Mechanism of the ethanol-based (C2H5OH2)+(SbF6)− salt formation by the superacid-catalyzed acetaldehyde hydrogenation

The mechanism of the acetaldehyde hydrogenation reaction yielding the ethanol-based (C2H5OH2)+(SbF6)− salt with the assistance of the HSbF6 (HF/SbF5) superacid catalyst was investigated theoretically using ab initio MP2 and CCSD(T) methods and the aug-cc-pVDZ basis set. (The effects of surrounding solvent were approximated by employing the polarized continuum solvation model and by the explicit inclusion of one solvent molecule.) The most important findings include the observation that the activation barriers calculated for both phases considered are significantly reduced (by 66%) in comparison with the uncatalyzed acetaldehyde hydrogenation process. The reaction begins with the spontaneous (barrier-less) protonation of acetaldehyde by the superacid and continues through the formation of ethanol molecule followed by its subsequent protonation by the recovered HSbF6. The overall process is exoenergetic by 63 kcal/mol and leads to the formation of the ethanol-based (C2H5OH2)+(SbF6)− ionic salt as the final product.