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Journal of the Korean Chemical Society (JKCS)

ISSN 1017-2548(Print)
ISSN 2234-8530(Online)
Volume 34, Number 3
JKCSEZ 34(3)
June 20, 1990 

Molecular Orbital Studies on the Reaction Path and Reactivity of SN2 Reactions. Determination of Reactivity by MO Theory (Part 69)

SN2 반응의 반응경로 및 반응성에 관한 분자궤도함수 이론적 연구
Ikchoon Lee, Jeoung Ki Cho, Hai Whang Lee, Hyuck Keun Oh

이익춘, 조정기, 이해황, 오혁근
The gas-phase SN2 reactions can be classified into neutral bimolecular, solvated, and ionic reactions; the neutral bimolecular reaction proceeds via retention mechanism whereas the ionic reaction produces inversion products. In the reaction of solvated nucleophile with one solvent molecule, a six-center transition state (TS) is formed and the two processes i.e., retention and inversion, are found to compete with a favored path depending on the electronic effect of the nucleophile and substituents in the substrate and on the steric requirement. In the ionic reaction, the difference in the energy barrier between the two processes reduces to a small value when the substrate methyl group is made bulky, leaving ability of the leaving group is improved and at the same time the negative charge of the nucleophile is dispersed. When the reaction center atom in the SN2 reaction is changed to a larger sized second row elements, the activation barrier decreases since the steric crowding in the penta-coordinated TS is relieved. However within the same row, the barrier was found to increase as the atomic size decreased. For the boron, B, the barrier height was the least since in addition to the relatively large atomic size compared to C and N, it forms tetra-coordinated TS so that the steric crowding becomes nearly negligible.
239 - 247
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