1. Dienes and Conjugated Systems
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
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

2. Kinetic versus thermodynamic control
of reactions
5.5 The Diels-Alde Cycloaddition
Reaction

3. 5.1 Classes and Nomenclature of Dienes
An alkadiene
(二烯烃):
Hydrocarbon containing two
carbon-carbon double bonds
Isolated dienes
1,4-pentadiene
Separated by one
or more sp3-C atom.
1,5-Cyclo-
octadiene
Alkadienes
Conjugated dienes:
Double bonds and
single bonds alternate
along the chain.
1,3-butadiene
1,3-cyclo-
hexadiene
Cumulated dienes
The C atom is common
for two double bonds
Allene(丙二烯)

4. Nomenclature: Suffix: ne diene Cis-trans isomers:P
Suffix: ne diene
Cis-trans isomers:
cis,cis –2,4-hexadiene
(2Z,4E)-2,4-hexadiene
cis,trans-
(2Z,4Z)-2,4-hexadiene
顺,顺-2,4-己二烯
(Z,Z)- 2,4-己二烯
顺,反-2,4-己二烯
(Z,E)-

5. 4 πelectrons are delocalized
5.2 Structure of Conjugated Dienes
1,3-Butadiene:
P128
4 C atoms are sp2-hybridized.
C2-C3 σbond: sp2-sp2overlap
4 C atoms are coplanar
πbond：2p-2p overlap
C2-C3 partially overlap
by 2p-2p orbital
4 πelectrons are delocalized
over 4 C atoms
Delocalization of πelectrons
lowers the energy.

6. Two possible planar conformation of
1,3-butadiene:
s-Cis conformation
s-Trans conformation

7. Molecular orbitals of 1,3-butadiene
Ch. P94.
(+)
(-)
Energy π1 π2
π3*
π4*
The four p orbitals
combine to form
a set of πmolecular
obitals:
Antibonding
molecular
orbitals
(反键轨道)
node
Bonding
molecular
orbitals
(成键轨道)
Two of the π
molecular orbitals
1,3-butadiene are
bonding molecular
orbitals.

8. πelectrons delocalization .
π2 - HOMO (The highest occupied orbital)
π3* - LUMO (the lowest unoccupied orbital)
π- πConjugate system:
The alternate system by single bond and
double bond.
Character of conjugate system:
πelectrons delocalization .

9. 5.3 Allylic Carbocations Relative Order of carbocation stability:
P129, 4.11
Allyl cation (烯丙基正离子)
Allylic cation(烯丙基型正离子)
Relative Order of carbocation stability:
Allylic
(烯丙型)
> 3° 2° 1°
Vinyl
Allyl
(烯丙基)

10. C atom with positive charge is sp2-hybridized. p orbital is vacant.δ + -
p -π overlap.
p -πconjugation
πelectrons delocalization
Resonance hybrid

11. Hyperconjugation (超共轭效应) σ-p Hyperconjugation:
Ch.P96
Hyperconjugation (超共轭效应)
σ-p Hyperconjugation:
C-H bond attached a
positive carbon delocalize
the positive charge
Obital overlap here H +
Vacant p orbital
sp3 hybride
sp2 hybride
The order of stability of carbocations:

12. C-H bond atttached a Carbon-carbon double bond
σ-πHyperconjugation
C-H bond atttached a Carbon-carbon
double bond
Delocalization of electrons
Orbital overlap here H
The stability of alkene is increased
by enhance of density of electronic cloud

13. 5.4 Reaction of conjugated Dienes
5.4.1 Electrophilic Attack on Conjugated
Dienes: 1,4-addition
1,3-butadiene
3-Bromo-1-butane (81%)
Mechanism of the addition:
1,2-addition
Step1.
The formation of
allylic carbocation
(烯丙基型正离子
的生成)
1-Bromo-2-butane (19%)
1,4-addition

14. Stability of allylic cations:
Secondary allylic cation
(I)
Primary Allylic cation (II)
Stability of allylic cations:
(I) > (II)
Step 2. Cation-anion combination
1,2-addition δ +
1,4-addition

15. Kinetic versus thermodynamic control of reactions:
(44%) (56%)
40℃
-80℃
r.t
(81%) (19%)
ΔG1,4
Active energy:
ΔG1,4 > ΔG1,2
ΔG1,2
Energy
Stability of product:
1,2-addition
< 1,4-addition
Reaction coordinate

16. Otto Diels and Kurt Alder
1,2-Addition product:
Rate control or kinetic
control product
Equilibrium control
or thermodynamic
control product
1,4-Addition product:
5.5 The Diels-Alder Cycloaddition
Reaction
The conjugate addition of an alkene to diene
Otto Diels and Kurt Alder
(Germany) receive the
1950 Nobel Prize in
chemistry
Diene Dienophile Adduct
(双烯体) (亲双烯体) (加成物)
Pericyclic reaction
(周环反应)

17. Otto Paul Hermann Diels
Diels was the scientific mentor
of Kurt Alder (see portrait) and together they
discovered the reaction which bears their names,
the Diels-Alder (DA) reaction. For this they jointly
received the 1950 Nobel Prize in Chemistry.
Diels was born in Hamburg, Germany into an
academic family (his father was a professor
of classical philology in Berlin, and two brothers also
became professors). He obtained the Ph.D. (1899)
at the University of Berlin with the great Emil Fischer
(see portrait), after which he was appointed
to the faculty there. In 1907 his textbook "Einführung
in die Organische Chemie“was published; it became
one of the most popular texts in its field and,
by 1962, had gone through 19 editions. In 1916 Diels
became director of the Institute of Chemistry at the
University of Kiel, where he remained until his retirement.
Besides the DA reaction, Diels is also known
for having discovered carbon suboxide (C3O2) and
for his early research on the structure of cholesterol,
especially for the use of selenium (rather than sulfur)
for dehydrogenations, and for Diels' hydrocarbon
(3'-methyl-1,2-cyclopentenophenanthrene)
which he obtained from cholesterol by that technique.
Otto Paul Hermann Diels

18. Alder is best known for his discovery, with Otto Diels
(see portrait)of the cycloaddition reaction that bears
their names, the Diels-Alder (DA) reaction.For this
discovery they received the 1950 Nobel Prize in
Chemistry.The reaction, which involves 1,4-addition
of a dienophile(examples they studied included maleic
anhydride,ethyl azodicarboxylate and benzoquinone)
to a conjugated diene (their examples included
cyclopentadiene, acyclic 1,3-dienes, furans and
anthracene) is still one of the most general methods
for constructing 6-membered rings.
Born in Königshütte, Germany (now in Poland), Alder
took his Ph.D. degree with Diels at theUniversity of
Kiel in For a time he worked in industry on
polymers and synthetic rubbers, then in 1940 joined
the faculty of the University of Cologne where he
remained until his untimely death after a brief illness,
shortly before his 56th birthday. He co-authored 173
papers which included, besides the stereochemistry
and mechanism of the DA reaction, studies on the
retro-DAreaction, the "ene" reaction, and applications
to natural products synthesis. In 1955 Alder signed,
with 17 other Nobel laureates, a declaration calling
on nations to renounce war as a means of settling
disputes.
Kurt Alder

19. Characteristic of the Diels-Alder Reaction:
1,3-Butadine Maleic anhydride adduct(100%)
(马来酐)
Maleic anhydride is used to identify conjugate
Dienes.
Characteristic of the Diels-Alder Reaction:
1. The reaction is reversible.
2. Dienophile: with electron-withdrawing
group

21. 3-Dimethyl-1,3-butadiene
3. Diene: with electron-releasing groups and
s-cis conformation
not
3-Dimethyl-1,3-butadiene
Adduct (100%)
4. Stereospecific: Syn-addition
Problem: ?
cis-

22. 5. Application:
a. Identification to conjugate dienes.
b. Cyclization by the formation
of C-C bond.

23. Problems to Chapter 5 Additional problems: P146 4.37 4.46 4.47
1.Electrophilic addition of Br2 to isoprene
(异戊二烯)yields the following product mixture:
Of the 1,2-addition
products, explain why
3,4-dibromo-3-methyl-
1-butene(21%) predominates
over 3,4-dibromo-1-butene(3%).
(3%) (21%)
76%)

24. 2. Draw the possible products resulting from
addition of 1 equivalent of HCl to 1-phenyl-
1,3-butadiene. Which would you expect to
predominate, and why?
3. How would you use Diels-alder reactions to
prepare the following products? Show the
starting diene, dienophilie in each case and
the reactions.
(1) (2)