Stereoelectronic effects in Menshutkin-type S_N2 reactions: Theoretical study based on through-space/bond orbital interaction analysis

Through-space/bond orbital interaction analysis has been applied to investigate the stereoelectronic effects on stabilizing the transition state of Menshutkin-type S_N2 reactions. The mechanism of how the substituent effects work on accelerating the reactions has been demonstrated from orbital interaction perspective. The geometrical structures and Mulliken charge distributions have been compared to elucidate the substituent effects for the S_N2 reaction center. It is found that the substituents lower the activation energies by strengthening the orbital interactions in the S _N2 reaction process. When electron-donating and electron-accepting substituents (-C₆H₅ and -CHO) are introduced to the same central carbon at the reaction center, the symmetry allows the π-π* interactions among the donor and acceptor in the transition state. It stabilizes the transition state much more than the reactant complex. And the π-π* interactions are estimated to decrease about 2.28 kcal/mol of the energy for transition state. The σ-like orbitals of the partial bond around the central carbon are reactive, and the σ-π* orbital interactions stabilize the reactant complex much more than the π-σ* interaction. When the σ-π* and π-σ* interactions are deleted from the system, the activation energy increases and turns close to the values of the systems which are without such substituents. It can be concluded that the π-π*, σ-π*, and π-σ* interactions cooperatively accelerates the S_N2 reaction by stabilizing its transition state.