Substituent effects on menshutkin-type reactions in the gas phase and solutions: Theoretical approach from the orbital interaction view

In this study, we developed a method to interpret the mechanism of acceleration for Menshutkin-type reactions in solutions theoretically, from the orbital interaction view, utilizing the through-space/bond (TS/TB) interaction analysis in the polarizable continuum model (PCM). Different method levels were tested to determine the substituent effects on the reactions of NH₃ attacking para-substituted benzyl bromide. The geometrical structures and Mulliken charge distributions were analyzed to elucidate the substituent effects on the S_N2 reaction center. The results of Mulliken charge analysis showed that the para-substituted benzyl group (-C₆H₄Y) received negative charge through the reaction process, and both electron-donating and electron-withdrawing substituents Y made -C ₆H₄Y groups receive greater charges. Solvent effects on the structures of transition states (T-S(s)) were significant. The structures of T-S(s) were found to be exhibiting longer bond lengths in solutions, especially in polar solvents such as water. Our TS/TB-PCM analysis method can predict the substituent effects in solutions by evaluating contributions from orbital interactions in question. The orbital interaction analysis results revealed that the key orbital interactions for stabilizing the T-S(s) of the systems with substituents Y = NH₂ and NO₂ in water were n(NH ₂)-π*(ph) (ph = phenyl) and π(ph)-π*(NO ₂) interactions, respectively. Stronger interactions between π*(ph) and σ*(C_α-Br) occurred because of the n(NH₂)-π*(ph) and π(ph)-π*(NO₂) interactions that resulted when para-substituents -NH₂ and -NO ₂, respectively, were added to the system. These stronger π*(ph)-σ*(C_α-Br) interactions stabilized the transition state and enabled the Br leaving group to leave more easily.