Competition between S _N 2 and E2 Pathways in CN ^– + RI/RF Systems: Effects of Reactive Centers, Substitution, and Leaving Groups

This study examines how substitution degrees in S_N2 reactions using CN^- and alkyl halides (RI/RF) are determined through detailed electronic structure calculations. The results reveal that for ambident nucleophile CN^-, sp³ hybridized C dominates S_N2 pathways at low substitution degrees (α = 1-2), while sp hybridized N demonstrates superior reactivity at high substitution degrees (α = 3). However, E2 pathways consistently favor C as the reactive center, regardless of the substitution degree. For CN^- + RI systems, S_N2 barriers increase significantly with α-methyl substitution, with activation strain model (ASM) analysis identifying strain energy as the primary influence of barrier heights, showing strong correlation with geometric distortion parameters (%<i>D</i>^‡, <i>R</i>² = 0.81-0.99). Conversely, E2 pathways maintain relatively stable geometric distortion through the concerted cleavage of C^α-I and H^β-C^β bonds, resulting in gradually decreasing barriers. Notably, the superior leaving group I leads to lower S_N2 transition state barriers than E2 at α = 1-2, attributable to the weak C-I bond and minimal steric hindrance. At α = 3, increased steric bulk stabilizes the E2 pathway, providing an explanation for the experimentally observed significant rate enhancement at α = 3. In contrast, for CN^- + RF systems, the barrier difference between E2 and S_N2 pathways becomes smaller with increasing substitution degrees. This suggests distinct substitution degree-dependent trends in rate constants between systems containing leaving groups F and I.