Effect of hydroxylamine buffers on apparent equilibrium constant for reversible conversion of N-hydroxyphthalimide to <i>o</i>-(N-hydroxycarbamoyl)- benzohydroxamic acid: Evidence for occurrence of general acid-base catalysis

Abstract

Hydroxylamine reacts rapidly and reversibly with N-hydroxyphthalimide (NHPH) to form <i>o-(N-</i>hydroxycarbamoyl) enzohydroxamic acid (P). The apparent equilibrium constants, <i>K</i>_app =[P]_e/[X]_e, where [P]_e,and [X]_e, represent the equilibrium concentrations of product, P, and reactants {both ionized (NHP^-) and nonio zed NHPH respectively, have been determined at different <i>p</i>H<i>. </i> The values of <i>K</i>_app obtained at fixed <i>p</i>H and different total hydroxylamine buffer concentrations ([Buf]_T) reveal a relationship: <i>K</i>_app = A₁[Buf]_T/(1+A₂[Buf]_T). The unknown parameters, A₁ and A₂, obtained at different pH are discussed in terms of the mechanistic aspects of the reaction. The suggested stepwise mechanisms involve an oxymonoanionic tetrahedral intermediate in both the hydroxylaminolysis of NHPH and NHP to form P and cyclization of P to form NHPH and NHP^-.