Bài 30. Ankađien

HOÁ HỌC HỮU CƠ
Organic Chemistry
CHƯƠNG 7 (t.t.)
HYDROCARBON CHƯA NO MẠCH HỞ
7.2. ALKADIEN

7.2.1. DANH PH�P & D?NG PH�N
7.2.2. C�C PHUONG PH�P DI?U CH?
7.2.3. TÍNH CH?T
7.2.4. ?ng d?ng
Alkadienes and Polyunsaturated Hydrocarbons
Alkadienes (referred to as dienes) contain two double bonds
Alkadiynes contain 2 triple bonds
Alkenynes contain a double and a triple bond

Alkadienes and Polyunsaturated Hydrocarbons
Cumulated, conjugated or isolated polyunsaturated compounds
(2E,5E)-2,5-heptadiene
(2E,4E)-2,4-heptadiene
3,4-heptadiene
Nomenclature
Conjugated Unsaturated Systems.
We are going to examine a special group of unsaturated compounds : species that have a p orbital on an atom adjacent to a double bond.
The p–orbital next to the double bond allows delocalization of the π bond, extending them over more than two nuclei. These systems are called conjugated unsaturated systems.
Allyl radical
Allyl cation
H2C=CH–CH=CH2
1,3-butadiene
1,3-Butadiene
Electron Delocalization
Bond Lengths
C-C : 1.54 Å
C=C : 1.34 Å



s bond between C2 and C3 made from sp2-sp2 overlap
Overlap between the C2-C3 p orbitals
Stability of Conjugated Dienes
1,3-butadiene has a lower heat of hydrogenation by 15 kJ mol-1 than two molecules of 1-butene
1,3-butadiene is more stable
These molecules can be compared directly because upon hydrogenation they lead to the same product
Molecular Orbitals of 1,3-Butadiene.
The central carbon atoms of 1,3-butadiene are close enough for overlap to occur between p orbitals of C2 andC3. It is not as great as that of the C1 and C2, but does give the central bond a partial double bond character and allows the four π electrons of 1,3-butadiene to delocalize over all atoms. The four p orbitals combine to form 4 molecular orbitals.
p589
PREPARATION OF DIENES
590-675°C
CH3CH2CH2CH3
chromia-
alumina
More than 4 billion pounds of 1,3-butadiene
prepared by this method in U.S. each year
used to prepare synthetic rubber (See "Diene
Polymers" box)
1,3-Butadiene
+
2H2
PREPARATION OF DIENES
KHSO4
heat
Dehydration of Alcohols
KOH
heat
Dehydrohalogenation of Alkyl Halides
252 kJ/mol
226 kJ/mol
1,3-pentadiene is
26 kJ/mol more
stable than
1,4-pentadiene,
but some of this
stabilization is
because it also
contains a more
highly substituted
double bond
Heats of Hydrogenation
252 kJ/mol
226 kJ/mol
126 kJ/mol
115 kJ/mol
Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
Heats of Hydrogenation
when terminal double bond is conjugated with other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolated
126 kJ/mol
111 kJ/mol
Heats of Hydrogenation
This extra 15 kJ/mol is known by several terms
stabilization energy
delocalization energy
resonance energy
Cumulated double bonds have relatively
high heats of hydrogenation
Heats of Hydrogenation
H2C
CH2
C
+
2H2
CH3CH2CH3
+
H2
CH3CH2CH3
PREPARATION OF DIENES
Isolated diene
Conjugated diene
less electron delocalization; less stable
more electron delocalization; more stable
s-trans
s-cis
Conformations of Dienes
Both conformations allow electron delocalization via overlap of p orbitals to give extended  system
s-trans is more stable than s-cis
12 kJ/mol
Interconversion of conformations requires two  bonds to be at right angles to each other and prevents conjugation
16 kJ/mol
12 kJ/mol
Isolated dienes: double bonds react independently
of one another
Cumulated dienes: specialized topic
Conjugated dienes: reactivity pattern requires
us to think of conjugated diene system as a
functional group of its own
Reactions of Dienes
Electrophilic Attack on Conjugated Dienes
1,4 Addition






Electrophilic Attack on Conjugated Dienes
Mechanism
Kinetic Control vs. Thermodynamic Control
Kinetic Control versus Thermodynamic Control of a Chemical Reaction
The temperature of reaction greatly affects the distribution of 1,2 and 1,4 products
Kinetic Control vs. Thermodynamic Control
Heating the 1,2-addition product leads to an equilibrium which favors the 1,4-addition product
Kinetic Control vs. Thermodynamic Control
At lower temperatures
The proportion of products determined by the relative rates of formation of product
The DG‡ for formation of 1,2-addition product is lower than for 1,4-addition product (Allyl cation intermeiate)
Fewer molecules have enough energy to overcome the higher DG‡ for formation of the 1,4-addition product
1,2-addition product is formed faster and is the major product
The reaction is said to be under kinetic control

Kinetic Control vs. Thermodynamic Control
At higher temperatures
when an equilibrium is established, the most stable product predominates
The 1,4 product: disubstituted double bond, 1,2-addition product: monosubstituted double bond
Enough energy is available to overcome DG‡ barriers for formation of 1,2- and 1,4-addition products and for the reverse reactions
An equilibrium situation exists and the most stable product is the major one
1,4-addition product is more stable and is the major product at high temperatures
The reaction is said to be under thermodynamic control
p601
1,3-butadiene shows 1,4- addition reactions with other electrophilic reagents. Addition of HBr and Br2 (without peroxides) are two examples.
Reactions of this type are quite general with other conjugated dienes.
HALOGEN ADDITION TO DIENS
The Diels-Alder Reaction
The Diels-Alder Reaction
1,4-Cycloaddition Reaction of Dienes





The general Diels-Alder reaction forms a cylohexene product

Examples.
Simplest Diels-Alder reaction takes place between 1,3-butadiene and ethene. This reaction is sluggish and must be performed at high temperature and pressure.
Diels-Alder reaction used in the preparation of an intermediate in the synthesis of anticancer drug Taxol®
p605
electron rich
electron poor
diene
dienophile
III. The Diels-Alder Reaction
A. Mechanism
Pericyclic reaction: concerted reaction that proceeds through
a cyclic array of electrons in the TS
cyclic array of
6 p e–s
The Diels-Alder Reaction
Factors Favoring the Diels-Alder Reaction
The simplest possible example: very low yield and high temperatures


To proceed in good yield and at low temperature: the dienophile should have electron-withdrawing groups
III. The Diels-Alder Reaction
B. Dienes and dienophiles
Works best with electron-poor dienophiles (A,B,Y,Z = C=O, CN):
III. The Diels-Alder Reaction
B. Dienes and dienophiles
III. The Diels-Alder Reaction
B. Dienes and dienophiles
The Diels-Alder Reaction
Stereochemistry of the Diels-Alder Reaction
syn addition,
configuration of the dienophile is retained in the product





The Diels-Alder Reaction
The Diels-Alder Reaction
Stereochemistry of the Diels-Alder Reaction
syn addition,
configuration of the dienophile is retained in the product
The diene must be in the s-cis conformation to react
The Diels-Alder Reaction
Cyclopentadiene spontaneously undergoes Diels-Alder reaction with itself at room temperature
This dimer can be “cracked” (undergo retro-Diels-Alder reaction) by heating and the cyclopentadiene product isolated by distillation.



The Diels-Alder Reaction
III. The Diels-Alder Reaction
C. Stereospecificity
The Diels-Alder Reaction
The Diels-Alder reaction occurs primarily in an endo rather than an exo fashion when the reaction is kinetically controlled
III. The Diels-Alder Reaction
D. Stereoselectivity
The Diels-Alder Reaction
Molecular Orbital Considerations that Favor an Endo Transition State
III. The Diels-Alder Reaction
Answer 10-5. Draw the structure of the Diels-Alder adduct in each of the following reactions.
III. The Diels-Alder Reaction
Answer 10-6. What diene and dienophile would be used to make each of the following compounds?
Intramolecular Diels-Alder Reactions
the reacting groups are part of the same molecule
Molecular Orbital Considerations that Favour an Endo Transition State.
In the Diels-Alder reaction of cyclopentadiene and maleic anhydride the endo stereochemistry is favoured by a secondary interaction between the orbitals of the diene and the orbitals of the carbonyl groups of maleic anhydride.
p608
p609
The transition state for the endo product thus has a lower activation energy due to this secondary interaction. It is the kinetic (and major) product of this Diels-Alder reaction.
DIENE POLYMERS
DIENE POLYMERS
DIENE POLYMERS
DIENE POLYMERS
DIENE POLYMERS
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