Hydrocarbon Reactivity in the Superacid SbF₅/HF:  an ab Initio Molecular Dynamics Study

The free energy profiles for the protonation of simple alkanes in SbF 5 /HF superacid solution have been studied by ab initio molecular dynamics simulations. It is shown that the presence of extremely reactive protons at high SbF 5 concentrations open reaction channels for the C−H protonation with a considerably decreased barrier with respect to dilute solutions and the gas phase. It is proposed that this, along with the progressive presence of neutral SbF 5, is responsible for the experimentally observed increased reactivity of these solutions as the concentration of SbF 5 is increased. It has also been found that in solution an electrostatic stabilization of the transition state contributes to the lowering of the reaction barrier. In the case of ethane C−H protonation is in competition with C−C protonation. In fact, nearby, the transition state of C−C protonation, alternative reaction pathways that lead to the C−H protonation are accessible.