Acidity of hydroxamic acids and amides

Abstract

The relatively strong acidity of hydroxamic acids was analyzed by means of isodesmic reactions in which this acid or its anion is formed from simpler precursors. Acidity of amides was analyzed in the same way. Energies of all compounds involved in the reactions were calculated at the B3LYP/AUG-cc-pVTZ//B3LYP/6-311+G(d,p) level; at this level a good agreement was reached with the sparse experimental data. Interpretation of the results was the same as in the recent discussion of the acidity of carboxylic acids, and the conclusions were similar: both amides and hydroxamic acids are stabilized with respect to simpler reference molecules of amines or N-alkylhydroxylamines, respectively. However, their anions are stabilized still more and are responsible for the acidity. This effect is stronger in hydroxamic acids or amides than in carboxylic acids. The problem of whether it is due to resonance depends on the definition of this term. Semiquantitative comparison suggests that resonance in hydroxamic acids is more important than in amides and still more than in carboxylic acids. The stronger acidity of hydroxamic acids compared to amides is due to the destabilizing inductive effect of the hydroxyl group in the acid molecule, not to any effect in the anion.