HOA

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HOAÙ HOÏC HÖÕU CÔ

• Organic Chemistry

CHÖÔNG 7 (t.t.) HYDROCARBON CHÖA NO MAÏCH HÔÛ
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7.2. ALKADIEN
7.2.1. DANH PHÁP Ð?NG PHÂN 7.2.2. CÁC PHUONG
PHÁP ÐI?U CH? 7.2.3. TÍNH CH?T 7.2.4. ?ng d?ng
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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

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Alkadienes and Polyunsaturated Hydrocarbons

• Cumulated, conjugated or isolated polyunsaturated
  compounds

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Nomenclature

• (2E,5E)-2,5-heptadiene
• (2E,4E)-2,4-heptadiene
• 3,4-heptadiene

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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.
Allyl radical
Allyl cation
1,3-butadiene
H2CCHCHCH2
The porbital next to the double bond allows
delocalization of the p bond, extending them
over more than two nuclei. These systems are
called conjugated unsaturated systems.
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1,3-Butadiene

• Electron Delocalization
• Bond Lengths
• C-C 1.54 Å
• CC 1.34 Å
• s bond between C2 and C3 made from sp2-sp2
  overlap
• Overlap between the C2-C3 p orbitals

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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

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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 p electrons of 1,3-butadiene to delocalize
over all atoms. The four p orbitals combine to
form 4 molecular orbitals.
p589
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PREPARATION OF DIENES
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1,3-Butadiene
590-675C
CH3CH2CH2CH3
chromia- alumina

2H2

• 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)

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PREPARATION OF DIENES
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Dehydration of Alcohols
KHSO4
heat
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Dehydrohalogenation of Alkyl Halides
KOH
heat
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Heats of Hydrogenation

• 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

252 kJ/mol
226 kJ/mol
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Heats of Hydrogenation
126 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol
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Heats of Hydrogenation
126 kJ/mol
111 kJ/mol

• when terminal double bond is conjugated with
  other double bond, its heat of hydrogenation is
  15 kJ/mol less than when isolated

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Heats of Hydrogenation
126 kJ/mol
111 kJ/mol

• This extra 15 kJ/mol is known by several
  terms stabilization energy delocalization
  energy resonance energy

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Heats of Hydrogenation
Cumulated double bonds have relatively high
heats of hydrogenation

C
2H2
CH3CH2CH3
H2C
CH2

CH3CH2CH3
H2
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PREPARATION OF DIENES
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Isolated diene
less electron delocalization less stable
more electron delocalization more stable
Conjugated diene
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Conformations of Dienes
s-trans
s-cis

• Both conformations allow electron delocalization
  via overlap of p orbitals to give extended ?
  system

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s-trans is more stable than s-cis

• Interconversion of conformations requires two ?
  bonds to be at right angles to each other and
  prevents conjugation

12 kJ/mol
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16 kJ/mol
12 kJ/mol
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Reactions of Dienes

• 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

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Electrophilic Attack on Conjugated Dienes

• 1,4 Addition

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Electrophilic Attack on Conjugated Dienes

• Mechanism

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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

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Kinetic Control vs. Thermodynamic Control

• Heating the 1,2-addition product leads to an
  equilibrium which favors the 1,4-addition product

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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

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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

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1,3-butadiene shows 1,4- addition reactions with
other electrophilic reagents. Addition of HBr and
Br2 (without peroxides) are two examples.
p601
Reactions of this type are quite general with
other conjugated dienes.
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HALOGEN ADDITION TO DIENS
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The Diels-Alder Reaction
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The Diels-Alder Reaction

• 1,4-Cycloaddition Reaction of Dienes
• The general Diels-Alder reaction forms a
  cylohexene product

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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
diene
dienophile
electron rich
electron poor
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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 es
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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

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III. The Diels-Alder Reaction
B. Dienes and dienophiles
Works best with electron-poor dienophiles
(A,B,Y,Z CO, C?N)
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III. The Diels-Alder Reaction
B. Dienes and dienophiles
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III. The Diels-Alder Reaction
B. Dienes and dienophiles
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The Diels-Alder Reaction

• Stereochemistry of the Diels-Alder Reaction
• syn addition,
• configuration of the dienophile is retained in
  the product

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The Diels-Alder Reaction
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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

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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.
  

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The Diels-Alder Reaction
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III. The Diels-Alder Reaction
C. Stereospecificity
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The Diels-Alder Reaction

• The Diels-Alder reaction occurs primarily in an
  endo rather than an exo fashion when the reaction
  is kinetically controlled

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III. The Diels-Alder Reaction
D. Stereoselectivity
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The Diels-Alder Reaction

• Molecular Orbital Considerations that Favor an
  Endo Transition State

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III. The Diels-Alder Reaction
Answer 10-5. Draw the structure of the
Diels-Alder adduct in each of the following
reactions.
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III. The Diels-Alder Reaction
Answer 10-6. What diene and dienophile would be
used to make each of the following compounds?
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Intramolecular Diels-Alder Reactions

• the reacting groups are part of the same molecule

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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.
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DIENE POLYMERS
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DIENE POLYMERS
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DIENE POLYMERS
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DIENE POLYMERS
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DIENE POLYMERS
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