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<log KBHB+ <2.75). In methanol, the extent of the cationic homoconjugation is markedly smaller. It encompasses a smaller number of systems and the values of the homoconjugation constants are also lower (0.87<log KBHB+ <2.03). The tendency towards cationic homoconjugation of the N-bases and the conjugate cationic acids increases in the solvents studied as follows: propylene carbonate<methanol<acetonitrile<nitromethane. Similar to other protophobic solvents, only in one of the systems of the type: a pyridine derivative - cationic acid conjugated with another pyridine derivative, the cationic heteroconjugation equilibrium is present in acetone (log K-BHB1(+) =2.13). In methanol, the tendency towards heteroconjugation in the systems studied is markedly greater. As a matter of fact, the heteroconjugation constants are lower (1.58<log K-BHB1(+) <2.29), but they are determinable in the majority of the systems under study. (C) 2000 Elsevier Science B.V. All rights reserved.