氣態溴化烷類與鈍氣分子雙分子親和取代(SN2)與消去(E2)反應的理論研究

Theoretical Study on the Gas-Phase Ion-Molecule SN2 and E2 Reaction of Bromoalkane and Noble Gas Molecules

王崇恩(CHONG-EN WANG)

指導教授 ： 胡維平

國立中正大學/理學院/化學暨生物化學系/碩士(2016年)

摘要

本篇論文一共分為三章，第一章和第二章我們主要探討在氣態中bromoalkane 與親核基的親核取代(nucleophilic substitution, SN2)與消去(nucleophilic elimination, E2)反應的穿隧效應(tunneling effects)對反應速率常數的影響、反應的競爭性以及動力學同位素效應(kinetic isotopes effects, KIE)。第三章則是探討類似「有機」鈍氣分子與氯離子的反應。
第一章我們探討直碳鏈鹵烷類RBr + CN(R=n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl)的SN2與E2 反應，結果顯示當SN2反應因立體障礙提高時，穿隧效應也會對反應速率常數有較明顯貢獻，但因SN2反應能障的上升使得主要反應也會逐漸變為E2反應，因此SN2與E2 路徑可能會產生競爭性。當R為iso-butyl bromide時，在300 K的反應速率常數，SN2反應只快E2反應2.2倍，而到400 K則更只差不到1.6倍，顯示二者具有相當的競爭性。
第二章節探討的主題為環鹵烷類RX (R=cyclopentyl(C5H9)、cyclohexyl(C6H11)；X=Br、I) 與CN或 SH的SN2與E2反應。我們發現由於環鹵烷相較直碳鏈鹵烷更具立體障礙，使SN2與E2反應更具有競爭性，二種反應能障只相差0.4至0.9 kcal/mol左右，此外我們也發現主要反應路徑不只受立體障礙的影響，也受到親核基的影響，在SH為親核基時反應會傾向進行SN2反應，而在CN時則傾向E2反應。而另外我們也發現具有較高反應能障的SN2反應，碳原子對穿隧效應也有相當的貢獻度，在Bromocyclohexane + CN反應，125 K時deuterium kinetic isotope effects的穿隧貢獻ηtunneling為1.14，而KIE(13C)為1.10，顯示出穿隧效應並非只有較輕原子的貢獻，其實碳原子也會有不小的貢獻。
在第三章我們在探討類似「有機」的鈍氣化合物是否會進行類似SN2及E2反應。我們模擬一個應該可以穩定存在的「有機」的鈍氣分子FKrCH2CH2Cl 與氯離子的 ion-molecule反應，而也確實發現四條可能進行的反應路徑，其中具有兩條為類似SN2反應，而另兩條則類似E2反應，顯示出「有機」的鈍氣分子確實有可能可以進行一般的有機化學反應。但反應能障又以斷C—Kr鍵較C－Cl鍵低上許多，因此雖然有4種反應途徑，但主要反應路徑還是為偏向斷C—Kr鍵的SN2與 E2反應。

關鍵字

溴化烷雙分子親核取代 ； 消去反應

並列摘要

This thesis consists of three chapters. In chapter one and chapter two, we studied the elimination (E2) and substitution (SN2) reactions of bromoalkane with nucleophiles in the gas phase. In particular, we investigated the following important properties: (1) the enhancement of tunneling effects on rate constants, (2) The competition of the two reaction pathways, and (3) the kinetic isotope effects. In chapters three, we studied the SN2 and E2 reactions of organic noble-gas molecules.
In chapter one, we studied the reactions of RBr + CN(R=n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl). The results showed that as the steric effect raises the barrier of the SN2 reactions, the tunneling effect also increases the rate constants. In the iso-butyl bromide + CNreaction the SN2 and E2 pathways are competitive at higher temperature.
In chapter two, we studied the reactions RX (R=cyclopentyl(C5H9)、cyclohexyl(C6H11)；X=Br、I) with CN or SH. The steric effect for cyclic alkyl halides was larger than linear alkyl halides. Therefore, the SN2 and E2 reaction pathways were more competitive. The differences in barrier heights between SN2 and E2 reactions ranged from 0.4 to 0.9 kcal/mol. The magnitude of barrier heights was affected not only by the steric factor but also the identity of the nucleophiles. When the nucleophile is SHthe reactions prefer the SN2 pathways. However, when the nucleophile is CNthe reactions prefer the E2 pathway. We also found that the tunneling effects of carbon atom contribute considerably to the rate constants for both pathways. In Bromocyclohexane + CN reaction, the tunneling contribution of KIE(D) was 1.14 and the contribution of tunneling for KIE(13C) was 1.10. It revealed the tunneling effect was important not only for light atoms but also for carbon atoms.
In chapter three, we studied whether organic noble-gas molecules and nucleophiles could also react by SN2 and E2 reaction pathways. Our model noble-gas molecule is FKrCH2CH2Cl and the nucleophile used is chloride ion. Our calculations identified four reaction paths. Two of them were SN2 reactions and the other two were E2 reactions. Our results suggest that the stable organic noble-gas molecules may also undergo common organic reactions.

並列關鍵字

SN2 and E2 reaction ； gas-phase ； cycloalkane

參考文獻

2. Minato, T.; Yamabe, S. J. Am. Chem. Soc. 1998, 110, 4586.

3. Meng, Q.; Gogoll, A.; Thibblin, A. J. Am. Chem. Soc. 1997, 119, 1217.

4. Gronert, S.; Fagin, A. E.; Wong, L. J. Am. Chem. Soc. 2007, 129, 5330.

5. Gronert, S. Depuy, C. H.; Bierbaum, V. M. J. Am. Chem. Soc. 1991, 113, 4009.

6. Gronert, S. Chemical reviews 2001, 101 (2), 329.