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Chapter 13: Conjugated -Systems
• Allylic Substitution—Allyl Radicals (Section 13.2)
• Allyl Radical Stability (Section 13.3)
• Allyl Cation/Anion (Section 13.4)
• Resonance Structures Revisited (Section 13.5)
• Alkadienes, Polyunsaturated Hydrocarbons (Section 13.6)
• 1,3 Butadiene (Section 13.7 – 13.8)
• UV-Vis Spectroscopy (Section 13.9)
• Electrophilic Attack: 1,4 Addition (Section 13.10)
• Diels-Alder Reactions (Section 13.11)
Allylic Substitution
• First Reaction Addition of Br2 to Alkene
• Second Reaction Allylic Substitution
• Illustrates Reaction’s Dependence Upon Conditions
Br2, CCl4
0 oCBr
Br
high temperature
or dilute X2
Br + HBr
Allylic Chlorination
• Allyl Choride Synthesis Known as “Shell Process”
• Radical Substitution Mechanism (Multi-Step)
Initiation
Propagation
Termination
Cl2, 400 oC
Gas Phase Cl
Allylic Chlorination: Mechanism
• Allylic C—H Bonds Relatively Ease to Dissociate
• Termination Arises from Any Combination of Radicals
Cl Cl 2 Cl
H Cl + HCl
Cl ClCl
Cl+
Allylic Bromination: NBS
• NBS: N-Bromosuccinimide (Low Br2 Concentration)
• Nonpolar Solvent, Dilute Conditions
• Primarily Get Allylic Substitution Product
NBr
O
O
+light or ROOR
CCl4
Br NH
O
O
+
Allylic Radical: MO Description
• Three p Orbitals Combine to Form 3 Molecular Orbitals
• One Unpaired Electron (Radical)
3
2
1
Molecular Orbitals: General Rules
• Molecular Orbitals are Symmetric
• Nodes Are Through Atoms or Bonds
• Nodes Represent an Orbital Phase Change (+/-)
• In Allyl Radical, Unpaired Electron on C1 and C3 (NOT C2)
• Molecular Orbitals Explain Resonance in Allyl Radical
• Same Orbital Picture for Same Carbon Scaffold(Orbital Occupancy Changes)
Allylic Radical: MO Description
• Same Orbitals as Allyl Radical (Different Occupancies)
3
2
1
3
2
1
Allyl Cation Allyl Anion
Resonance: The Carbonate Ion
C
O
O O
2-
CO
O
O
2-
CO O
O
2-
• Double headed arrows indicate resonance forms
• Red “Curved Arrows” show electron movement
• Curved Arrow notation used to show electron flow in resonancestructures as well as in chemical reactions: we will usethis electron bookkeeping notation throughout the course
Rules for Drawing Resonance Structures
1. Hypothetical Structures; “Sum” Makes Real Hybrid Structure
2. Must be Proper Lewis Structures
3. Can Only Generate by Moving Electrons (NO Moving Atoms)
4. Resonance Forms are Stabilizing
5. Equivalent Resonance Structures Contribute Equally to Hybrid
C
O
O O
2-
CO
O
O
2-
CO O
O
2-
Rules for Drawing Resonance Structures
6. More Stable Resonance Forms Contribute More to Hybrid
Factors Affecting Stability
1. Covalent Bonds
2. Atoms with Noble Gas (Octet) Configurations
3. Charge Separation Reduces Stability
4. Negative Charge on More Electronegative Atoms
O CH3H2C vs. O CH3H2C
Alkadienes (Polyunsaturated HCs)
• Follow the General IUPAC Rules We’ve Used This Semester
H2C CH2
CH2CH2
CH2
1,2-Propadiene 1,3-Butadiene
(3Z)-Penta-1,3-diene (3E)-Penta-1,3-diene
(2Z,4E)-Hexa-2,4-diene Pent-1-en-4-yne
Alkadienes: 1,3-Butadiene
• Conformations Not True cis/trans (Single Bond Rotomers)
• Conformations Will be Important for Diels-Alder Reactions
1,3-Butadiene
1.34Å 1.34Å1.47Å
s-cis Conformation s-trans Conformation
Alkadienes: 1,3-Butadiene MOs
What Would Butadiene Cation/Anion Occupancies Look Like?
HOMO
LUMO
ANTI-BONDING Orbitals
BONDING Orbitals
UV-Vis Spectroscopy
• Measures Absorbance at Wavlengths Spanning UV/Vis Regions
• UV: Ultraviolet Vis: Visible
• Typically Record Solvent Spectrum First, Subtract From Sample
• Intensity (y-axis) is the Molar Absorptivity (Extiction Coefficient)
• Conjugated Dienes Have Absorptions Detectable by UV-Vis
• Absorbances of Conjugated Dienes Typically > 200nm
• More Conjugation (# of Bonds) Greater Wavelength
• Smaller HOMO/LUMO Gap Greater Wavelength (E=hc/)
UV-Vis Absorption Spectrum
6000
4000
2000
0
/
M–1
cm–1
353025201510
h /103 cm
–1
1000 800 700 600 550 500 450 400 380 360 340 320 300
(nm)
Representative UV Spectrum: Top Axis is Nanometers, Bottom cm-1
NN
t-Bu
Hy2Na
max
1,4 Addition in Conjugated Dienes
HCl
25 oC
Cl
Cl+
78% 22%
Cl
1,2 additionH
1,4 additionCl
H
• 1,4 Addition Due to Stability and Delocalization in Allyl Cation
• Look at the Intermediate (Carbocation) Observed in Reaction
1,4 Addition in Conjugated Dienes
H+
• Resonance Forms (Hybrid) Explain Possible Addition Products
• 1,4 Product is Thermodynamic Product: Lower Energy
• 1,2 Product is Kinetic Product: Reaction Occurs Faster
Elevated Temperatures Favor Thermodynamic Addition Products
Diels-Alder Reactions: 1,4 Cycloadditions
Diene(s-Cis)
Dienophile
1,4 Cycloaddition
Diels-Alder Adduct
• Diels-Alder Reactions are 1,4 Cycloadditions
• Diene (4 e¯) and Dienophile (2 e¯) Form Cyclic Structure
• Usually Requires Elevated Temperature Conditions
• Usually Energetically Favored (2 Bonds Stronger than 2 )
Diels-Alder Reactions: 1,4 Cycloadditions
Representative Diels-Alder Reactions
Diene(s-Cis)
Dienophile
1,4 Cycloaddition
Diels-Alder Adduct
O O
OCH3
O
O
+
O
OOCH3
AlCl3Et2O
25 oC
Diels-Alder Reactions: 1,4 Cycloadditions
…That’s All Folks! (For Slides, Anyway)
GENERAL NOTES ON DIELS-ALDER REACTIONS:
• Stereospecific: Syn Additions, Retain Dienophile Configuration
• Diene Must React in s-Cis Conformation (Strain in New Ring)
• Under Kinetic Conditions, Endo Products are Favored
H
R
R
H
Endo Exo