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Chapter 13: Conjugated p-Systems

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Allyl Radical Stability (Section 13.3) Allyl Cation/Anion (Section 13.4) ... What Would Butadiene Cation/Anion Occupancies Look Like? UV-Vis Spectroscopy ... – PowerPoint PPT presentation

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Title: 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

10
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

11
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)

16
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
19
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)

20
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

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