高等有机化学第四章亲电加成反应.ppt

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1、1,第四章 亲电加成反应,2,（1）亲电加成(electrophilic addition)（2）亲核加成(nucleophilic addition)（3）自由基加成(radical addition)（4）环加成(cycloaddition),碳碳重键(multiple bonds)的加成反应,3,一、碳-碳双键的亲电加成反应,烯烃活泼的原因：电子受核束缚力小，容易极化。,4,1.烯烃的亲电加成反应历程,5,Mechanism（1）involves prior dissociation of the electrophile and implies that a carbocation i

2、s generated that is free of the counterion Y-at its formation.,（1）Prior dissociation of electrophile and formation of carbocation intermediate,6,（2）Formation of carbocation ion pair from alkene and electrophile.,7,Mechanism（2）also involves a carbocation intermediate,but it is generated in the presen

3、ce of an anion and exists initially as an ion pair.Depending on the mutual reactivity of the two ions,they might or might not become free of one another before combining to give product.,8,（3）Formation a bridged carbonic intermediate from alkene and electrophile.,9,Mechanism（3）leads to a bridged int

4、ermediate that undergoes addition by a second step in which the ring is opened by a nucleophile.Mechanism（3）implies stereospecific anti addition.,Mechanisms（1）,（2）and（3）are all AdE2 reactions:they are bimolecular electrophilic additions.,10,（4）Concerted addition of electrophile and nucleophile in a

5、termolecular reaction,11,Mechanism（4）is a process that has been observed for several electrophilic additions and implies concerted transfer of the electrophilic and nucleophilic components of the reagent from two separate molecules.It is a termolecular electrophilic addition,AdE3,a mechanism that im

6、plies formation of a complex between one molecule of the reagent and the reactant and also is expected to result in anti addition.,12,碳正离子历程,v=k isopropeneHCl,（2）双分子历程,13,生成结构稳定的碳正离子中间体,碳正离子中间体的AdE2历程的条件,叔碳正离子中间体、苄基型碳正离子,14,通常不具有立体选择性,15,有时有重排产物出现,重排产物的出现可作为经碳正离子中间体历程的证据之一,16,鎓型离子历程,立体化学为反式加成,17,“Br

7、omine with two bonds and a positive charge?In a three-membered ring?In 1937,two Columbia chemists proposed just such a species as an intermediate in the reaction of ethylene with bromine.They were right.”,18,溴鎓离子具有八隅体结构，碳正离子为六电子体，前者比后者稳定。,Br2,I2,IN3,RSCl or ArSCl,Hg(OCOCH3)2,亲电试剂的原子半径要足够大,思考：为什么形成溴鎓

8、离子而不是碳正离子？,哪些亲电试剂与烯烃可形成鎓型离子中间体？,19,鎓离子存在的直接证据？,Biadamantylidene bronomium,20,（3）三分子历程（AdE3）,某些非共轭烯烃与HX加成按AdE3历程进行。,立体化学通常为反式加成,21,complex,22,23,2.烯烃亲电加成反应的立体化学,（1）反式加成（Antiaddition）,24,AdE2：涉及环状鎓离子的形成,反式加成,25,反式加成,26,反式加成,27,AdE3：一分子烯烃和一分子烯烃与HX形成的络合物反应。,反式加成,28,（2）顺式加成（synaddition）,碳碳双键与苯基共轭的烯烃和HX的加

9、成反应主要给出顺式加成产物,29,Ion pair,Owing to the greater stability of the benzylic carbocations formed in these reactions,concerted attack by halide ion is not required for protonation.,30,对比立体选择性,31,顺式加成,p,硼烷与烯烃的加成为顺式加成,32,（3）无立体选择性的加成,33,实例1：,解释双键碳原子上连有苯基的烯烃与溴加成时，顺式加成产物增多。如：溴与反-1-苯基丙烯反应所得反、顺式加成产物之比为88:12；当双

10、键碳原子所连的苯基上有给电子基团时，顺式加成产物的比例也明显升高。,34,反式加成产物（88%）,顺式加成产物（12%）,35,反式加成产物（63%）,顺式加成产物（37%）,36,某些芳烃与卤素加成，主要生成顺式加成产物。,（35%）,（10%）,实例2：,37,提示：,38,邻基参与（注意）,当双键邻位有带未共用电子对的基团存在时，该基团会对碳正离子中间体进行亲核进攻。,39,例如：,40,对比：,41,42,某些五、六元环状内酯或环醚的形成也和邻基参与有关。,43,44,3.烯烃亲电加成反应的活性,（1）烯烃结构对加成速率的影响,45,结论：Z为推电子基，使双键电子云密度增加；稳定碳正离

11、子中间体，降低活化能,46,结论：Z为吸电子基，使双键电子云密度降低；使碳正离子中间体更不稳定，增加活化能,47,Z为强吸电子基时，烯烃不发生亲电加成反应，发生亲核加成反应！,48,49,（2）亲电试剂对加成速率的影响,对于特定烯烃，卤化氢的加成速率与酸性强弱一致。,HI HBr HCl HF,对于特定烯烃，混合卤素的加成速率与其异裂难易程度相符,ICl IBr I2,50,4.烯烃亲电加成反应的定向规律,区域选择性影响因素：电子效应，空间效应。,51,（1）电子效应,52,“Markovnikov 规则”？,“围绕碳正离子的-CC单键旋转，当带正电荷碳原子的p轨道轴和-CH键的轨道轴在同一平

12、面时，这两个轨道可发生部分重叠，使部分正电荷分散到甲基上，起稳定碳正离子的作用，这种现象称超共轭作用”。,53,对含强吸电子基团的烯烃与不对称亲电试剂的加成，从表面上看是反马氏规则的。,54,也适用于苯乙烯类与卤化氢等不对称试剂的加成的区域选择性，且当苯环上连有强吸电子基时，其区域选择性与吸电子基和双键碳原子直接相连时相似。,55,（2）立体效应,56,二、炔烃和丙二烯类的亲电加成反应,1.炔烃的亲电加成,57,Since alkynes have type orbitals,it is not surprising that there is a good deal of similarit

13、y to the reactivity of alkenes.,The fundamental questions about additions to alkynes include the following:How reactive are alkynes in comparison with alkenes?What is the stereochemistry of additions to alkynes?,58,What is the regiochemistry of additions to alkynes?The important role of bridged ions

14、 in addition reactions of alkenes raises the question of whether similar species are involved with alkynes,where the ring includes a double bond and bridged intermediates and would be expected to be substantially more strained.,59,The basic mechansims that are considered to be involved in electrophi

15、lic additions to alkynes:,vinyl cation,syn+anti,AdE2,60,This mechanism involves a discrete vinyl cation.In general,this reaction will lead to a mixture of the two stereoisomeric addition products.,61,Bridged intermediate,Bridged intermediate,AdE3,AdE2,62,Mechanisms B and C depict bridged intermediat

16、es formed without or with participation of a second electrophilic molecule.They should lead to anti addition.,63,Mechanism D is a termolecular process that would be expected to be a stereospecific anti addition.,AdE3,64,In general,alkynes are somewhat less reactive than alkenes toward many electroph

17、iles.A major reason for this difference in reactivity is the substantially higer energy of the vinyl cation intermediate that is formed by an electrophilic attack on an alkyne.,Reactivity,65,For additions that proceed through bridged intermediates,the alkynes are also less reactive because of additi

18、onal strain in the intermediate.,Reactivity,66,Example 1.Hydrohalogenation,Aryl-substituted acetylenes give mainly the syn addition product.,vinyl cation(as ion pair),67,Alkyl-substituted acetylenes can react with HCl by either the AdE3 or the AdE2 mechanism,depending on the reactant structure and t

19、he reaction conditions.The stereochemistry is anti addition.,68,anti,The presence of Br-greatly accelerates the reaction,69,Example 2.Halogenation of alkynes,Disubstituted(internal),anti,70,2.丙二烯类的亲电加成（注意）,allene,allylic carbocation?,vinyl carbocation,71,The kinetically favored protonation at a sp2

20、carbon leads to the vinyl cation intermediate.,The reason is stereoelectronic:the allene structure is nonplanar,so that a protonation of the center carbon leads to a twisted structure that lacks of allylic conjugation.,+,72,+,中间过程缺乏烯丙基型碳正离子的共轭,73,（1）丙二烯与卤化氢加成时，主要生成卤素加到中间碳原子上的产物.,主产物,74,（2）丙二烯与卤素和汞离子

21、可形成桥状离子中间体，亲核试剂一般进攻其烯丙位。,75,3.共轭二烯的亲电加成反应,76,1,2-,77,9.3 共轭二烯的亲电加成反应,78,低温，动力学控制（考察中间体的稳定性），主要生成1,2-加成产物；高温，热力学控制（考察产物的稳定性），主要生成1,4-加成产物。,在同样的反应条件下，高温加热两异构体可得相同含量的混合物,卤素和共轭多烯加成也可生成多种加成产物,79,9.4 亲电加成反应在有机合成中的应用,形成CX键、CO键、CN键、CC键等,1.CX键的形成加X2，HX，HOX,2.CO键的形成加H2O，醇，羧酸,3.CN键的形成加NOCl，NO2Cl,80,4.CC键的形成加RX，RCOCl,81,习题：,（1）比较下列化合物与HCl反应的反应活性：,（2）比较下列化合物与Br2反应的反应活性：,（3）完成下列反应方程式：,