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Acid-base equilibria of substituted pyridines in nitromethane
In the framework of our studies on acid-base equilibria in systems comprising substituted pyridines and nonaqueous solvents, acid dissociation constants have been determined potentiometrically for a variety of cationic acids conjugated with pyridine and its derivatives in the polar protophobic aprotic solvent nitromethane. The potentiometric method enabled a check as to whether and to what extent cationic homoconjugation equilibria of the BH+/B type, as well as cationic heteroconjugation equilibria in BH+/B1 systems without proton transfer, are set up in nitromethane. The equilibrium constants were compared with those determined in water and two other polar protophobic aprotic solvents, propylene carbonate and acetonitrile. The pKa values of acids conjugate to the N-bases in nitromethane fall in the pKa range of 5.84 to 17.67, i.e., 6 to 7 pKa units, on average, higher than in water, 1 to 2 units higher than in propylene carbonate, and less than 1 unit lower than in acetonitrile. This means that the basicity of the pyridine derivatives increases on going from propylene carbonate through nitromethane to acetonitrile. Further, it was found that the sequence of the pKa changes of the protonated amines was consistent in all three media, thus providing the basis for establishing linear correlations among these values. In the majority of the BH+/B systems in nitromethane, cationic homoconjugation equilibria have been established. The cationic homoconjugation constants, log KBHB+, are relatively low, falling in the range 1.60-2.89. A comparison of the homoconjugation constants in nitromethane with those in propylene carbonate and acetonitrile shows that nitromethane is a more favorable solvent for the cationic homoconjugation equilibria than the other two solvents. Moreover, results of the potentiometric measurements revealed that cationic heteroconjugation equilibria were not present in the majority of the BH+/B1 systems in nitromethane. The heteroconjugation constant could be determined in one system only, with log KBHB1+ = 2.56.
Note that a catalyst decreases the activation energy for both the forward and the reverse reactions and hence accelerates both the forward and the reverse reactions.Quality Control of 2,4-Dimethylpyridine, you can also check out more blogs about108-47-4
Reference£º
Chiral nitrogen ligands in late transition metal-catalysed asymmetric synthesis¡ªI. Addressing the problem of ligand lability in rhodium-catalysed hydrosilations,
Nitrogen-Containing Ligands for Asymmetric Homogeneous and Heterogeneous Catalysis