Chapter 14: Conjugated Dienes
Chapter 14: Conjugated Dienes Coverage: 1. Conjugated vs Nonconjugated dienes and Stability 2. MO picture of 1,3-butadiene 3. Electrophilic addition to Dienes 4. Kinetic vs Thermodynamic Control 5. Diels-Alder Reaction Problems: 20,21,24,25,26,31,32,34,35 Goals: 1. Know relative stabilites of conjugated vs nonconjugated dienes 2. Be able to draw an orbital picture showing pi system (overlapping p orbitals) for 1,3butadiene. 3. Know the difference between the s-cis and s-trans conformation. Know which conformation is preferred and why. 4. Be able to draw a energy diagram for the MO system of 1-3-butadiene. 5. Understand the definitions of kinetic and thermodynamic products as they apply to the electrophilic addition products of 1,3-butadiene and related compounds. 6. Be able to predict the products of Diels-Alder reaction. Know the mechanism of this reaction. Also be able to predict what reactants are required to synthesise a Diels-Alder adduct.
Dienes – two double bonds a. Nonconjugated diene – double bonds are separated by at least two C-C single bonds. 1,4-pentadiene
b. Conjugated diene – double bond separated by only one C-C single bond.
1,3-pentadiene
c. Allenes - cumulated double bonds.
H
H C C C
H
1,2-pentadiene
CH2CH3
Stabilites of dienes – measured by heats of hydrogenation, -∆H -∆H, kcal/mol H2, Pt
30.0 H2, Pt
27.4
H2, Pt
60.2 H 2, Pt
Are the values for dienes what you expect?
53.7
• 1,4-pentadiene contains two monosubstituted double bonds. Predicted: -∆H = 2 x 30.0 = 60.0 kcal/mol Actual: -∆H = 60.2 kcal/mol
• 1,3-pentadiene contains one monosubstituted double bond and one disubstituted double bond. Predicted: -∆H = 1 x 30.0 + 1 x 27.4 = 57.4 kcal/mol Actual: -∆H = 53.7 kcal/mol Conclusion: 1,3-pentadiene is more stable than predicted. Why? Answer: Conjugation
sp2 sp2 sp2 sp2
sp3
Why? Conjugation – the two double bonds form a continuous overlap of the p orbitals. This results in delocalization of the pi electrons and extra stability
sp2 sp2 sp2 sp2
sp3
Conformations of 1,3-butadiene H2 C
1
CH
CH
2
3
CH2
4
1,3-butadiene exists in two conformations that are in equilibrium 180o
s-trans
s-cis
s - single bond s-cis - double bonds are cis with respect to single bond s-trans – double bonds are trans with respect to single bond All atoms (including C and H) lie in the same plane for these conformations. Any nonplanar conformations results in disruption of the continuous overlap of p orbitals and raises the energy.
Bond Rotation of Butadiene S-Trans
S-Trans 180 0 0.0 kcal/mol
600 3.40 kcal/mol
S-Cis
150 0 1.62 kcal/mol
300 2.80 kcal/mol
900 5.35 kcal/mol
S-Cis 00 3.46 kcal/mol
Energy, kcal/mol
6 5 4 3 2
Ea
S-Trans
S-Cis
1 0 180
150
120
90
60
Dihedral Angle
30
0
Space-Filling Model of S-Cis Conformation Steric Repulsion of Hydrogen Atoms
Steric repulsion makes the s-cis conformation adopt a slightly nonplanar conformation.
1,2- and 1,4-addition reactions to 1,3-butadiene
H2C
CH
CH
CH2
H
Br
H2C H
H2C H
CH
CH
CH2
1,2-product
Br
CH
CH
CH2
1,4-product
Br
Mechanism: Markovnikov Addtion of HBr (see Alkenes)
H2C
CH
CH
CH2
H
Br
H2C H
+
CH
CH
CH2
H2C H
CH
CH
Allylic carbocation – resonance stabilized Br-
+
CH2
H2C H
-80 C +40 C
CH Br
CH
CH2
H2C
CH
CH
H
80% 15%
CH2 Br
20% 85%
The product distribution is temperature dependent. Low Temperature: The product that forms the fastest will be the major product. This product is termed the kinetic product. Thus, 1,2-product is the kinetic product and forms faster than than the 1,4-product. High Temperature: The product that is more stable will be the major product. This product is termed the thermodynamic product. The 1,4-product is the thermodynamic product and is more stable.
Further explanation: • At low temperature, the reaction is not reversible. The 1,2-product forms faster because attack by Br- at the 20 carbon, which bears a larger positive charge, has a lower energy of activation, Ea. So the 1,2-product builds up and it does not revert back to reactant. • At high temperature, the reaction is reversible. Although the 1,2-product forms faster, once it forms, it reverts back to reactant, which then reacts to form 1,4 product, which is more stable. The 1,2- and 1,4-product are in equilibrium at high temperature, with the 1,4-product predominating.
Molecular Orbital Picture of 1,3-butadiene Nodes
π4* Antibonding
____
π3*Antibonding
____
LUMO
π2 Bonding
____
HOMO
π1
____
3
2
E
Bonding
1
0
UV-Visible Spectroscopy UV Visible
200-400 nm 400-800 nm
• Electronic transitions of π or nonbonding electrons • Follows Beer’s Law A=εcl A = absorbance ε = molar absorptivity of molecule c = concentration l = path length of light • Absorption characterized by λ max – wavelength of maximum absorption • λ max increases with 1. Conjugation - 30 nm per double bond 2. Alkyl substitution - 5 nm per alkyl group
π4* Antibonding
____
___
π3*Antibonding
____
___ UV
E
217 nm π2 Bonding
____
___
π1
____
___
Bonding
Ground State
Excited State
495 nm
483 nm
603 nm
Why are my jeans so blue?
Oxidized form of Indigo responsible for blue color
Reduced water-soluble yellow form of Indigo applied to jeans
Phenolphthalein – Acidic and Basic Forms Phenolphthalein is an pH indicator dye used in titrations. It is also an ingredient in Ex-lax! When the solution is acidic, the molecule is colorless. Under basic conditions, it turns red. Increased conjugation is responsible for the red color. sp3 Acid Form –less conjugation, absorbs in UV and colorless
OH-
Basic Form – more flat and conjugated, absorbs in visible region and red sp2
3-D structures
Diels-Alder Reaction
Otto Diels
• Method for synthesis of 6-membered ring • One-step, conerted reaction • Termed [4+2] cycloaddition reaction where 4π and 2π electrons react. σ π
+
σ
Diene – electron-rich nucleophile electron-donating groups make it more reactive. H3C H3C
MeO
Kurt Alder
Dienophile – electron poor electrophile electron-withdrawing groups make it more reactive O
CO2CH3 C
CN
O O
acrylonitrile
C CO2CH3
maleic anhydride
Reaction: CN
CN
+ MeO MeO
CO2CH3 O
+
C C CO2CH3
O CO2CH3 CO2CH3
Mechanism of Diels-Alder: one-step, simultaneous (concerted) bond-making and bond-breaking involving 6 π electrons
Diels Alder Movie
Stereochemical Requirements of Diels-Alder Reaction 1. Diene must be in the s-cis conformation in order to react.
+ s-trans unreactive
s-cis
Explain the following reactivities of dienes.
Very Reactive!
Unreactive
100% s-cis
Cannot adopt s-cis
2. Syn Stereochemistry – due to its concerted nature, the reaction is syn with respect to both the diene and dienophile. A
A T T
A
T
A
T
T
A T
+
T A
A
T
CO2CH 3
A
CO2CH 3
+ CO2CH 3
cis
T = toward you in product A = away from you in product
CO2CH 3
cis
H3CO 2C
CO2CH3
+ CO2CH3
trans
CO2CH3
trans
Conclusion: The reaction stereospecific with respect to the dienophile. cis cis trans trans
What about the diene? CH3
CH3
CO2CH 3
CO2CH3
+ CO2CH3 CO2CH 3
CH3
CH3
trans, trans
cis CH3
CO2CH 3 CH 3
CO2CH3
+
CO2CH3 CO2CH 3
CH3
CH3
cis, trans
trans severe steric crowding
s-cis
CH3 CH3
cis, cis Unreactive!!!! Why?
CH3 H3C
s-trans - much more stable
Conclusion: Diels-Alder is stereospecific with respect to the diene. trans, trans à cis product trans, cis à trans product cis, trans à trans product cis, cis à unreactive 3. Endo Rule –when substituted bicyclic structures form ,the endo product is favored over the exo product.
+ H
CHO
OHC
Endo favored
CHO H
Exo
Explanation of Endo Rule: There are two possible approaches between the diene and dienophile in this reaction. HOMO diene
H
H
H
H
H H H
H H
H
H
H
H
LUMO dienophile
H
O
H H
H O
H H
H
Addition interaction lowers Ea – reacts faster
CHO
H
OHC
Endo - major
H
Exo - minor
Dienes – two double bonds a. Nonconjugated diene – double bonds are separated by at least two C-C single bonds. 1,4-pentadiene
b. Conjugated diene – double bond separated by only one C-C single bond.
1,3-pentadiene
c. Allenes - cumulated double bonds.
H
H C C C
H
1,2-pentadiene
CH2CH3
Stabilites of dienes – measured by heats of hydrogenation, -∆H -∆H, kcal/mol H2, Pt
30.0 H2, Pt
27.4
H2, Pt
60.2 H 2, Pt
Are the values for dienes what you expect?
53.7
• 1,4-pentadiene contains two monosubstituted double bonds. Predicted: -∆H = 2 x 30.0 = 60.0 kcal/mol Actual: -∆H = 60.2 kcal/mol
• 1,3-pentadiene contains one monosubstituted double bond and one disubstituted double bond. Predicted: -∆H = 1 x 30.0 + 1 x 27.4 = 57.4 kcal/mol Actual: -∆H = 53.7 kcal/mol Conclusion: 1,3-pentadiene is more stable than predicted. Why? Answer: Conjugation
sp2 sp2 sp2 sp2
sp3
Why? Conjugation – the two double bonds form a continuous overlap of the p orbitals. This results in delocalization of the pi electrons and extra stability
sp2 sp2 sp2 sp2
sp3
Conformations of 1,3-butadiene H2 C
1
CH
CH
2
3
CH2
4
1,3-butadiene exists in two conformations that are in equilibrium 180o
s-trans
s-cis
s - single bond s-cis - double bonds are cis with respect to single bond s-trans – double bonds are trans with respect to single bond All atoms (including C and H) lie in the same plane for these conformations. Any nonplanar conformations results in disruption of the continuous overlap of p orbitals and raises the energy.
Bond Rotation of Butadiene S-Trans
S-Trans 180 0 0.0 kcal/mol
600 3.40 kcal/mol
S-Cis
150 0 1.62 kcal/mol
300 2.80 kcal/mol
900 5.35 kcal/mol
S-Cis 00 3.46 kcal/mol
Energy, kcal/mol
6 5 4 3 2
Ea
S-Trans
S-Cis
1 0 180
150
120
90
60
Dihedral Angle
30
0
Space-Filling Model of S-Cis Conformation Steric Repulsion of Hydrogen Atoms
Steric repulsion makes the s-cis conformation adopt a slightly nonplanar conformation.
1,2- and 1,4-addition reactions to 1,3-butadiene
H2C
CH
CH
CH2
H
Br
H2C H
H2C H
CH
CH
CH2
1,2-product
Br
CH
CH
CH2
1,4-product
Br
Mechanism: Markovnikov Addtion of HBr (see Alkenes)
H2C
CH
CH
CH2
H
Br
H2C H
+
CH
CH
CH2
H2C H
CH
CH
Allylic carbocation – resonance stabilized Br-
+
CH2
H2C H
-80 C +40 C
CH Br
CH
CH2
H2C
CH
CH
H
80% 15%
CH2 Br
20% 85%
The product distribution is temperature dependent. Low Temperature: The product that forms the fastest will be the major product. This product is termed the kinetic product. Thus, 1,2-product is the kinetic product and forms faster than than the 1,4-product. High Temperature: The product that is more stable will be the major product. This product is termed the thermodynamic product. The 1,4-product is the thermodynamic product and is more stable.
Further explanation: • At low temperature, the reaction is not reversible. The 1,2-product forms faster because attack by Br- at the 20 carbon, which bears a larger positive charge, has a lower energy of activation, Ea. So the 1,2-product builds up and it does not revert back to reactant. • At high temperature, the reaction is reversible. Although the 1,2-product forms faster, once it forms, it reverts back to reactant, which then reacts to form 1,4 product, which is more stable. The 1,2- and 1,4-product are in equilibrium at high temperature, with the 1,4-product predominating.
Molecular Orbital Picture of 1,3-butadiene Nodes
π4* Antibonding
____
π3*Antibonding
____
LUMO
π2 Bonding
____
HOMO
π1
____
3
2
E
Bonding
1
0
UV-Visible Spectroscopy UV Visible
200-400 nm 400-800 nm
• Electronic transitions of π or nonbonding electrons • Follows Beer’s Law A=εcl A = absorbance ε = molar absorptivity of molecule c = concentration l = path length of light • Absorption characterized by λ max – wavelength of maximum absorption • λ max increases with 1. Conjugation - 30 nm per double bond 2. Alkyl substitution - 5 nm per alkyl group
π4* Antibonding
____
___
π3*Antibonding
____
___ UV
E
217 nm π2 Bonding
____
___
π1
____
___
Bonding
Ground State
Excited State
495 nm
483 nm
603 nm
Why are my jeans so blue?
Oxidized form of Indigo responsible for blue color
Reduced water-soluble yellow form of Indigo applied to jeans
Phenolphthalein – Acidic and Basic Forms Phenolphthalein is an pH indicator dye used in titrations. It is also an ingredient in Ex-lax! When the solution is acidic, the molecule is colorless. Under basic conditions, it turns red. Increased conjugation is responsible for the red color. sp3 Acid Form –less conjugation, absorbs in UV and colorless
OH-
Basic Form – more flat and conjugated, absorbs in visible region and red sp2
3-D structures
Diels-Alder Reaction
Otto Diels
• Method for synthesis of 6-membered ring • One-step, conerted reaction • Termed [4+2] cycloaddition reaction where 4π and 2π electrons react. σ π
+
σ
Diene – electron-rich nucleophile electron-donating groups make it more reactive. H3C H3C
MeO
Kurt Alder
Dienophile – electron poor electrophile electron-withdrawing groups make it more reactive O
CO2CH3 C
CN
O O
acrylonitrile
C CO2CH3
maleic anhydride
Reaction: CN
CN
+ MeO MeO
CO2CH3 O
+
C C CO2CH3
O CO2CH3 CO2CH3
Mechanism of Diels-Alder: one-step, simultaneous (concerted) bond-making and bond-breaking involving 6 π electrons
Diels Alder Movie
Stereochemical Requirements of Diels-Alder Reaction 1. Diene must be in the s-cis conformation in order to react.
+ s-trans unreactive
s-cis
Explain the following reactivities of dienes.
Very Reactive!
Unreactive
100% s-cis
Cannot adopt s-cis
2. Syn Stereochemistry – due to its concerted nature, the reaction is syn with respect to both the diene and dienophile. A
A T T
A
T
A
T
T
A T
+
T A
A
T
CO2CH 3
A
CO2CH 3
+ CO2CH 3
cis
T = toward you in product A = away from you in product
CO2CH 3
cis
H3CO 2C
CO2CH3
+ CO2CH3
trans
CO2CH3
trans
Conclusion: The reaction stereospecific with respect to the dienophile. cis cis trans trans
What about the diene? CH3
CH3
CO2CH 3
CO2CH3
+ CO2CH3 CO2CH 3
CH3
CH3
trans, trans
cis CH3
CO2CH 3 CH 3
CO2CH3
+
CO2CH3 CO2CH 3
CH3
CH3
cis, trans
trans severe steric crowding
s-cis
CH3 CH3
cis, cis Unreactive!!!! Why?
CH3 H3C
s-trans - much more stable
Conclusion: Diels-Alder is stereospecific with respect to the diene. trans, trans à cis product trans, cis à trans product cis, trans à trans product cis, cis à unreactive 3. Endo Rule –when substituted bicyclic structures form ,the endo product is favored over the exo product.
+ H
CHO
OHC
Endo favored
CHO H
Exo
Explanation of Endo Rule: There are two possible approaches between the diene and dienophile in this reaction. HOMO diene
H
H
H
H
H H H
H H
H
H
H
H
LUMO dienophile
H
O
H H
H O
H H
H
Addition interaction lowers Ea – reacts faster
CHO
H
OHC
Endo - major
H
Exo - minor
