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)
Allylic Substitution
Br
Br2, CCl4
Br
0 oC
high temperature
Br
or dilute X2
+ HBr
• First Reaction  Addition of Br2 to Alkene
• Second Reaction  Allylic Substitution
• Illustrates Reaction’s Dependence Upon Conditions
Allylic Chlorination
Cl2, 400 oC
Gas Phase
• Allyl Choride Synthesis Known as “Shell Process”
• Radical Substitution Mechanism (Multi-Step)
 Initiation
 Propagation
 Termination
Cl
Allylic Chlorination: Mechanism
Cl
Cl
H
Cl
2 Cl
Cl
Cl
+ HCl
Cl
+
• Allylic C—H Bonds Relatively Ease to Dissociate
• Termination Arises from Any Combination of Radicals
Cl
Allylic Bromination: NBS
O
O
+
NBr
light or ROOR
Br +
NH
CCl4
O
• NBS: N-Bromosuccinimide (Low Br2 Concentration)
• Nonpolar Solvent, Dilute Conditions
• Primarily Get Allylic Substitution Product
O
Allylic Radical: MO Description
p3
p2
p1
• Three p Orbitals Combine to Form 3 p Molecular Orbitals
• One Unpaired Electron (Radical)
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)
Allylic Radical: MO Description
Allyl Cation
Allyl Anion
p3
p3
p2
p2
p1
p1
• Same Orbitals as Allyl Radical (Different Occupancies)
Resonance: The Carbonate Ion
2-
2-
O
C
O
O
O
2-
O
O
C
C
O
O
O
• 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
2-
2-
O
C
O
O
O
2-
O
O
C
C
O
O
O
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
H2C
O
CH3
vs.
H2C
O
CH3
3. Charge Separation Reduces Stability
4. Negative Charge on More Electronegative Atoms
Alkadienes (Polyunsaturated HCs)
H2 C
CH2
1,2-Propadiene
1,3-Butadiene
CH2
(3Z)-Penta-1,3-diene
CH2
(3E)-Penta-1,3-diene
CH2
(2Z,4E)-Hexa-2,4-diene
Pent-1-en-4-yne
• Follow the General IUPAC Rules We’ve Used This Semester
Alkadienes: 1,3-Butadiene
1.47 Å
1.34 Å
1.34 Å
1,3-Butadiene
s-cis Conformation
s-trans Conformation
• Conformations Not True cis/trans (Single Bond Rotomers)
• Conformations Will be Important for Diels-Alder Reactions
Alkadienes: 1,3-Butadiene MOs
ANTI-BONDING Orbitals
LUMO
HOMO
BONDING Orbitals
What Would Butadiene Cation/Anion Occupancies Look Like?
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)
UV-Vis Absorption Spectrum
1000
800
700
600
550
500
360
340
320
300
N
N
6000
Hy2Na
–1
–1
400 380
(nm)
t-Bu
 / M cm
450
4000
lmax
2000
0
10
15
20
25
3
h /10 cm
30
35
–1
Representative UV Spectrum: Top Axis is Nanometers, Bottom cm-1
1,4 Addition in Conjugated Dienes
Cl
HCl
+
25 oC
78%
Cl
22%
Cl
1,2 addition
1,4 addition
H
H
Cl
• 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
1,4 Cycloaddition
Diene Dienophile
(s-Cis)
Diels-Alder Adduct
• 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)
Diels-Alder Reactions: 1,4 Cycloadditions
O
O
1,4 Cycloaddition
Diene Dienophile
(s-Cis)
Diels-Alder Adduct
O
AlCl3
Et2O
25 oC
+
OCH3
O
O
OCH3
Representative Diels-Alder Reactions
O
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
R
H
H
R
Endo
Exo
…That’s All Folks! (For Slides, Anyway)