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Journal of the Korean Chemical Society (JKCS)

ISSN 1017-2548(Print)
ISSN 2234-8530(Online)
Volume 17, Number 2
JKCSEZ 17(2)
April 20, 1973 

Determination of Reactivities by Molecular Orbital Theory (Ⅴ). Sigma Molecular Orbital Treatment of SN Reactivities of Alkylchlorides.

화학반응성의 분자궤도론적 연구 (제5보). 염화알킬의 친핵성치환 반응성에 대한 시그마 분자궤도론적 연구
Ikchoon Lee, Bon-Su Lee, Kwang-Su Kim

이익춘, 이본수, 김광수
일련의 염화알킬(염화메틸, 에틸, 이소-프로필, 트란스 n-부틸, sec-부틸, tert-부틸)의 바닥 상태 전자구조와 친핵성 치환반응성을 근사적 시그마 분자궤도법(σ-MO)인 EHT 와 CNDO/2 법을 이용하여 비교 검토하였다. 염화알킬처럼 상호 구조적 차이가 현저하지 못한 경우에 있어서는 EHT법이 CNDO/2 법에 비하여 비교적 부정확함을 알았다. CNDO/2 계산에 의하면, 쌍극자모멘트의 계산치가 실험치 보다 약간 크게 주어지며 대체로 primaryGround electronic structures and SNreactivities of a series of alkylchlorides (methyl,ethyl, iso-propyl, trans n-butyl, sec-butyl, tert-butylchloride) have been studied using approximate σ-MO method, such as EHT and CNDO/2. It was found that CNDO/2 gives better results for the systems such as alkylchlorides whose structural differences are not remarkable, in comparison with EHT method. According to CNDO/2 results, calculated dipole moments for alkylchlorides are slightly higher than observed values, showing the order of primary < secondary < tertiary alkylchlorides. It was also found that highest occupied(HO) MO's are completely or nearly degenerate, and show relatively weak π-antibonding nature between α-carbon and Cl atoms. Furthermore, the electrons in this MO are largely confined to Cl atom, and hence these behaves as likely as p-lone pair electrons of Cl atom. On the contrary, lowest unoccupied (LU) MO's show strong σ-antibonding nature between α-carbon and Cl atoms whose electron clouds are directed along the C-Cl axis. It has been discussed that the SN2 reactivities of alkylchlorides may largely be controlled by σ* LUMO, and the antibonding strength between α-carbon and Cl atoms in this MO may become the measure of SN2 reactivity. The relationship between SN2 reactivity and C-Cl bond polarizability has also been discussed. It has been suggested that the unique structure factors determining SN1 reactivities may be π-antibonding strength between α-carbon and Cl atoms in HOMO and C-Cl bond strength in ground state.
95 - 104
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