Temperature-Dependent Acid Dissociation Constants (Ka, ΔHa, ΔSa) for a Series of Nitrogen-Substituted Hydroxamic Acids in Aqueous Solution

Article abstract of DOI:10.1021/jo00346a005

The acid dissociation constants (K_a) of a series of eight substituted N-phenylacetohydroxamic acids, CH3C-(O)N(OH)C6H4X (X=H, 4-CH3, 4-Cl, 4-I, 3-I, 3-CN, 4-CN, 4-C(O)CH3), have been determined in aqueous solution (I=2.0) for a range of temperatures.The pKa data at 25 deg C exhibit a small variation with the substituent X in the direction expected according to their Hammett substituent constants (ρ ca. 0.1).These small variations in pKa values are due to compensating trends in ΔH_a and ΔS_a, which show significant variation with substituent.These results are discussed in terms of the substituent's influence on hydroxamate anion-solvent interactions and the relative influence on pKa of a substituted phenyl group attached to the C or N end of the hydroxamate moiety.