Temperature-Dependent Acid Dissociation Constants (KaΔHa, ΔSa) for some C-Aryl Hydroxamic Acids: The Influence of C and N Substituents on Hydroxamate Anion Solvation in Aqueous Solution

The acid dissociation constants (Ka) of a series of substituted N-methylbenzohydroxaraic acids, 4-XC6H4C-(O)N(OH)CH3(X = H, CH3O, CH3, NO2) and 4-methoxybenzohydroxamic acid, 4-CH3OC6H4C(O)N(OH)H, have been determined in aqueous solution (I = 2.0) over a range of temperatures. The pKavalues at 25 °C are as follows: 4-XC6H4C(O)N(OH)CH3, X = H (8.28), X = CH3O (8.67), X = CH3(8.50), X = NO2(7.94); 4-CH3OC6H4C(O)N(OH)H, (8.76). The substituted N-methylbenzohydroxamic acids exhibit a trend in pKavalues that is consistent with the Hammett σ substituent parameters but with a ρ value of 0.6. ΔHaand ΔSavalues fall in a narrow range (ΔHa= 1.1-2.2 kcal/mol; ΔSa= -31 to -36 cal/(K mol)) and represent minimum values for these parameters when compared with other C-and N-substituted hydroxamic acids. These results suggest that the C and N substituents influence the water solvation of the hydroxamate moiety-[C(=O)N(O-)-] and that the N-methylhydroxamate anions are the most highly solvated. © 1985, American Chemical Society. All rights reserved.

Duke Scholars

Author Alvin L. Crumbliss Chemistry