Kinetics and mechanism for the formation of o-carboxy(N-methyl)-benzohydroxamic acid in the cleavage of ethyl N-[o-(N-methyl-N-hydroxycarbamoyl)-benzoyl]carbamate in N-methylhydroxylamine, acetate, and phosphate buffers

摘要

The rate of cleavage of ethyl N-[o-(N-methyl-N-hydroxycarbamoyl)benzoyl]-carbamate (ENMBC) in the buffer solutions containing N-methylhydroxylamine, acetate +N-methylhydroxylamine, and phosphate + N-methylhydroxylamine followed an irreversible consecutive reaction path: ENMBC -> A -> B where A and B represent N-hydroxyl group cyclized product of ENMBC and o-(N-methyl-N-hydroxycarbamoyl)benzoic acid, respectively. Both rate constants kappa_(1 obs) and kappa_(2 obs) showed the presence of buffer catalysts, but buffer catalysis turned out to be weak in the presence of N-methylhydroxylamine buffer, while it was strong in the presence of acetate and phosphate ones. Buffer-independent rate constants kappa_(10) and kappa_(20) increased linearly with the increase in a_(OH) with definite intercepts. The values of molar absorption coefficient of A, obtained under varying total buffer concentrations at a constant pH, showed the presence of a fast equilibrium: A + CH_3NHOH <=> C, where C represents N-[o-(N-methyl-N-hydroxycarbamoyl)methyl]benzohydroxamic acid.