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 Br 2 to Alkene Second Reaction  Allylic Substitution Illustrates Reaction’s Dependence Upon Conditions

Allylic Chlorination Allyl Choride Synthesis Known as “Shell Process” Radical Substitution Mechanism (Multi-Step)  Initiation  Propagation  Termination

Allylic Chlorination: Mechanism Allylic C—H Bonds Relatively Ease to Dissociate Termination Arises from Any Combination of Radicals

Allylic Bromination: NBS NBS: N-Bromosuccinimide (Low Br 2 Concentration) Nonpolar Solvent, Dilute Conditions Primarily Get Allylic Substitution Product

Allylic Radical: MO Description Three p Orbitals Combine to Form 3  Molecular Orbitals One Unpaired Electron (Radical)

 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 C 1 and C 3 (NOT C 2 )  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)

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

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

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

Alkadienes (Polyunsaturated HCs) Follow the General IUPAC Rules We’ve Used This Semester

Alkadienes: 1,3-Butadiene Conformations Not True cis/trans (Single Bond Rotomers) Conformations Will be Important for Diels-Alder Reactions

Alkadienes: 1,3-Butadiene MOs What Would Butadiene Cation/Anion Occupancies Look Like?

UV-Vis Spectroscopy Measures Absorbance at Wavlengths Spanning UV/Vis Regions UV: UltravioletVis: 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 Representative UV Spectrum: Top Axis is Nanometers, Bottom cm -1 max

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

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

Diels-Alder Reactions: 1,4 Cycloadditions 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

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