Potentiometric determination of standard acidity and cationic standard homoconjugation constants of substituted pyridine N-oxide systems in (acetonitrile  +  methanol)
✍ Scribed by R. Wróbel; E. Kaczmarczyk; L. Chmurzyński
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 142 KB
- Volume
- 31
- Category
- Article
- ISSN
- 0021-9614
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✦ Synopsis
An attempt has been made to determine potentiometrically standard acidity K o a (BH + ) constants of protonated substituted pyridine N-oxides and cationic standard homoconjugation constants K o f (BHB + ) of these acids conjugated with the N-oxides in a binary solvent system. The system consisted of solvents with different proton-donating properties, namely, the aprotic protophobic acetonitrile (CH 3 CN) and amphiprotic methanol. The measurements were carried out in solutions with mole fractions of acetonitrile ranging between 0 and x(CH 3 CN) = 1. The standard acidity constants expressed as pK a (BH + ) values in these systems were determined for cations resulting from protonation of a variety of substituted pyridine N-oxides, namely those of 4-N ,N -dimethylaminopyridine, 4-methylpyridine, pyridine, and 4-nitropyridine. Whenever the respective equilibria were established, the cationic standard homoconjugation constants of the protonated N-oxides with conjugate bases were also calculated. For comparison, the equilibrium constants were also determined for quinoline N-oxide, trimethylamine N-oxide, and pyridine, representing, respectively, binuclear heterocyclic amines, aliphatic amines, and mononuclear heterocyclic amines. Inspection of the determined standard acidity dissociation constants showed them to be nonlinearly related to x(CH 3 CN) in the binary solvent in all the acid-base systems studied. The character of the relations was specific for a particular cationic acid. It was also found that the cationic standard homoconjugation constants were determinable over the whole composition range of the (acetonitrile + methanol) system for the strongest bases only, namely trimethylamine N-oxide and 4-N ,N -dimethylaminopyridine N-oxide. Nonetheless, inspection of the standard homoconjugation constants determined in the majority of the remaining acid-base systems revealed that, similar to the pK a (BH + ) values, logarithms of the cationic standard homoconjugation constants do vary nonlinearly as a function of composition of the solvent system.