Question

Show a qualitative sketch and explain in words what you think that happens molecular-orbital-wise when the nucleophile attacks the electrophile in the SN2 reaction mechanism.

Answer 1

In an SN2 reaction, an occupied n orbital of the nucleophile (HOMO = highest occupied moecular orbital; lone electron pair) interacts with the unoccupied, antibonding σ* orbital of the substrate's C-L bond (LUMO = lowest unoccupied molecular orbital). As a result, a new bonding, as well as a new antibonding molecular orbital are developed. The bonding molecular orbital has a lower energy level than the initial n orbital of the nucleophile does. As only the two electrons of the nucleophile have to be distributed among the new molecular orbitals, the antibonding molecular orbital is not occupied. Therefore, the system is stabilized by the HOMO/LUMO interaction. Thus, the HOMO/LUMO interaction leads to a bonding interaction between the substrate and the nucleophile. If the nucleophile's n orbital interacts with the bonding, occupied σ orbital of the C-L bond, the new antibonding molecular orbital would also have to be occupied. The system would then not be stabilized by this HOMO/HOMO interaction. Thus, the HOMO/HOMO interaction does not lead to a bonding interaction between the substrate and the nucleophile.In an SN2 reaction, an occupied n orbital of the nucleophile (HOMO = highest occupied moecular orbital; lone electron pair) interacts with the unoccupied, antibonding σ* orbital of the substrate's C-L bond (LUMO = lowest unoccupied molecular orbital). As a result, a new bonding, as well as a new antibonding molecular orbital are developed. The bonding molecular orbital has a lower energy level than the initial n orbital of the nucleophile does. As only the two electrons of the nucleophile have to be distributed among the new molecular orbitals, the antibonding molecular orbital is not occupied. Therefore, the system is stabilized by the HOMO/LUMO interaction. Thus, the HOMO/LUMO interaction leads to a bonding interaction between the substrate and the nucleophile. If the nucleophile's n orbital interacts with the bonding, occupied σ orbital of the C-L bond, the new antibonding molecular orbital would also have to be occupied. The system would then not be stabilized by this HOMO/HOMO interaction. Thus, the HOMO/HOMO interaction does not lead to a bonding interaction between the substrate and the nucleophile.