DIRECT DETERMINATION OF PK(A) VALUES OF CATIONIC ACIDS CONJUGATED TO HETEROCYCLIC AMINE N-OXIDES IN POLAR APROTIC AND AMPHIPROTIC SOLVENTS

The applicability of the direct method of pK(a) determination in the c ase of protonated heterocyclic amine N-oxides in a series of polar non -aqueous (aprotic and amphiprotic) solvents has been tested. The metho d is based on the pH determination of the non-aqueous solution of comp lex salt (the semiperchlorate in this case) formed by the N-oxides stu died. The direct method not only provides for quick (one data point pe r each pK(a) determined), but also relatively accurate estimates of ac idic dissociation constants. It has been experimentally shown on the e xample of substituted pyridine N-oxides that this method is precise en ough in all studied non-aqueous solvents when applied to compounds of not too low basicity (the pK(a) being of the order of 5 or higher). To prove this, the pK(a) values of protonated monocyclic N-oxides obtain ed by the direct method have been compared with those resulting from t he potentiometric titration curve. The agreement between the results f ound by using both methods is very good in most cases, the differences being within standard deviations. Based upon this observation it can be inferred that the pK(a) values of protonated bicyclic N-oxides in s olvents studied determined by using the direct method can be also cons idered reliable, especially in the case of polar aprotic solvents.