THE SN2 REACTION: A THEORETICAL-COMPUTATIONAL ANALYSIS OF A SIMPLE AND VERY INTERESTING MECHANISM

CAPURSO, MATÍAS; GETTE, RODRIGO; RADIVOY, GABRIEL; DORN, VIVIANA

Santiago de Compostela

Simposio; 23th International Electronic Conference on Synthetic Organic Chemistry; 2019

ECSOC

Bimolecular nucleophilic substitution (SN2) reaction is one of the most frequently process choose as mechanism model to introduce undergraduate chemistry students in the computational chemistry methodology.1 Initially, simple computational calculations are proposed, to then increase its complexity. In this way, students correlate concepts already learned in previous subjects, with the results obtained from the computational calculations.In this work, we performed a computational analysis for the ionic SN2 reaction, applying DFT methods with the Gaussian09 program. In this mechanism, 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 using the B3LYP functional, applying the 6-31+G* basis set for all atom except iodine (6-311G*) and the solvents effect (acetonitrile and cyclohexane) were evaluated with the PCM model as implemented in Gaussian09. 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. We analyzed the atomic charge (ESP) and the bond distance throughout the process. Vacuum and solvent analyze were performed in order to analyze the solvation effects on the reactivity of the different nucleophiles. As is known, in the gas phase (vacuum), fluoride was the best nucleophile. All the structures were shown to be more stable energetically under solvated conditions than ones in vacuum. Despite the notable difference in the solvation capacity between the both solvents study (acetonitrile and cyclohexane), as indicated by the Hughes-Ingold rules, an increase in polarity solvent produces a small decrease on the rates of the reaction, due to the dispersal of charges in the transition state. On the other hand, we thought it would be enriching, to carry out a reactivity analysis from the point of view of molecular orbitals. So, we analyzed the OM HOMO and the OM LUMO of the different stationary states of the PES, for the SN2 reaction between methyl chloride and fluoride and bromide as nucleophiles, both in a vacuum and in acetonitrile as the solvent.