Reactions d'addition nucléophile sur les cétones : Addition de l'hydroxylamine: Mise en évidence de facteurs steriques par la mésure de l'éffet isotopique sécondaire du deutérium

Abstract

Resume The rates of addition of NH2OH, in water at 25°, to seventeen ketones and their α-deuterated homologs, have been measured. The secondary kinetic isotope effects are expressed by the ratio kH/kD. The reaction rates have been measured in an acidic medium, and in these conditions the rate determining step is the attack of the free nucleophile on the carbonyl group of the ketone. The isotope effects are discussed in terms of electronic and steric factors. For this reaction the transition state is “reactant-like”. as we have shown elsewhere. As already observed in the addition of SO3= ion, the hyperconjugative factor is still operating in this case; however due to the shortness of the incipient CN bond, two kinds of steric effects art superimposed on the hyperconjugation. These steric effects, that we have called “effet sterique d'approche” and “effet sterique de torsion”, stem from the interaction between the α CH bonds and the incipient nucleophile and are, generally, mutally exclusive. The “effet steriquc de torsion” can arise when the NH2OH molecule approaches the substrates along a line perpendicular to the plane of the CO group. In the other cases the “effet sterique d'approche” is operative.