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Dominant Direct Mechanism at Low Collision Energy Induced by the Nucleophile.
Hongyi Wang1,2, Gang Fu1, Shiyue Liang3
1Xinjiang Key Laboratory for Value-added Utilization of Heavy Carbon Resources, School of Chemistry and Chemical Engineering, Yili Normal University, Yining 835000, P. R. China.
The Journal of Physical Chemistry. A
|March 18, 2026
Summary
The dominant mechanism for SN2 reactions changes from indirect to direct when the nucleophile is hydroxide (OH-) and the leaving group is varied. This highlights the influence of nucleophiles and leaving groups on reaction pathways.
Area of Science:
- Chemical Dynamics
- Reaction Mechanisms
- Computational Chemistry
Background:
- SN2 reactions typically proceed via an indirect mechanism at room temperature due to dynamic bottlenecks.
- Recent findings suggest a shift towards a direct mechanism for SN2 reactions involving hydroxide (OH-) as the nucleophile.
Purpose of the Study:
- To investigate the cause of the mechanism transition in SN2 reactions when the nucleophile is hydroxide.
- To compare the dynamic properties of various SN2 reactions (X- + CH3Y) to understand mechanism-changing regularities.
Main Methods:
- Direct dynamic simulations using the B3LYP/ECP/d method.
- Exploration of the OH- + CH3Br reaction.
- Comparative analysis of X- + CH3Y reactions (X = OH, F; Y = Cl, Br, I).
Main Results:
- For fluoride (F-) reactions, indirect mechanisms remain dominant, though their proportion decreases with less electronegative leaving groups (Cl to I).
- For hydroxide (OH-) reactions, a mechanism conversion occurs: the dominant pathway shifts from indirect to direct as the leaving group changes from chloride (Cl) to bromide (Br) or iodide (I).
Conclusions:
- The nucleophile and leaving group critically influence the dominant mechanism of SN2 reactions.
- Kinetic and dynamic factors, including energy barriers, electronegativity, and dipole moments, are crucial in determining SN2 reaction mechanisms.
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