5.4取代反应.ppt

1、5.4.1 Aliphatic Nuclephilic Substitution 5.4.2 Aromatic Electrophilic Substitution 5.4.3 Aromatic Nuclephilic Substitution,5.4 Substitution Reactions,5.4.1 Aliphatic Nuclephilic Substitution,Mechanism,SN2,Backside attack (Walden inversion) One-step process with no intermediate Linear TS,vk R-LNu,Wal

2、den inversion,Linear TS,SN1,slow,vk1 R-L,k1,k2,R+ is assisted by solvent Departure of L is also assisted by solvent,The ion pairs concept predicts that SN1 reactions can display either complete racemization or partial inversion!,Ion Pairs in the SN1,intimate,loose,Solvent-separated,Significant bondi

3、ng,Reactivity,Substrate Attacking Nucleophile Leaving Group Medium,(一) Substrate,SN2, 位阻小的反应快，烯丙基反应快。,RX Relative Rate RX Relative Rate Me 30 i-Bu 0.03 Et 1 Me3CCH2 10-5 n-Pr 0.4 Allyl 40 i-Pr 0.025 Bn 120,RX Relative Rate Et 0.26 i-Pr 0.69 CH2=CHCH2 8.6 PhCH2 100 Ph2CH 105 Ph3C 1010,能够形成稳定碳正离子的反应快。

4、,SN1,SN1 reactions should be possible with large rings.,The rates of SN1 reactions are independent of the identity of Nu,(二) Attacking Nucleophile,A Nu with a negative charge is always a more powerful Nu than its conjugate acid OH-H2O, NH2-NH3 In the same row of the periodic table, Nu is approximate

5、ly in order of basicity R3C-R2N-RO-F- Going down the periodic table, Nu increases. I-Br-Cl-F- (solvent-dependent) Steric influences OEt-Me3CO-,The Leaving group comes off more easily the more stable it is as a free entity. This is usually inverse to its basicity. ROHRO- , H2OOH-,(三) Leaving Group,To

6、sylate Brosylate Nosylate Mesylate (ROTs) (ROBs) (RONs) (ROMs),Triflate,Nonaflate,Tresylate,Solvent For most reactions SN1 rates go up and SN2 rates go down in solvents of increasing polarity aprotic polar solvent SN2,(四) Medium,Phase Transfer Catalysis (PTC),SN1,SN2,Tertiary alkyl Primary alkyl Poo

7、r nucleophile Good nucleophile Polar solvent Nonpolar solvent(protic solvent) (aprotic polar solvent) Good Leaving group Poor leaving group,Summary,Neighboring-Group Participation,threo threo Participated atom “O”，“N”，“S”，“X”， ability: I Br Cl,Rate increase Retention of configuration,(一）C=C as a nei

8、ghboring-group,（二）Aromatic rings as neighboring-groups,（三）C-C as a neighboring-groups,5.4.2 Aromatic Electrophilic Substitution,Arenium Ion,-complex,-complex,-complex,Bromination,Diazoniation,Diazonium Coupling,Example:,Nitration,Sulfonation,1) 同位素效应很小，因为决速步骤中没有CH断裂 2) 芳正离子的分离：,Orientation and React

9、ivity in Monosubstituted Benzene Rings,ortho meta para,These effects are a combination of Resonance and Inductive effects,各种效应在芳烃亲电取代反应中的表现,Electronic effect,ortho-/para- ratio,Steric effect,Other effect,1）Eletronic effect-If a strong activation group competes with a weaker one or with a deactivatin

10、g group, the former controls.,Orientation in Benzene Rings with More than One substitutent,Exp.,O，NH2，OH，NR2 OR, OCOR, NNHCORR ,Ar X m-direction groups,2）Steric hindrance-All other things being equal, a third group is least likely to enter between two groups.,Thermodymatic control,（一）SNAr Mechanism（

11、加成-消除）,Step 2:,Step 1:,5.4.4 Aromatic Nuclephilic Substitution,Benzene rings contain groups with both I and C effect on Ortho- or para- position.(exp:-NO2, CN, -CF3 etc.),Meisenheimer-Jackson salt,A change in leaving group do not have much effect on the reaction rate，exp：,X Cl, Br, I, SOPh, 时， 速度差别不大,(二) SN1Mechanism：,1) good leaving group（exp：OTf）with Ortho-position containing bulky groups（exp:t-BuO, SiR3）,2) diazonium salts,Step1,Step2,slow,(Sch