The S_n2 Reaction: A Theoretical-Computational Analysis of a Simple and Very Interesting Mechanism

Bimolecular nucleophilic substitution (S_N2) reaction is one of the most frequently processes chosen as model mechanism to introduce undergraduate chemistry students to computational chemistry methodology. In this work, we performed a computational analysis for the ionic S_N2 reaction, where the nucleophile charged (X⁻; X=F, Cl, Br, I) attacks the carbon atom of the substrate (CH₃Cl) through a backside pathway, and simultaneously, the leaving group is displaced (Cl⁻). The calculations were performed applying DFT methods with the Gaussian09 program, the B3LYP functional, the 6-31+G* basis set for all atoms except iodine (6-311G*), and the solvents effects (acetonitrile and cyclohexane) were evaluated with the PCM model. We evaluated the potential energy surface (PES) for the mentioned reaction considering the reactants, the formation of an initial complex between the nucleophile and the substrate, the transition state, a final complex where the leaving group is still bound to the substrate and the products. We analyzed the atomic charge (ESP) and the bond distance throughout the process. Gas phase and solvent studies were performed in order to analyze the solvation effects on the reactivity of the different nucleophiles. We observed that increasing solvent polarity, decreases reaction rates. On the other hand, we thought it would be enriching, to carry out a reactivity analysis from the point of view of molecular orbitals. Therefore, we analyzed the MOs HOMO and the MOs LUMO of the different stationary states on PES, both in a vacuum (gas phase) and in acetonitrile as the solvent.