Chatper 11 Radical Reactions '13 BW
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1 Chapter 11. Radical Reactions Junha Jeon Department of Chemistry University of Texas at Arlington Arlington, Texas 76019 Chem 2321, Fall ‘13 11.1 Ionic Reactions (Ion) vs. Radical Reactions (Radical) Radicals and Ions Structure and Geometry of Carbocation and Carbanion Structure and Geometry of Radicals Structure and Geometry of Radicals
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Transcript of Chatper 11 Radical Reactions '13 BW
1!
Chapter 11. Radical Reactions
"Junha Jeon!
Department of Chemistry!University of Texas at Arlington!
Arlington, Texas 76019!"!
Chem 2321, Fall ‘13!
11.1 Ionic Reactions (Ion) vs. Radical Reactions (Radical)!
Radicals and Ions! Structure and Geometry of Carbocation and Carbanion!
Structure and Geometry of Radicals! Structure and Geometry of Radicals!
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Stability: Similar to Stability of Carbocation – Hyperconjugation!
• Free radicals do not have a formal charge but are unstable because of an incomplete octet.!
• Groups that can push (donate) electrons toward the free radical will help to stabilize it. Why and how? !
Stability: Similar to Stability of Carbocation – Hyperconjugation!
• Free radicals do not have a formal charge but are unstable because of an incomplete octet.!
• Groups that can push (donate) electrons toward the free radical will help to stabilize it. Why and how? hyperconjugation!
Stability! Stability!
8 kJ/mol!
Stability?! Resonance Structures: Allylic Radical!
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Resonance Structures: Allylic Radical! Resonance Structures: Benzylic Radical!
Resonance Structures: Benzylic Radical! Resonance Stabilized Radicals Are Even More Stable Than 3° Radicals"
Resonance Stabilized Radicals Are Even More Stable Than 3° Radicals"
40–50 kJ/mol!
• What is a bigger factor, hyperconjugation or resonance?!
Resonance Stabilized Radicals Are Even More Stable Than 3° Radicals"
• What is a bigger factor, hyperconjugation or resonance?!
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Vinylic Radical Is Even Less Stable Than a Primary Radical!No resonance!stabilization!
vs.!vs.!
resonance!stabilization!
Vinylic Radical Is Even Less Stable Than a Primary Radical!No resonance!stabilization!
vs.!vs.!
resonance!stabilization!
• What type of orbital is the vinylic free radical located in, and how does that affect stability? to the board!
Vinylic Radical Is Even Less Stable Than a Primary Radical!No resonance!stabilization!
resonance!stabilization!
11.2 Common Patterns in Radical Mechanisms!
Carbocation vs. Radical"
Common Patterns in Radical Mechanisms!
1. Homolytic cleavage!
2. Addition to a π bond!
3. Hydrogen abstraction!
4. Halogen abstraction!
5. Elimination!
!6. Coupling!
Common Patterns in Radical Mechanisms!
1. Homolytic cleavage!
2. Addition to a π bond!
3. Hydrogen abstraction!
4. Halogen abstraction!
5. Elimination!
!6. Coupling!
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Common Patterns in Radical Mechanisms!
1. Homolytic cleavage!
2. Addition to a π bond!
3. Hydrogen abstraction!
4. Halogen abstraction!
5. Elimination!
!6. Coupling!
Common Patterns in Radical Mechanisms!
1. Homolytic cleavage!
2. Addition to a π bond!
3. Hydrogen abstraction!
4. Halogen abstraction!
5. Elimination!
!6. Coupling!
Summary: Common Patterns in Radical Mechanisms! Initiation… Propagation… Termination!
Generating!Radicals!
Destroying!Radicals!
Regenerating!Radicals!
Initiation… Propagation… Termination!
Generating!Radicals!
Destroying!Radicals!
Regenerating!Radicals:!
radicals are moved from one loca/on to another.!
Initiation… Propagation… Termination!
Generating!Radicals!
Destroying!Radicals!
Regenerating!Radicals!
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11.3 Chlorination ! Chlorination: Mechanism!
Chlorination: Mechanism – Propagation Step Is Crucial!!
Radical Reaction: Chain Reaction!
Chlorination: Mechanism!
• The propagation steps give the net reaction:!
!
!
1. Initiation produces a small amount Cl• radical.!
2. H abstraction consumes the Cl• radical.!
3. Cl abstraction generates a Cl• radical, which can go on to start another H abstraction.!
• Propagation steps are self-sustaining.!
Polychlorination: How to Prevent This?! Polychlorination: How to Prevent This?!
Use excess methane and use a small amount of Cl2!
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Initiation Requires the Energy!!
∆H° = 243 kJ/mol!
Bond Dissociation Energy!No resonance!stabilization!
Initiation Requires the Energy!!
∆H° = 243 kJ/mol!Need several hundred °C!
!
Or!
Radical Initiators !
A radical initiator: a compound w/ a weak bond that undergoes!homolytic bond cleavage with greater ease!
Radical Initiators !
A radical initiator: a compound with a weak bond that undergoes!homolytic bond cleavage with greater ease!
∆H° = 159 kJ/mol!
Radical Initiators !
A radical initiator: a compound with a weak bond that undergoes!homolytic bond cleavage with greater ease!
∆H° = 121 kJ/mol: needs only 80 °C!
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Radical Inhibitors or Radical Scavengers! Radical Inhibitors or Radical Scavengers!
11.4 Thermodynamic Aspects for Radical Halogenation!
∆ G is positive: starting material will be favored.!∆ G is negative: product will be favored.!
Thermodynamic Aspects for Radical Halogenation!
∆ S is negligible!
Thermodynamic Aspects for Radical Halogenation!
∆ G ≈ ∆ H!Now all about the bond dissociation energy.!
Bond Dissociation Energies!
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Bond Dissociation Energies: Dihalogens! Calculation of the Enthalphy!
Calculations of the Enthalphy! Calculations of the Enthalphy!
435! 159! 456! 569!
∆ G = – {(456+569)–(435+159)} = –431 kJ/mol!
Calculations of the Enthalphy!
Thermodynamically unfavorable: No reaction!
Enthalphy: Chlorination and Bromination!
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Compare ∆H of Each Propagation Step For Chlorination and Bromination!
• Consider chlorination and bromination in more detail.!
• Chlorination is more product-favored than bromination.!
–21!
–96!
–117!
+126!
–176!
–50!
Energy Diagram for Two Propagation Steps: Rate Determining Step!
RDS!
• Which step in the mechanism is the slow step? !
• Which reaction has a faster rate?!
– Both steps are exothermic!– First step is endothermic and
Second step is exothermic!
11.5 Regioselectivity of Halogenation! Regioselectivity of Halogenation!
Statistically,!
Regioselectivity: Chlorination! Regioselectivity: Bromination!
Why does the bromination more selective than the chlorination?!
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Energy Diagram for Two Propagation Steps: Rate Determining Step!
RDS!
Hammond Postulate!
Hammond Postulate!
HHCH
H
HHCH
H HHCH
Why does the Bromination More Selective Than the Chlorination? !
HHCH
H
HHCH
H HHCH
The transition state in the bromination is more sensitive to the nature of the !substrate, which means the stability of the resulting radical.!
Difference in Transition States: Activation Energy!
HHCH
H HHCH
Another Example!
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Inverse Relationship Between Reactivity and Selectivity!
“Reagents that are the least reactive will generally !be the most selective.”!
11.6 Stereochemistry of Halogenation!
Structure and Geometry of Radicals! Existing Chirality Center?!
After the First Propagation Step…! After the Second Propagation Step…!
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11.7 Let’s Learn Allylic Bromination.!
So far, we have learned the radical reactions of alkanes.!!
Now we are going to explore the radical halogenation of alkenes.!
Resonance Stabilized Radicals Are Even More Stable Than 3° Radicals"
Vinylic Radical Is Even Less Stable Than a Primary Radical!No resonance!stabilization!
Bond Dissociation Energies!
Bond Dissociation Energies! Allylic Bromination Reaction!
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Competition Reaction: Ionic Bromination (Chapter 9.8)!
vs."
Competition Reaction: Ionic Bromination (Chapter 9.8)!
Any idea to prevent/minimize this competition reaction?!
Competition Reaction: Ionic Bromination (Chapter 9.8)!
Any idea to prevent/minimize this competition reaction?!
The amount of Br–Br in solution is minimal, so the competing addition reaction is minimized.!
Use NBS Instead of Br2 : Keep Bromine in a Low Concentration !
No Br2 Exists in the First Propagation Step!! In the Second Propagation Step!
Br2 is generated via the reaction between!NBS and HBr (ionic mechanism).!
No need to learn at this moment…but if you want, I can tell you.!
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11.8 Atmospheric Chemistry and Ozone Layer!
Shielding us from harmful UV radiation!
Conversion!
The Hole in the Ozone Layer over Antarctica! Chlorofluorocarbons (CFCs): Used as Refrigerants, Propellants…!
Solution?!
• CFC subs/tutes that generally decompose before reaching the O3 layer include hydrochlorofluorocarbons.
• Hydrofluorocarbons don’t even form the chlorine radicals that interfere with
the O3 cycle.
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11.9 Autooxidation!
• Autooxidation is the process by which compounds react with molecular oxygen.!
• The process is generally very slow. !
Mechanism!
Antioxidants! 11.10 Markovnikov Addition"
Anti-Markovnikov Addition" Mechanism: Initiation!
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Mechanism: Propagation – Generation of More Stable Radical! Mechanism: Termination!
More Stable Intermediates Result In The Products! Anti-Markovnikov Addition vs. Allylic Bromination!
+ Br•
Br
•
•
+ HBr
Br• adds to the double bond
Br• removes an allylic hydrogen!
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When the NBS/hv condition was employed, the amount of H–Br in solution is minimal. Therefore, the competing addition reaction is minimized.!
Thermodynamics!
v Markovnikov (Ionic) Mechanism: favorable in HCl, HBr, and HI.!
v Anti-Markovnikov (Radical) Mechanism: favorable in HBr only."
Calculations of the Enthalphy: Reaction With Alkanes!
435! 569! 456! 569!
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Thermodynamics: Addition to Alkenes!
v Markovnikov (Ionic) Mechanism: favorable in HCl, HBr, and HI.!
v Anti-Markovnikov (Radical) Mechanism: favorable in HBr only."
Thermodynamics!
v Markovnikov (Ionic) Mechanism: favorable in HCl, HBr, and HI.!
v Anti-Markovnikov (Radical) Mechanism: favorable in HBr only."
Thermodynamics!
v Markovnikov (Ionic) Mechanism: favorable in HCl, HBr, and HI.!
v Anti-Markovnikov (Radical) Mechanism: favorable in HBr only."
Thermodynamics!
v Markovnikov (Ionic) Mechanism: favorable in HCl, HBr, and HI.!
v Anti-Markovnikov (Radical) Mechanism: favorable in HBr only."
Remember: A Racemic Mixture of Enantiomers! Radical Polymerization!
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Radical Polymerization Mechanism: Initiation! Radical Polymerization Mechanism: Propagation!
Radical Polymerization Mechanism: Propagation! Radical Polymerization Mechanism: Termination!
Hold on! Recall: Chlorination ! Chain Branching!
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Common Polymers! Petrochemistry!
v Cracking:!
v Hydrocracking:!
v Reforming:!
Applications!
