To bridge or not to bridge: The role of sulfuric acid in the Beckmann rearrangement

Based on ab initio calculations using the Gaussian-4 method, we propose a new catalytic mechanism for the Beckmann rearrangement in concentrated H₂SO₄. Our calculations suggest that H₂SO₄ catalyzes the 1,2-proton-shift step via a cyclic transition structure, in which H₂SO₄ acts as a proton-transfer bridge. The reaction barrier for this mechanism is lower by 48.1 kJ mol^-1 than the barrier for the previously suggested catalytic mechanism, which involves a strained 3-memberd-ring transition structure. According to the previous mechanism the 1,2-proton shift has the highest activation energy, while in the revised mechanism the highest activation energy is obtained for the ensuing rearrangement/dehydration step.