PK-values

For strong acids, values are large and pK values are negative, for example pA., for hydrochloric acid is -7. 

Angyal and Angyal measured the pK of 2-aminothiazoles and 2-imino-4-thiazolines to obtain the protomeric equilibrium constants (Scheme l4) (74). The higher pK values of the imino derivative (9.65) compared with that of 2-aminothiazole (5.68) prove that the amino form is highly predominant, Xp = 2x 10" (72), The limitations of such a method of Kp determination are discussed by Elguero. Marzin and Katntzky in a review of protomeric equilibria in heterocycles (75). 

TABLE VI-L REPRESENTATIVE pK VALUES OF 2-AMINOTHIAZOLES AND 2-IMlNO-4-THIAZOLINES... 

Besides the well-known lower basicity of ethanol, these data illustrate the greater acidity of benzoxazolium compared with benzothiazolium. The relative pK. values of the quaternary salts obtained in acetonitrile when treated with tetrabutylammonium hydroxide are 18.3 and 17.6, respectively (25). Those of 2-methyl 4-phenyl thiazolium and 2.4-dimethyl thiazolium are 20.5 and 21.8 under the same conditions (25). 

The easier elimination of pyridine compared to quinoline-4 may be related to the pK value of 4-methylthiazole, which is between those of lepidine and 2-picoline (25. 55). This reaction explains also why a neutrodimethine cyanine is obtained with such good yields when reacting together a quaternary salt, ketomethylene, and o-ester in a basic medium. As the reaction proceeds, the trimethine cyanine is attacked by the ketomethylene. The resulting 2-methyl quaternary salt is transformed into trimethine cyanine, consuming the totality of the ketomethylene (1, p. 512 661). The mesosubstituted neutrodimethine cyanine is practically pure. 

Relative equilibrium ion-pair acidities have been determined by Streit-wieser and Scannon (434) for thiazole and several heterocyclic compounds by reference to hydrocarbon indicators. The pK values for the... 

How do we know that water IS a weaker acid than phe noP What are their respec tive pK values ... 

Some ammo acids have side chains that bear acidic or basic groups As Table 27 3 indicates these ammo acids are characterized by three values The third pK reflects the nature of the side chain Acidic ammo acids (aspartic and glutamic acid) have acidic side chains basic ammo acids (lysine arginine and histidine) have basic side chains The isoelectric points of the ammo acids m Table 27 3 are midway between the pK values of the zwitterion and its conjugate acid Take two examples aspartic acid and lysine Aspartic acid has an acidic side chain and a pi of 2 77 Lysine has a basic side chain and a pi of 9 74... 

Table 8.11 pK, Values for Proton-Transfer Reactions in Nonaqueous Solvents 8.81... 

TABLE 8.8 pK, Values of Organic Materials in Water at 25°C Continued)... 

---