《加州大学伯克利分校有机化学课件79548》由会员分享，可在线阅读，更多相关《加州大学伯克利分校有机化学课件79548（52页珍藏版）》请在金锄头文库上搜索。
1、Chapter 14 Delocalized Pi SystemsTrigonalTrigonalThe The bond is bond is e e- - rich: rich: E E+ +attackattack, , RR addadd) ) The lobes of the The lobes of the p p- - orbitals:orbitals: PerpendicularPerpendicular to the to the sigma frame and sigma frame and parallelparallel to each to each other.o
2、ther.Recall the double bondRecall the double bond2-Propenyl (Allyl)Question:Question: What about adding a What about adding a thirdthird p p -orbital adjacent to the double bond?-orbital adjacent to the double bond?Is there something special?Is there something special?Or: Is there any special Or: Is
3、 there any special reactivity at the carbons reactivity at the carbons adjacent to a double bond?adjacent to a double bond?Ha.a.b.b.c.c.S SNN1 reactivity of 1 reactivity of allylicallylic carbon like that carbon like that of of R RsecsecX X, , even though it is primary!even though it is primary!p pK
4、 Ka a 40: 40: Acidic!Acidic!5050S SNN1 187 kcal mol87 kcal mol-1-1: : Weak!Weak!101 kcal mol101 kcal mol-1-1Replacing one of the hydrogens in Replacing one of the hydrogens in etheneethene with with another another spsp2 2-hybridized carbon gives an -hybridized carbon gives an allylicallylic system.
5、system. AllylicAllylic position positionObservations:Observations:HLH BBHHClearly: Allylic are stabilized.- -+ +Short notation: Dotted linesShort notation: Dotted linesWhy? Why? Resonance!Resonance!MO Picture of 2-Propenyl (Allyl)3 3 p pOsOs 3 MOs 3 MOsRecallRecall: Bonds made by overlap: Bonds made
6、 by overlapHH. .HH. .H HH H+ + +HH+ + +HH+ +HH+ +HHIn phaseIn phase+ +HH+ +- -HH- -HH+ +HH Out of Out of phasephaseBondingBondingAnti-Anti- bondingbondingNodeNodeSign of the wave function,Sign of the wave function, Not charge!Not charge!E EE E * * * *CH2CH2What happens to this picture What happens t
7、o this picture when we interact with another when we interact with another p p orbital? orbital?1)1)Interaction with the Interaction with the bonding orbital causes bonding orbital causes energy splitting: the energy splitting: the p p orbital level moves up and orbital level moves up and the the bo
8、nding level moves bonding level moves down.down.2)2)Interaction with the Interaction with the antibondingantibonding orbital causes orbital causes the energy level of the the energy level of the p p orbital to move down to orbital to move down to where it was originally and where it was originally a
9、nd the energy level of the the energy level of the antibondingantibonding orbital to move orbital to move up. The two effects on the up. The two effects on the p p orbital cancel each other orbital cancel each other out and the two out and the two orbitals orbitals are pushed apart.are pushed apart.
10、Interactions of a singly occupied p-orbital with each of the molecular orbitalsp p E E * *0 0 * *upupdowndownunchangedunchangedEtheneEtheneAllylAllylp p Orbital OrbitalNonbonding Nonbonding MOMOp p E E# of e # of e depends depends on on + +, , , ,- - * *0 0Resulting picture:Resulting picture:And And
11、 location location at at terminiterminiReactivity of Allylic PositionA. Radical A. Radical HalogenationHalogenationCHCH2 2CHCH CHCH3 3+ + BrBr2 2CHCH2 2CH CH CH CH2 2BrBr + + HBrHBrFaster than addition!Faster than addition!Low conc.Low conc. Mech.Mech.1. 1.2.2.BrBr2 22 Br2 Br. .h h or or Initiation:
12、Initiation:Propagation:Propagation:CHCH2 2CHCH CHCH3 3+ + BrBr. .CHCH2 2CH CH CH CH2 2. .CHCH2 2CH CH CH CH2 2. .+ +HBrHBrCHCH2 2CH CH CH CH2 2. .CHCH2 2CH CH CH CH2 2. .BrBr2 2CHCH2 2CH CH CH CH2 2Br +Br + BrBr. .Termination:Termination:3.3.BrBr. .+ +. .CHCH2 2CH CHCH CH2 2BrCHBrCH2 2CH CHCH CH2 2B
13、rBr. .BrBr. .+ +BrBr2 2CHCH2 2CH CHCH CH2 2. .2 2CHCH2 2CH CHCH CH2 2CHCH2 2CHCH CHCH2 2Anything that traps radicals, including the “dirt” Anything that traps radicals, including the “dirt” on the walls of the flask, contributes to on the walls of the flask, contributes to termination.termination.Tr
14、acesTracesLow Low conc.conc.PropenePropene generates a symmetrical generates a symmetrical allylicallylic radical, radical, but this is not always the case. For but this is not always the case. For unsymmetrical systems: mixtures. unsymmetrical systems: mixtures. Ratios Ratios depend on % radical ch
15、aracter on each carbon depend on % radical character on each carbon and and TSsTSs leading to products. leading to products. Br2Br +B. SB. SNN1: The 1: The AllylicAllylic Cation is Stabilized Cation is StabilizedCHCH3 3CH CHCHCH CHCH2 2ClClHH2 2OO- - ClCl- -CHCH3 3CH CH CHCH CH CH2 2+ +CHCH3 3CH CH CHCH CH CH2 2+ +CHCH3 3CH CHCHCH CHCH2 2OHOHCHCH3 3CHCH CHCHCH CH2 2OHOHfastfastslowslowThermodynamic Thermodynamic pro
