Acid-base equilibria in systems involving substituted pyridines in polar aprotic protophobic media and in the amphiprotic methanol

Acid dissociation, as well as cationic homo- and heteroconjugation constants have been determined by potentiometric titration in systems involving substituted pyridines and conjugate cationic acids in the polar protophobic aprotic solvent acetone and in polar amphiprotic methanol. The values of the constant were compared with those previously determined in other polar protophobic aprotic solvents, acetonitrile, nitromethane and propylene carbonate. The pK(a) values of the protonated pyridine derivatives in acetone range between 2.69 and 12.69 and are on average 2-3 orders of magnitude higher than those determined in water. The pK(a) values in methanol vary between 1.02 and 10.37, and are only slightly higher than those in water, the difference not exceeding one order of magnitude. A comparison of the acid dissociation constants determined in all the non-aqueous solvents considered shows that the strength of the cationic acids increases on going from acetonitrile through nitromethane, propylene carbonate and acetone to methanol. In almost all systems of the type: a pyridine derivative its conjugate acid, the cationic homoconjugation equilibrium is present in acetone (1.60