Medium effect on acidity constants of some heterocyclic nitrogen azomethines

The effects of different organic solvent-water mixtures on the acidity constants of some azomethines derived from 3-amino-1,2,4-triazol; Zamino-1,3-pyrimidine and also 2-amino-4-methyl-1,3-pyrimidine have been examined. Two pK(a) values for the o-OH derivatives were determined; one is assigned to the deprotonation of the nitrogen azomethine group and the other one is ascribed to the ionization of the OH group. The p-OH derivatives give one pK(a) due to the ionization of the OH group. On the other hand, it was observed that with the increase in the amount of organic solvent in the medium, the pK(a) of the compound studied is decreased. This behaviour can be accounted for in terms of the high stabilization of both the non-protonated and ionic forms of these compounds by dispersion forces rather than by hydrogen bonding. Also, the effects of medium polarizability on the pK(a) values and thus on the spectra of the charge transfer band observed have been discussed.