Temperature-Dependent Acid Dissociation Constants (Ka, ΔHa, ΔSa) for a Series of Nitrogen-Substituted Hydroxamic Acids in Aqueous Solution
The acid dissociation constants (Ka) 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 °C exhibit a small variation with the substituent X in the direction expected according to their Hammett substituent constants (ρ ≈ 0.1). These small variations in pKa values are due to compensating trends in ΔHa and ΔSa, 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. © 1982, American Chemical Society. All rights reserved.
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