Abstract
Bimolecular nucleophilic substitution (SN2) 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 SN2 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.
Highlights
Computational chemistry has become a very useful technique in organic chemistry
We evaluated the potential energy surface (PES) for the mentioned reaction starting from the reactants, the formation of an initial complex between the nucleophile and the CH3Cl, a transition state, a final complex where the leaving group is still bound to the substrate and the products
It is known that the SN2 reaction on a carbon center is a process that occurs through a doublewell potential energy surface (PES) in the gas phase (Figure 1, blue line)
Summary
Computational chemistry has become a very useful technique in organic chemistry. Since it is not part of the contents of the degree subjects in our institution, students approach the research groups to acquire basic knowledge about computational chemistry. In this way, they learn to look for reaction intermediates and transition states, evaluate bond distances and atomic charges, graph and analyze molecular orbitals; the energy calculations help them to conceptualize thermodynamic and kinetic factors in a reaction coordinate, integrating computational theoretical chemistry with the concepts acquired during the undergraduate degree. In Scheme 1 can be seen the general mechanism of the SN2 reaction, where
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