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