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1、第二章有机中间体与机理,有机反应活性中间体(Reactive intermediates),第一节.正碳离子(Carbocations),第二节.负碳离子(Carbanions),第三节.自由基(Free radicals),第四节.碳烯（卡宾）(Carbenes),第五节.氮烯（乃春）(Nitrenes),第六节.苯炔(Benzyne),有机化学反应历程,13.过渡态要用标准格式书写。一步反应的反应机理必须写出过渡态。14.必要时，考虑共振和分子轨道理论,Carbocations(碳阳离子),The central carbon atom in a carbocation is electr
2、on deficient;it has only six electrons in its outside energy level.,正电荷越分散，越稳定,1.诱导效应:FCH2CH2+CH3CH2+,2.共轭效应,-I,稳;+I,稳,2.1 P-共轭效应 p95,(Conjugated effects)：,二.The relative stabilities of carbocations：,2.2-P超共轭,共轭体系的数目越多越稳定,参与超共轭的C-H键越多越稳定,北理工01,bcad,中科院01,三.正碳离子的反应活性：与稳定性基本一致,1.RBr SN1 水解速率比较,醇：,按E1反
3、应活性由大到小排列：,按E1反应活性由大到小排列：,20,More substituted alkenes are more stable.H2C=CH2 R-CH=CH2 R-CH=CH-R R-CH=CR2 R2C=CR2 unsub.monosub.disub.trisub.tetra sub.Alkyl group stabilizes the double bond.,2.烯烃对HX的亲电加成反应活性,四.正碳离子的生成：,1)直接生成,异裂,2)对不饱和键的加成,3)由其它正离子转化,4）重排反应,R+RX,+CH2CH2OH Ethylene oxide,RC+=O RCOX,R
4、C+HOH RCHO,+CH2OH CH2O,+CH2CHCOR CH2=CHCOR,合成子,等效试剂,五.Reactions of Carbocations,4)Rearrangement,3)Combination with a nucleophile,2)Elimination of a proton,Addition to an unsaturated linkage,Usually the rearrangement was classified by the electron property of the moving group,such as Nucleophilic rea
5、rrangement,Electrophilic rearrangement,Radical rearrangement,etc.,Molecular Rearrangements:it refers those reactions in which the carbon skeleton or the position of functional group changed.,Alkyl groups and hydrogen can migrate in rearrangement reactions to give more stable intermediate carbocation
6、s.,Hydride shift:H-on adjacent carbon bonds with C+.Methyl shift:CH3-moves from adjacent carbon if no Hs are available.,The type of rearrengement,1.亲电加成与亲电取代机理 P67,烯烃与HX加成，H+首先加到CC电子云密度较高的碳原子上，形成较稳定的碳正离子，然后，X-再加成上去。如果加成得到的碳正离子不稳定，有可能重排为较稳定的碳正离子，生成取代或消除产物。,What is it Mars Rule?-The addition of HX to
7、an alkenes,the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms.,【解】,当芳环上取代基中的X或CO处于适当的位置时,均可发生分子内反应。如：,AlCl3,当芳环上电子云密 度高的易发生亲电取代反应。,主要产物,科大,a.SN1：烯丙醇、苄醇、叔醇、仲醇。,2、亲核取代反应机理：P 265,SN1 Reaction,Unimolecular nucleophilic substitution.Two step
8、 reaction with carbocation intermediate.Rate is first order in the alkyl halide,zero order in the nucleophile.Racemization occurs.【rsimizein】,nucleophilic,nju:kliufilik,-碳有支链的伯或仲醇和卤化氢反应时，发生分子重排。,科大,3.醇脱水成烯,Zaitsev消除取向,H催化：有重排Al2O3：不重排,醇的反应活性：3 2 1反应取向：查依采夫规则强酸催化下的醇消除是碳正离子历程,Acid Catalyzed Dehydratio
9、n of Secondary and tertiary alcohols,Protonation,establishing of good leaving group.,Elimination of water to yield carbocation in rate determining step.,Elimination of H+from carbocation to yield alkene.,醇的b消除,protonation,prutnein,eliminationlmnen,主要得双键上烃基取代较多的烯,氢少消氢,Orientation of elimination,E1 re
10、actions faithfully follow Zaitsevs rule!This means that the major product should be the product that is the most highly substituted.E1 reactions do not require an anti coplanar orientation of H and X.,E1反应机理,SN1反应机理,慢,慢,快,快,进攻-H,进攻C+,碱性强，升温对E1有利。中性极性溶剂对SN1有利。,E1反应机理和SN1反应机理的比较,4.Pinacol Rearrangemen
11、t:Nucleophilic Rearrangement*p116,an example of stabilization of a carbocation by an adjacent lone pair.,-H2O,H+,H+,1.Protonation of a hydroxyl group,Mechanism of the pinacol rearrangement,-H2O,2.Elimination of water to yield tertiary carbocation.,Resonance-stablized carbocation,Deprotonation,a.不对称的
12、取代乙二醇中，质子化,电子云密度大的羟基优先,动力:氧正离子,更稳定,满足八隅体结构,b.迁移亲核性强的基团优先，能提供电子的基团优先,规律：,Phenyl group move first,Phenyl with electron donor group move first,How to decide which will be major?First decide which carbocation is more stableThen consider the relative migratory aptitude of the groups that will undergo 1,2
13、-shift.Migratory aptitude arylalkyl usually Halkyl,Hydride generally migrates because of the generation of a more stable carbocation,Stereochemistry of migration:The migrating group retains configuration.That means migrating group is never totally detached,CH3迁移,环重排,00科大,The steric chemistry of Pinacol Rearrangements,Leaving group and shifting group must be anti-coplanar,中山05,中科大,03山大,中山05,中科院02,作业1,06中科院,作业2,03浙江,作业3,作业4 03大连理工,作业5,谢谢大家！,
