Mechanism of the Beckmann rearrangement in sulfuric acid solution

Abstract

Ab initio calculations have been performed to probe the mechanism of the Beckmann rearrangement of formaldehyde oxime in concentrated sulfuric acid or in oleum solution (H₂SO₄ + S₂O₇ ). In the gas phase, the most favoured reaction path is: protonation of oxime → N-protonated oxime → O-protonated oxime → fragmentation products, in which the 1,2-H-shift connecting both protonated forms constitutes the rate-determining step. Reaction field calculations using two different models [Onsager self-consistent reaction field (SCRF) and polarizable continuum model (PCM)] indicate that the non-specific interaction of the solvent exerts only a small effect on both the energetic and geometrical parameters of the considered reaction path. Formation of the sulfate ester, H₂CN–O–SO₃H, also appears to play a negligible role in marginally affecting the 1,2-H-shift. In contrast, a specific interaction between solvent molecules and substrates seems to be the dominant factor in reducing substantially the energy barrier to 1,2-H-shift. Using a neutral H₂SO₄ molecule as a simple model for solvent molecules, MP2/6-311G(d,p) energy calculations based on HF/6-31G(d)-geometries of the supermolecule reveal that the barrier to 1,2-H-shift is decreased by 115 kJ mol^-1 with respect to the gas phase value, when a H₂SO₄ molecule interacts specifically with the protonated oxime and thereby assists the hydrogen migration. The calculated results thus suggest a strong case of active solvent participation in which the solvent molecules exert a catalytic effect.