophoresis or by thin layer electrophoresis as well. If one or more cation forming groups, (amino group, guanidino group, imidazole) are present, electrophoresis in an acidic solvent, such as dilute acetic acid, is most revealing. Compounds with free carboxyl groups are best run under basic conditions to allow differentiation by the number of carboxylate anions. The acidic character of the phenolic hydroxyl in tyrosine should be included in these considerations. 

Many of the inorganic oxoacids are strong (i.e. have negative PX3 values) in aqueous solution. But, as we have seen, use of a solvent with a lower proton affinity than water (for example pure ethanoic (acetic) acid makes it possible to differentiate between the strengths of these acids and measure pX values. The order of strength of some typical oxoacids is then found to be (for H X -> H , X- + H") ... 

Trifluoromethyl-l-phenylethyl tosylate has been used to differentiate as shown in Table 1, the solvolytic power of three fluorinated solvents and to compare these with formic and acetic acids The three fluorinated solvents are trifluoroacetic acid, trifluoroethanol, and 1,1,1,3,3,3-hexafluoroisopropyl alcohol [55]... 

Determinations in non-aqueous solvents are of importance for substances which may give poor end points in normal aqueous titrations and for substances which are not soluble in water. They are also of particular value for determining the proportions of individual components in mixtures of either acids or of bases. These differential titrations are carried out in solvents which do not exert a levelling effect. 

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