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Chapter 13: Conjugated p -Systems

Chapter 13: Conjugated p -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)

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Chapter 13: Conjugated p -Systems

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  1. Chapter 13: Conjugated p-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)

  2. Allylic Substitution • First Reaction  Addition of Br2 to Alkene • Second Reaction  Allylic Substitution • Illustrates Reaction’s 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 C—H 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 • Hypothetical Structures; “Sum” Makes Real Hybrid Structure • Must be Proper Lewis Structures • Can Only Generate by Moving Electrons (NO Moving Atoms) • Resonance Forms are Stabilizing • 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 We’ve 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 > 200nm • More Conjugation (# of p Bonds)  Greater Wavelength • Smaller HOMO/LUMO Gap  Greater Wavelength (E=hc/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 …That’s All Folks! (For Slides, Anyway)

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