Chap 12 Conjugated unsaturated systems 12. 1 Introduction Systems that have a p orbital on an atom adjacent to a double bond---molecules with delocalized pi bonds-are called conjugated unsaturated systems 122 Allylic substitution and the allyl radical(烯丙基取代和 烯丙基自由基) CCl Low temp H2C=CHCH3 X H2C—cHcH3 (addition reaction) high temp XX H2C=CHCH3 2 H,C--CHCH2X HX (substitution reaction
Chap 12 Conjugated unsaturated systems 12.1 Introduction Systems that have a p orbital on an atom adjacent to a double bond---molecules with delocalized pibonds– are called conjugated unsaturated systems. 12.2 Allylic substitution and the allyl radical (烯丙基取代和 烯丙基自由基） H2C CHCH3 + X2 CCl 4 (addition reaction) H2C CHCH3 X X Low temp H2C CHCH3 + X2 (substitution reaction) H2C CHCH2X + HX high temp
12.2A Allylic chlorination (烯丙基氯化) ( High temperature) 400° high temp H2C=CHCH3 CI H,C=CHCH2Cl HCl (substitution reaction) 3-Chloropropene (allyl chloride) This is a free radical substitution reaction
12.2A Allylic chlorination （烯丙基氯化） （High temperature) H2C CHCH3 + Cl 2 (substitution reaction) H2C CHCH2Cl 400 high temp o + HCl 3-Chloropropene (allyl chloride) This is a free radical substitution reaction
The mechanism for allylic substitution is the same as the chain mechanism for alkane halogenations 1. Chain-Initiating Step high temp CI Cl r2 CI or hy 2. Chain-Propagating step H2=CHCH3 CI H2C= CHCH2°+HCl H2C=cHcH2°+Cl2一 H2C=CHCH2Cl+ Cl
The mechanism for allylic substitution is the same as the chain mechanism for alkane halogenations 1. Chain-Initiating Step Cl Cl high temp or hv 2 Cl H2 C CHCH3 2. + Cl H2 C CHCH2 + HCl Chain-Propagating step H2 C CHCH2 + Cl 2 H2C CHCH2Cl + Cl
Relative stability of free radical groups(自由基相对稳定性) allylic or allyl 烯丙基自由基 VImy 乙烯基自由基 CH,=CH CH3CH2CH2· Allyl radical H2C=CHCH2· CH3CCH3 CH3CHCH CH DH=85 kcal mol DH=91 kcal mol Dho=94.5 kcal mol- DHo=98 kcal mor-1 DH =98 kcal moll H2C=CHCH CHcHcH CH3 CH,CH3 CH3CH2CH3 CH2=CH2 CH3
Relative stability of free radical groups (自由基相对稳定性） allylic or allyl 烯 丙 基 自 由 基 3 o 2 o 1 o vinyl 乙 烯 基 自 由 基 H2C CHCH2 H2C CHCH3 DH o = 85 kcal mol -1 Allyl radical DH o = 91 kcal mol -1 CH3CHCH3 CH3 CH3CCH3 CH3 DH o = 94.5 kcal mol -1 CH3CH2CH3 CH3CHCH3 DH o = 98 kcal mol -1 CH3CH2CH3 CH3CH2CH2 DH o = 98 kcal mol -1 CH2 =CH2 CH2 =CH
12.2B Allylic bromination with n- bromosuccinimide(烯丙基的溴 代与N-溴代丁二酰亚胺NBS) Propene undergoes allylic bromination when it is treated with N-bromosuccinimide nbs) in CCl4 in the presence of peroxides or light light or roor CH,=CHCH3+ CH,=CHCH, Br NH CCl4 Propene 3-Bromopropene N-Bromosuccinimide Succinimide (NBS) insoluble in CClg
12.2B Allylic bromination with Nbromosuccinimide （烯丙基的溴 代与N-溴代丁二酰亚胺NBS) Propene undergoes allylic bromination when it is treated with N-bromosuccinimide (NBS) in CCl4 in the presence of peroxides or light. CH2=CHCH3 + N O O Br N-Bromosuccinimide (NBS) light or ROOR CCl4 CH2=CHCH2Br + NH O O Propene 3-Bromopropene Succinimide insoluble in CCl4
light or ROOR CH2=CHCH,CH,CH3 CH,=CHCHCH,CH3+ CCl4 3-Bromopentene pentene N-Bromosuccinimide (NBS) Su succinimide insoluble in CCl4 light or roor CCI Cyclohexene N-Bromosuccinimide 3-Bromocyclohexene Succinimide (NBS) insoluble in CCI4
+ N O O Br N-Bromosuccinimide (NBS) light or ROOR CCl4 + NH O O pentene 3-Bromopentene Succinimide insoluble in CCl4 CH2=CHCH2CH2CH3 CH2=CHCHCH2CH3 Br + N O O Br N-Bromosuccinimide (NBS) light or ROOR CCl 4 + NH O O Cyclohexene 3-Bromocyclohexene Succinimide insoluble in CCl 4 Br
12.3 The stability of the allyl radical (烯丙基自由基的稳定性) 12.3A Molecular orbital description of the allyl radical H2C-ATWHH2c 小H
12.3 The stability of the allyl radical (烯丙基自由基的稳定性） 12.3A Molecular orbital description of the allyl radical C H C H H H H2C sp 2 C H H H2C sp 2 sp 2 sp 2 H H2 C H C C H H
The molecular orbital of the allyl radica Node node H antibonding orbital ↑↑↑ nonbonding E orbital Three isolated p orbitals (with an elctron in each) Bonding orbital H
The molecular orbital of the allyl radical H2C H C C H H H2C H C C H H H2C H H Bonding orbital Nonbonding orbital Three isolated p orbitals (with an elctron in each) antibonding orbital E Node Node
12.3B Resonance description of the ally radical (烯丙基自由基的共振论的描述) 123 H2C=G—CH2 2CH CH2
12.3B Resonance description of the allyl radical (烯丙基自由基的共振论的描述） H2 C C H CH2 H2 C C H CH2 1 2 3 1 2 3
12.5 Summary of rules for resonance 12.5A Rules for writing resonance structures 1. Resonance stuctures exist only on paper. 2. In writing resonance structures we are only allowed to move electrons 23 H1C=C—ch H1C♀=cHt CH3 CHCH=CH,2 chrCHCHCH CH,=CH
12.5 Summary of rules for resonance 12.5A Rules for writing resonance structures 1. Resonance stuctures exist only on paper. 2. In writing resonance structures we are only allowed to move electrons. H2 C C H CH2 H2 C C H CH2 1 2 3 1 2 3 + + CH3 CHCH=CH2 CH3 CH=CHCH2 + CH2 CH2 CH=CH2