Acid Dissociation Constants (Ka) and Their Temperature Dependencies (AHa, ASa) for a Series of Carbon- and Nitrogen-Substituted Hydroxamic Acids in Aqueous Solution
The acid dissociation constants (Ka) of a series of six C- and N-substituted hydroxamic acids, R1C(O)N(OH)R2 (R1 = CH3, C6H5; R2 = H, CH3, C6H5), have been determined in aqueous solution (I = 2.0) for a range of temperatures. In contrast to many other organic acids, large variations in AHa and ASa were found to exist for these hydroxamic acids. The pAa data at 25 °C exhibit random variations with changes in the Ri and R2 substituents, while regular variations are seen in Aifa and ASa. A plot of Aifa vs. ASa is linear and suggests that the ionization processes for all six hydroxamic acids are the same. This requires that CH3C(O)N(OH)H and C6H6C(O)N(OH)H ionize in aqueous solution by the loss of the O-H proton rather than the N-H proton. This is the first instance in which this differentiation has been possible and is a direct result of the temperature-dependence study. A model is presented which qualitatively rationalizes the variations in the Afía and ASa values. This model is based upon the changes in solvation with changes in molecular dipole which accompany ionization. It is concluded that the greater the increase in the molecular dipole upon ionization thelarger the increase in solvation. This lowers AHa and ASa through anion-solvent interaction. Data obtained in less polar nonaqueous solvents indicate that the A-methyl acids [CH3C(O)N(OH)CH3 and C6H5C(O)N(OH)CH3] exist in equilibrium with a dimer formed through H bonding. © 1980, American Chemical Society. All rights reserved.
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