Competition between Elimination and Substitution for Ambident Nucleophiles CN– and Iodoethane Reactions in Gaseous and Aqueous Medium

Nucleophilic substitution (SN2) and elimination (E2) reactions between ambident nucleophiles have long been considered as typical reactions in organic chemistry, and exploring the competition between the two reactions is of great importance in chemical synthesis. As a nucleophile, CN– can use its C and N atoms as the reactive centers to undergo E2 and SN2 reactions, but related research is currently limited. This study uses the CCSD­(T)/pp/t//MP2/ECP/d electronic structure method to perform detailed investigations on the potential energy profiles for SN2 and E2 reactions between CN– and CH3CH2I in gaseous and aqueous media. The potential energy profiles reveal that the energy barriers for SN2 and E2 reactions with the C atom as the reactive center are consistently lower than those with the N atom, indicating that the C atom has a stronger nucleophilic ability and stronger basicity. Furthermore, the potential energy profiles in both gas and aqueous environments show that the barriers of SN2 reactions are lower than those for E2 reactions with both C and N as the attacking atom. By using the frontier molecular orbital and activation strain models to explain the interesting phenomenon, the transition from the gas phase to solution was investigated, specifically in the gas–microsolvation–water transition. The results show that water molecules reduce the nucleophilicity and basicity of CN–, while strain energy (ΔE strain) causes a greater increase in the energy barrier for E2 reactions. This study provides new insights and perspectives on the understanding of CN– as a nucleophile in SN2 reactions and serves as theoretical guidance for organic synthesis.