Chapter 13: Conjugated p-Systems

1
Chapter 13 Conjugated p-Systems

• Allylic SubstitutionAllyl 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)

2
Allylic Substitution

• First Reaction ? Addition of Br2 to Alkene
• Second Reaction ? Allylic Substitution
• Illustrates Reactions Dependence Upon
  Conditions

3
Allylic Chlorination

• Allyl Choride Synthesis Known as Shell
  Process
• Radical Substitution Mechanism (Multi-Step)
• Initiation
• Propagation
• Termination

4
Allylic Chlorination Mechanism

• Allylic CH Bonds Relatively Ease to Dissociate
• Termination Arises from Any Combination of
  Radicals

5
Allylic Bromination NBS

• NBS N-Bromosuccinimide (Low Br2
  Concentration)
• Nonpolar Solvent, Dilute Conditions
• Primarily Get Allylic Substitution Product

6
Allylic Radical MO Description

• Three p Orbitals Combine to Form 3 p Molecular
  Orbitals
• One Unpaired Electron (Radical)

7
p Molecular Orbitals General Rules

• p 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)
• p Molecular Orbitals Explain Resonance in Allyl
  Radical
• Same Orbital Picture for Same Carbon Scaffold
• (Orbital Occupancy Changes)

8
Allylic Radical MO Description

• Same Orbitals as Allyl Radical (Different
  Occupancies)

9
Resonance The Carbonate Ion

• Double headed arrows indicate resonance forms
• Red Curved Arrows show electron movement
• Curved Arrow notation used to show electron
  flow in resonance
• structures as well as in chemical reactions we
  will use
• this electron bookkeeping notation throughout
  the course

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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

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Rules for Drawing Resonance Structures

• More Stable Resonance Forms Contribute More to
  Hybrid
• Factors Affecting Stability
• Covalent Bonds
• Atoms with Noble Gas (Octet) Configurations
• Charge Separation Reduces Stability
• Negative Charge on More Electronegative Atoms

12
Alkadienes (Polyunsaturated HCs)

• Follow the General IUPAC Rules Weve Used This
  Semester

13
Alkadienes 1,3-Butadiene

• Conformations Not True cis/trans (Single Bond
  Rotomers)
• Conformations Will be Important for Diels-Alder
  Reactions

14
Alkadienes 1,3-Butadiene MOs
What Would Butadiene Cation/Anion Occupancies
Look Like?
15
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 gt
  200nm
• More Conjugation ( of p Bonds) ? Greater
  Wavelength
• Smaller HOMO/LUMO Gap ? Greater Wavelength
  (Ehc/l)

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UV-Vis Absorption Spectrum
lmax
Representative UV Spectrum Top Axis is
Nanometers, Bottom cm-1
17
1,4 Addition in Conjugated Dienes

• 1,4 Addition Due to Stability and
  Delocalization in Allyl Cation
• Look at the Intermediate (Carbocation) Observed
  in Reaction

18
1,4 Addition in Conjugated Dienes

• 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
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Diels-Alder Reactions 1,4 Cycloadditions

• Diels-Alder Reactions are 1,4 Cycloadditions
• Diene (4 p e) and Dienophile (2 p e) Form
  Cyclic Structure
• Usually Requires Elevated Temperature
  Conditions
• Usually Energetically Favored (2 s Bonds
  Stronger than 2 p)

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Diels-Alder Reactions 1,4 Cycloadditions
Representative Diels-Alder Reactions
21
Diels-Alder Reactions 1,4 Cycloadditions

• 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

Thats All Folks! (For Slides, Anyway)