Determination of pKa’s of hydroxamic acids by nucleophilic substitution reaction

Abstract

Acid dissociation constant (pKa) of some para-substituted benzohydroxamic, 4-XC₆H₄CONHOH, and N-methyl para-substituted benzohydroxamic acids, 4- XC₆H₄CON(OH)CH₃, where X = H, CH₃, CH₃O, NO₂, Cl, have been determined spectrophotometrically by nucleophilic substitution reactions of p-nitrophenyl acetate with hydroxamate ions at 27± 0.1ºC. All reactions in this study follow pseudo-first order kinetics under condition of excess nucleophile. Good correlation has been observed between pKa and substitutent constants pointing out the validity of the Hammett equation. The kinetics results have been discussed on the basis of pKa and ⍺-effect of hydroxamic acids.