Collect. Czech. Chem. Commun. 1993, 58, 1109-1121

Dissociation of Hydroxamic Acids: Solvent Effects

Otto Exnera, Martin Hradilb and Jiří Mollinb

a Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 166 10 Prague 6, Czech Republic
b Department of Inorganic and Physical Chemistry, Palacký University, 771 46 Olomouc, Czech Republic


The dissociation constants of benzohydroxamic, 4-chlorobenzohydroxamic, and 4-nitrobenzohydroxamic acids, and their N-methyl and O-methyl derivatives, were measured spectrophotometrically or potentiometrically in mixtures of 2-propanol and water. The results were extrapolated to zero ionic strength. The ratio of dissociation constants of the N-methyl and O-methyl derivatives can be taken to represent - with some approximation - the ratio of NH and OH acidities of the parent acid. This ratio increases with substitution by electron-attracting substituents, and decreases with solvent permittivity: some irregularities might be attributable to the effects of mixed solvents, It follows that 4-nitrobenzohydroxamic acid behaves essentially as N-acid in all solvents, 4-chlorobenzohydroxamic acid only in 90% 2-propanol or 80% methyl cellosolve. In benzohydroxamic acid the NH and OH acidities are comparable, the latter prevails slightly in water, the former in less polar solvents. Some apparent discrepancies in the literature can be explained in the same terms, only a few results have not yet been explained.