- 學位論文
環鹵烷雙分子親核取代(SN2)與消去(E2)反應的理論研究
Theoretical study on the gas-phase ion-molecule SN2 and E2 reactions of halides of cycloalkanes
摘要
本論文共有三章，重點在探討環鹵烷與非環鹵烷與陰離子的親核基的消去 (nucleophilic elimination, E2)及親核取代(nucleophilic substitution, SN2) 反應的競爭性以及動力學同位素效應的比較。 第一章中我們討論RX + SH ( R=ethyl (C2H5), isopropyl (C3H7), tert-butyl (C4H9), cyclopentyl (C5H9), cyclohexyl (C6H11), n-propyl (C3H7), n-butyl (C4H9), n-pentyl (C5H11), n-hexyl (C6H13)；X=Cl、Br、I)，研究結果顯示以上反應除了R為tert-butyl時以及與RX 為C5H9I時會進行E2反應，其餘系統均較容易進行SN2反應。當R為cyclopentyl，SN2反應與E2反應的反應能障相差僅約0.5 kcal/mol ~ 2.6 kcal/mol，兩反應路徑間有競爭的可能性。 第二章討論的主題為RX + CN ( R=ethyl (C2H5), isopropyl (C3H7), tert-butyl (C4H9), cyclopentyl (C5H9), cyclohexyl (C6H11), n-propyl (C3H7), n-butyl (C4H9), n-pentyl (C5H11), n-hexyl (C6H13)；X=Cl、Br、I)，研究結果顯示，當R為cyclopentyl，E2反應較容易進行， C5H9X (cyclopentyl，X=Cl、Br、I) + CN系統的E2反應數率常數高於SN2反應常數約1.6~15.6倍。當R為C6H11(cyclohexyl，Cl、Br、I)系統的E2反應數率常數高於SN2反應常數約2.6~6倍。由計算出的反應速率常數來看，當R為C5H9X (cyclopentyl) + CN及C6H11 (cyclohexyl，Cl、Br、I) + CN系統時，反應雖然可能以E2反應為主，但兩種反應路徑應該會互相競爭。 在第三章中我們探討環鹵烷與非環鹵烷的SN2反應能障差異，為避免不同的反應能量造成干擾，此章主要討論identity exchange反應。我們將探討各種環鹵烷與非環鹵烷的、反應能障、速率常數、同位素效應的差異性、以及環張力對於SN2的影響。此章中研究的反應為RX+X( R=ethyl (C2H5)，isopropyl (C3H7)，tert-butyl (C4H9)，cyclopentyl (C5H9)，cyclohexyl (C6H11)；X=Cl、Br、I)，我們計算得到的結果顯示當R group為環戊烷基時其SN2反應性卻高於當R group為異丙烷基，如在C5H9Cl (Chlorocyclopentane) + Cl的反應能障(4.4 kcal/mol)低於C3H7Cl (2-Chloropropane) + Cl反應能障(6.3 kcal/mol)約1.9 kcal/mol，顯示當環戊烷的反應中，環張力並沒影響SN2反應性。我們預測出的KIEs 在環鹵烷與非環鹵烷呈現些許的normal 效應，但在環鹵烷反應中KIEs會較非環鹵烷略高。
並列摘要
This thesis consists of three chapters. We studied the bimolecular nucleophilic substitution (SN2) and bimolecular elimination (E2) reactions of cyclic and noncyclic alkyl halides. In Chapter 1, we studied RX + SH( R=ethyl (C2H5), isopropyl (C3H7), tert-butyl (C4H9), cyclopentyl (C5H9), cyclohexyl (C6H11), n-propyl (C3H7), n-butyl (C4H9), n-pentyl (C5H11), n-hexyl (C6H13)；X=Cl、Br、I). For reactions of C4H9X (tert-butyl，X=Cl、Br、I) + SH and C5H9I (Iodocyclopentane) + SH, the elimination reactionas were favored . The substitution reactions were the most feasible pathway for the other reactions. When the reactant is cyclopentyl halide, the differences in barrier heights between substitution and elimination reactions ranged from 0.5 to 2.6 kcal/mol. The elimination reactions would compete with the substitution reactions. In chapter 2, we studied RX + CN( R=ethyl (C2H5), isopropyl (C3H7), tert-butyl (C4H9), cyclopentyl (C5H9), cyclohexyl (C6H11), n-propyl (C3H7) , n-butyl (C4H9), n-pentyl (C5H11), n-hexyl (C6H13)；X=Cl、Br、I) reactions. According to theoretical study of transition state theory (TST), the elimination reactions were favored over the substitution reactions for the C5H9X (cyclopentyl，X=Cl、Br、I) + CN. Especially, the rate constants of the elimination reactions were higher than theose of the substitution reactions by 1.6 to 15.6 times for the reactions of C5H9X (cyclopentyl，X=Cl、Br、I) + CN. The rate constants of the elimination reactions were higher than theose of the substitution reactions by 2.6 to 6 times for the reactions of C6H11X (cyclohexyl, X=Cl、Br、I) + CN. The substitution and the elimination pathways are competitive for the reactions of C5H9X and C6H11 X.l. In Chaper 3, we studied the identity exchange SN2 reactions of RX + X(R=ethyl (C2H5), isopropyl (C3H7), tert-butyl (C4H9), cyclopentyl (C5H9), cyclohexyl (C6H11), X=Cl、Br、I). The energy barrier of C5H9Cl + Cl(4.4kcal/mol) is lower than that of C3H7Cl + Cl(6.3 kcal/mol) by 1.9 kcal/mol. The deuterium KIEs of both cyclic and noncyclic halides were predicted to be normal with those of cyclic halides being slightly higher.