Transcript

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 Br2 to Alkene

• Second Reaction Allylic Substitution

• Illustrates Reaction’s Dependence Upon Conditions

Br2, CCl4

0 oCBr

Br

high temperature

or dilute X2

Br + HBr

Allylic Chlorination

• Allyl Choride Synthesis Known as “Shell Process”

• Radical Substitution Mechanism (Multi-Step)

Initiation

Propagation

Termination

Cl2, 400 oC

Gas Phase Cl

Allylic Chlorination: Mechanism

• Allylic C—H Bonds Relatively Ease to Dissociate

• Termination Arises from Any Combination of Radicals

Cl Cl 2 Cl

H Cl + HCl

Cl ClCl

Cl+

Allylic Bromination: NBS

• NBS: N-Bromosuccinimide (Low Br2 Concentration)

• Nonpolar Solvent, Dilute Conditions

• Primarily Get Allylic Substitution Product

NBr

O

O

+light or ROOR

CCl4

Br NH

O

O

+

Allylic Radical: MO Description

• Three p Orbitals Combine to Form 3 Molecular Orbitals

• One Unpaired Electron (Radical)

3

2

1

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 C1 and C3 (NOT C2)

• 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)

3

2

1

3

2

1

Allyl Cation Allyl Anion

Resonance: The Carbonate Ion

C

O

O O

2-

CO

O

O

2-

CO O

O

2-

• Double headed arrows indicate resonance forms

• Red “Curved Arrows” show electron movement

• Curved Arrow notation used to show electron flow in resonancestructures as well as in chemical reactions: we will usethis 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

C

O

O O

2-

CO

O

O

2-

CO O

O

2-

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

O CH3H2C vs. O CH3H2C

Alkadienes (Polyunsaturated HCs)

• Follow the General IUPAC Rules We’ve Used This Semester

H2C CH2

CH2CH2

CH2

1,2-Propadiene 1,3-Butadiene

(3Z)-Penta-1,3-diene (3E)-Penta-1,3-diene

(2Z,4E)-Hexa-2,4-diene Pent-1-en-4-yne

Alkadienes: 1,3-Butadiene

• Conformations Not True cis/trans (Single Bond Rotomers)

• Conformations Will be Important for Diels-Alder Reactions

1,3-Butadiene

1.34Å 1.34Å1.47Å

s-cis Conformation s-trans Conformation

Alkadienes: 1,3-Butadiene MOs

What Would Butadiene Cation/Anion Occupancies Look Like?

HOMO

LUMO

ANTI-BONDING Orbitals

BONDING Orbitals

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 Bonds) Greater Wavelength

• Smaller HOMO/LUMO Gap Greater Wavelength (E=hc/)

UV-Vis Absorption Spectrum

6000

4000

2000

0

/

M–1

cm–1

353025201510

h /103 cm

–1

1000 800 700 600 550 500 450 400 380 360 340 320 300

(nm)

Representative UV Spectrum: Top Axis is Nanometers, Bottom cm-1

NN

t-Bu

Hy2Na

max

1,4 Addition in Conjugated Dienes

HCl

25 oC

Cl

Cl+

78% 22%

Cl

1,2 additionH

1,4 additionCl

H

• 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

Diene(s-Cis)

Dienophile

1,4 Cycloaddition

Diels-Alder Adduct

• 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

Diene(s-Cis)

Dienophile

1,4 Cycloaddition

Diels-Alder Adduct

O O

OCH3

O

O

+

O

OOCH3

AlCl3Et2O

25 oC

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

H

R

R

H

Endo Exo


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