H2C CH CH2
CH3CH CH CHCH3
Allyl cation (烯丙基正离子) Allylic cation(烯丙基型正离子)
Relative Order of carbocation stability:
C
CH3CH CH CHCH3 > C C > H2C CH CH2
C
H
C
> C C > C C > CH2 CH3
Kinetic versus thermodynamic control of reactions 5.5 The Diels-Alde Cycloaddition
Reaction
5.1 Classes and Nomenclature of Dienes
An alkadiene (二烯烃):
Hydrocarbon containing two carbon-carbon double bonds
1,3-butadiene 1,3-cyclo-
Cumulated dienes
hexadiene
The C atom is common H2C C CH2
for two double bonds Allene(丙二烯)
Nomenclature: • Suffix: ne diene P127.4.10
πbond：2p-2p overlap
C2-C3 partially overlap
H H
H
C
C
C
C H
by 2p-2p orbital
H 4 πelectrons are delocalized
H
over 4 C atoms
Delocalization of πelectrons
lowers the energy.
π- πConjugate system: The alternate system by single bond and
double bond.
Character of conjugate system: πelectrons delocalization .
5.3 Allylic Carbocations P129, 4.11
Chapter 5 Dienes and Conjugated Systems
5.1 Class and Nomenclature of Dienes 5.2 Structure of Conjugated Dienes
Molecular obitals 5.3 Allylic Carbocations
πelectrons delocalization
(CH3)2C CH CH2 (CH3)2C CH CH2
Resonance hybrid
Hyperconjugation (超共轭效应)
Ch.P96
C-H bond attached a σ-p Hyperconjugation:
(Z,Z)- 2,4-己二烯
(Z,E)-
5.2 Structure of Conjugated Dienes
1,3-Butadiene:
P128
4 C atoms are sp2-hybridized.
H H
H
C
C
C
C H
HC2-C3 σbond: sp2-sp2overlap H 4 C atoms are coplanar
Two of the π molecular orbitals
C
(+) (-)
C
(-)
C(+) C
1,3-butadiene are
(+)
bonding molecular orbitals.
C
CC (-)
C
π2 Bonding
molecular orbitals
(成键轨道)
π1
π2 - HOMO (The highest occupied orbital) π3* - LUMO (the lowest unoccupied orbital)
combine to form C
(-)
C
(+)
C
(-)
C

a set of πmolecular (-) (+) (-) (+)
obitals:
(+) (-) (-) ((++))
C
(-)
CC
(+) (+)
C
(-)
π4*
Antibonding molecular orbitals
(反键轨道)
π3* node
Hyperconjugation σ-p Hyperconjugation σ-πHyperconjugation 5.4 Reaction of conjugated Dienes 5.4.1 Electrophilic Attack on Conjugated Dienes: 1,4-addition Mechanism of conjugate addition
Isolated dienes
CCCC C
1,4-pentadiene
Separated by one or more sp3-C atom.
1,5-Cyclooctadiene
Alkadienes Conjugated dienes: C C C C
Double bonds and single bonds alternate along the chain.
• Cis-trans isomers:
H
H
H3C C
C C
C CH3
H
H
H
CH3
H3C C
C C
C H
H
H
cis,cis –2,4-hexadiene (2Z,4E)-2,4-hexadiene
(2Z,4Z)-2,4-hexadiene cis,trans-
顺,顺-2,4-己二烯
顺,反-2,4-己二烯
Two possible planar conformation of 1,3-butadiene:
s-Cis conformation s-Trans conformation
Molecular orbitals of 1,3-butadiene Ch. P94.
The four p orbitals (+)
H
H
Allylic
(烯丙型)
>
3°>
Allyl
(烯丙基)
>
2°>
1°>Vinyl
C CC
C atom with positive charge is sp2-hybridized. p orbital is
vacant.
δ-
C C C δC+ C C p -π overlap.
p -πconjugation