《哥伦比亚大学有机化学-第8章-替代和消除作为相互竞争的反应8.13》由会员分享，可在线阅读，更多相关《哥伦比亚大学有机化学-第8章-替代和消除作为相互竞争的反应8.13（15页珍藏版）》请在金锄头文库上搜索。
1、8.13 Substitution and Elimination as Competing Reactions,Alkyl halides can react with Lewis bases in two different ways; nucleophilic substitution or elimination.,H,X,+,Y,H,+,b-elimination,nucleophilic substitution,Two Reaction Types,How can we tell which reaction pathway is followed for a particula
2、r alkyl halide?,H,X,+,Y,H,+,b-elimination,nucleophilic substitution,Two Reaction Types,A systematic approach is to choose as a reference point the reaction followed by a typical alkyl halide (secondary) with a typical Lewis base (an alkoxide ion).,The major reaction of a secondary alkyl halide with
3、an alkoxide ion is elimination by the E2 mechanism.,Elimination versus Substitution,NaOCH2CH3 ethanol, 55C,CH3CH=CH2,+,(87%),(13%),Example,Br,E2,Figure 8.11,Br,SN2,Figure 8.11,Given that the major reaction of a secondary alkyl halide with an alkoxide ion is elimination by the E2 mechanism, we can ex
4、pect the proportion of substitution to increase with:,1) decreased crowding at the carbon that bears the leaving group,When is substitution favored?,Decreased crowding at carbon that bears the leaving group increases substitution relative to elimination.,primary alkyl halide,CH3CH2CH2Br,NaOCH2CH3 et
5、hanol, 55C,CH3CH=CH2,+,CH3CH2CH2OCH2CH3,(9%),(91%),Uncrowded Alkyl Halides,primary alkyl halide + bulky base,CH3(CH2)15CH2CH2Br,KOC(CH3)3,tert-butyl alcohol, 40C,+,CH3(CH2)15CH2CH2OC(CH3)3,CH3(CH2)15CH=CH2,(87%),(13%),But a crowded alkoxide base can favor elimination even with a primary alkyl halide
6、.,Given that the major reaction of a secondary alkyl halide with an alkoxide ion is elimination by the E2 mechanism, we can expect the proportion of substitution to increase with:,1) decreased crowding at the carbon that bears the leaving group 2) decreased basicity of the nucleophile,When is substi
7、tution favored?,Weakly basic nucleophile increases substitution relative to elimination,KCN,pKa (HCN) = 9.1,(70%),DMSO,secondary alkyl halide + weakly basic nucleophile,Weakly Basic Nucleophile,Weakly basic nucleophile increases substitution relative to elimination,NaN3,pKa (HN3) = 4.6,(75%),seconda
8、ry alkyl halide + weakly basic nucleophile,Weakly Basic Nucleophile,I,N3,Tertiary alkyl halides are so sterically hindered that elimination is the major reaction with all anionic nucleophiles. Only in solvolysis reactions does substitution predominate over elimination with tertiary alkyl halides.,Tertiary Alkyl Halides,+,+,2M sodium ethoxide in ethanol, 25C,1%,99%,Example,
