Theoretical Study of the Solvent Effect on Elimination Reactions: Hybrid-DFT Study and NBO Analysis

In elimination reactions, the arrangement of leaving groups plays a crucial role in facilitating the reaction. On the other hand, equatorial and axial stereoisomers of a compound have a different share of stability. The presence of a solvent plays a different role in the stability of each stereoisomer. LC-ωPBE and B3LYP methods and the Natural Bond Orbital (NBO) interpretation are used to investigate 3-chloro-8-methyl-8-azabicyclo [3.2.1] octane (1), 3-bromo-8-methyl-8-azabicyclo [3.2.1] octane (2), and 8-methyl-8-azabicyclo [3.2.1] octan-3-yl 4-methylbenzenesulfonate (3). The investigation of thermodynamic parameters shows that equatorial stereoisomers are more stable than axial ones, and by changing the gas phase to n-hexane and water, the stability of equatorial stereoisomers is increased. The stability of the equatorial stereoisomers is the result of the electrostatic model involving dipole-dipole interactions and the combined effects of hyperconjugation interactions and the total steric exchange energy (TSEE). Therefore, it can be expressed that the equatorial stereoisomers of compounds 1-3 show the superior performance in the elimination reaction of 1, 4 in the presence of a solvent, and this performance becomes more evident by changing the polarity of the solvent.