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Issue 4, 2011
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The lipophilicity and hydrogen bond strength of pyridine-N-oxides and protonated pyridine-N-oxides

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Abstract

Watersolvent partition coefficients for 4-nitropyridine-N-oxide have been determined in six solvent systems, and solubilities measured in water and 37 solvents. The combined data are used to obtain Abraham descriptors for 4-nitropyridine-N-oxide as follows: E = 0.934, S = 1.92, A = 0.21, B = 0.76, V = 0.9082 and L = 5.246; literature data on partition coefficients were used to calculate descriptors for 11 other pyridine-N-oxides as well as quinoline-N-oxide and isoquinoline-N-oxide. In all cases, the N-oxides were shown to be stronger hydrogen bond bases than the corresponding pyridines, thus leading to a decrease in lipophilicity. An unexpected observation is that of the hydrogen bond acidity of the N-oxides due, we suggest, to the positive nitrogen atom in the moiety N(+) → O(−). This leads to an even sharper decrease in the N-oxide lipophilicity in non-polar solvents. We have also used literature data to determine the 1 ∶ 1 hydrogen bond basicity, βH2, of a number of pyridine-N-oxides; again, this basicity is significantly larger than that of the corresponding pyridines. Protonation of pyridine-N-oxides removes all hydrogen bond basicity (B = 0.00), but the protonated compounds are strong hydrogen bond acids. For protonated pyridine-N-oxide itself, E = 0.514, S = 2.62, A = 1.51, B = 0.00, V = 0.7555 and the ionic descriptor J+ = 0.894; protonated 4-nitropyridine-N-oxide with A = 2.05 is a very strong hydrogen bond acid indeed.

Graphical abstract: The lipophilicity and hydrogen bond strength of pyridine-N-oxides and protonated pyridine-N-oxides

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Publication details

The article was received on 11 Nov 2010, accepted on 07 Feb 2011 and first published on 28 Feb 2011


Article type: Paper
DOI: 10.1039/C0NJ00893A
Citation: New J. Chem., 2011,35, 930-936
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    The lipophilicity and hydrogen bond strength of pyridine-N-oxides and protonated pyridine-N-oxides

    M. H. Abraham, L. Honcharova, S. A. Rocco, W. E. Acree, Jr and K. M. De Fina, New J. Chem., 2011, 35, 930
    DOI: 10.1039/C0NJ00893A

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