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A Comparison of Basicities of Substituted Pyridines and Pyridine N-Oxides

Makowski M., Kozak A., Chmurzyński L.

Polish Journal of Chemistry

2002

Vol. 76 / nr 4

581-588

The basicity of pyridine and its derivatives, as representatives of monocyclic heterocyclic N-bases, has been compared with that of pyridine N-oxide and its derivatives representing the class of heterocyclic monocyclic oxygen-containing bases. The basicities have been compared both in the gas phase and in solution. To do this, correlated were both calculated (at RHF, MP2 levels and using SCRF and PCM methods) energy parameters, _Eprot and _Gprot, with experimental pKa values for a particular class of the bases, as well as theoretical and experimental characteristics of the two classes of compounds studied. Results of the correlations paved the way to discussion of the effect of the medium on basicity of both classes of compounds, as well as enabled to compare the basicities of these very important classes of organic bases in a quantitative way.

A Potentiometric Study of the Acid Dissociation and Cationic Homoconjugation Equlibria in Substituted Pyridine N-Oxide Systems in Binary Polar Solvents, Acetonitrile-Acetone and Acetonitrile-Dimethyl Sulfoxide

Wróbel R.

Polish Journal of Chemistry

2001

Vol. 75, nr 9

1341-1350

Acid dissociation constants of protonated substituted pyridine N-oxides and cationic homoconjugation constants of these acids with conjugate N-oxides have been determined by potentiometric titration in mixed polar non-aqueous solvents (acetonitrile - acetone and acetonitrile - dimethyl sulfoxide) with increasing (from 0 to 1) mole fraction of acetonitrile. In acetonitrile - dimethyl sulfoxide mixtures, the acid dissociation constants (expressed as pKa) and cationic homoconjugation constants (expressed as log KBHB+) vary non-linearly with increasing mole fraction of acetonitrile. In systems with the least basic N-oxides, local minima of pKa have been observed. In acetonitrile-acetone mixtures, both the pKa and the cationic homoconjugation constant values changed irregularly in the acid-base systems studied and the character of those variations could not be unambiguously determined.