A possible re-interpretation of the mechanisms of oximation of carbonyl compounds

Abstract

Differential temperature—time curves obtained during the reactions of propanal and cyclohexanone with excess of hydroxylamine in aqueous solutions over a wide range of pH values show that addition of hydroxylamine does not become rate-determining except in very strongly acidic solutions (pH < 0.4), which contradicts the mechanism that is generally accepted. An alternative mechanism is proposed; it involves (a) the equilibrium between hydroxylamine and hydroxylammonium ion, (b) a reversible reaction of the carbonyl compound with hydroxylamine, yielding a carbinolamine adduct, (c) equilibria among the neutral carbinolamine and its mono- and diprotonated forms, (d) dehydration of the monoprotonated and neutral forms of the carbinolamine, and (e) equilibrium between the protonated and neutral forms of the oxime. Values were obtained for enthalpy changes accompanying all of the elementary steps; for the equilibrium constants and entropy changes for all of the steps save those involving dehydration, which appear to be complete under the conditions studied; and for the rate constants for all of the steps save those involving proton-transfer, which appear to be instantaneous under the conditions studied. So many of these values appear reasonable that further inquiry into such a mechanism seems justified, but the nature of step (b) could not be fully elucidated.