Mechanism of the Beckmann rearrangement in sulfuric acid solution

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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.