1. 11.1 Free Radicals Free radicals form when bonds break homolytically
Note the single-barbed or fishhook arrow used to show the electron movement
Copyright 2012 John Wiley & Sons, Inc.

2. 11.1 Free Radicals
Recall the orbital hybridization in carbocations and carbanions
Copyright 2012 John Wiley & Sons, Inc.

3. 11.1 Free Radicals
Free radicals can be thought of as sp2 hybridized or quickly interconverting sp3 hybridized
Copyright 2012 John Wiley & Sons, Inc.

4. 11.1 Free Radical Stability
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? HOW? Consider hyperconjugation
Copyright 2012 John Wiley & Sons, Inc.

5. 11.1 Free Radical Stability
Use arguments that involve hyperconjugation, and an energy diagram to explain the differences in bond dissociation energy below
Practice with conceptual checkpoint 11.1
Copyright 2012 John Wiley & Sons, Inc.

6. 11.1 Free Radical Resonance
Drawing resonance for free radicals using fishhook arrows to show electron movement
Remember, for resonance, the arrows don’t ACTUALLY show electron movement. WHY?
Draw the resonance hybrid for an allyl radical
HOW and WHY does resonance affect the stability of the free radical?
Copyright 2012 John Wiley & Sons, Inc.

7. 11.1 Free Radical Resonance
The benzylic radical is a hybrid that consists of 4 contributors
Draw the remaining contributors
Draw the hybrid
Copyright 2012 John Wiley & Sons, Inc.

8. 11.1 Resonance Stabilization
How does resonance affect the stability of a radical? WHY?
What is a bigger factor, hyperconjugation or resonance?
Practice with SkillBuilder 11.1
Copyright 2012 John Wiley & Sons, Inc.

9. 11.1 Resonance Stabilization
Vinylic free radicals are especially unstable
What type of orbital is the vinylic free radical located in, and how does that affect stability?
Practice with SkillBuilder 11.2
No resonance stabilization
Copyright 2012 John Wiley & Sons, Inc.

10. 11.2 Radical Electron Movement
Free radical electron movement is quite different from electron movement in ionic reactions
For example, free radicals don’t undergo rearrangement
There are SIX key arrow-pushing patterns that we will discuss
Copyright 2012 John Wiley & Sons, Inc.

11. 11.2 Radical Electron Movement
Homolytic Cleavage: initiated by light or heat
Addition to a pi bond
Hydrogen abstraction: NOT the same as proton transfer
Halogen abstraction
Copyright 2012 John Wiley & Sons, Inc.

12. 11.2 Radical Electron Movement
Elimination: the radical from the α carbon is pushed toward the β carbon to eliminate a group on the β carbon (reverse of addition to a pi bond)
The –X group is NOT a leaving group. WHY?
Coupling: the reverse of homolytic cleavage
Note that radical electron movement generally involves 2 or 3 fishhook arrows
Copyright 2012 John Wiley & Sons, Inc.

13. 11.2 Radical Electron Movement
Note the reversibility of radical processes
Practice with SkillBuilder 11.3
Copyright 2012 John Wiley & Sons, Inc.

14. 11.2 Radical Electron Movement
Radical electron movement is generally classified as either initiation, termination, or propagation
Initiation occurs when radicals are created
Termination occurs when radicals are destroyed
Copyright 2012 John Wiley & Sons, Inc.

15. 11.2 Radical Electron Movement
Propagation occurs when radicals are moved from one location to another
Copyright 2012 John Wiley & Sons, Inc.

16. 11.3 Chlorination of Methane
Let’s apply our electron-pushing skills to a reaction
We must consider each pattern for any free radical that forms during the reaction
Is homolytic cleavage also likely for CH4?
Copyright 2012 John Wiley & Sons, Inc.

17. 11.3 Chlorination of Methane
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18. 11.3 Chlorination of Methane
Why are termination reactions less common than propagation reactions?
Copyright 2012 John Wiley & Sons, Inc.

19. 11.3 Chlorination of Methane
The propagation steps give the net reaction
Initiation produces a small amount Cl• radical
H abstraction consumes the Cl• radical
Cl abstraction generates a Cl• radical, which can go on to start another H abstraction
Propagation steps are self-sustaining
Copyright 2012 John Wiley & Sons, Inc.

20. 11.3 Chlorination of Methane
Reactions that have self-sustaining propagation steps are called chain reactions
Chain reaction: the products from one step are reactants for a different step in the mechanism
Polychlorination is difficult to prevent, especially when an excess of Cl2 is present. WHY?
Practice with SkillBuilder 11.4
Copyright 2012 John Wiley & Sons, Inc.

21. 11.3 Chlorination of Methane
Draw a reasonable mechanism that shows how each of the products might form in the following reaction
Which of the products above are major and which are minor? Are other products also possible?
Copyright 2012 John Wiley & Sons, Inc.

22. 11.3 Radical Initiators
An initiator starts a free radical chain reaction
Which initiator above initiates reactions most readily? WHY?
The acyl peroxide will be effective at 80 °C
Copyright 2012 John Wiley & Sons, Inc.

23. 11.3 Radical Inhibitors
Inhibitors act in a reaction to scavenge free radicals to stop chain reaction processes
Oxygen molecules can exist in the form of a diradical, which reacts readily with other radicals. Use arrows to show the process
How can reaction conditions be modified to stop oxygen from inhibiting a desired chain reaction?
Copyright 2012 John Wiley & Sons, Inc.

24. 11.3 Radical Inhibitors
Hydroquinone is also often used as a radical inhibitor
Draw in necessary arrows in the reactions above
Copyright 2012 John Wiley & Sons, Inc.

25. 11.4 Halogenation Thermodynamics
If we want to determine whether a process is product favored, we must determine the sign (+/-) for ΔG
In a halogenation reaction, will ΔH or -TΔS have a bigger effect on the sign of ΔG? WHY?
Copyright 2012 John Wiley & Sons, Inc.

26. 11.4 Halogenation Thermodynamics
Because the -TΔS term should be close to zero, we can simplify the free energy equation
Considering the bonds that break and form in the reaction, estimate ΔHhalogenation for each of the halogens in the table below
Copyright 2012 John Wiley & Sons, Inc.

27. 11.4 Halogenation Thermodynamics
Copyright 2012 John Wiley & Sons, Inc.

28. 11.4 Halogenation Thermodynamics
Fluorination is so exothermic that it is impractical. WHY does that make it impractical?
Iodination is nonspontaneous, so it is not product favored
Copyright 2012 John Wiley & Sons, Inc.

29. 11.4 Halogenation Thermodynamics
Consider chlorination and bromination in more detail
Chlorination is more product favored than bromination
Copyright 2012 John Wiley & Sons, Inc.

30. 11.4 Halogenation Thermodynamics
Which step in the mechanism is the slow step? Which reaction has a faster rate? Which is more product favored?
First step is endothermic
Both steps are exothermic
Second step is exothermic
Copyright 2012 John Wiley & Sons, Inc.

31. Study Guide for Sections 11.1-11.4
DAY 1, Terms to know:
Sections Free radical, homolytic, hyperconjugation, benzylic, vinylic, hydrogen abstraction, halogen abstraction, coupling, initiation, propagation, termination, chain reaction, radical halogenation, inhibitors
DAY 1, Specific outcomes and skills that may be tested on exam 1:
Sections
Be able to predict whether homolytic cleavage is exo or endo thermic.
Be able to use fishhook arrows to show correct movement of electrons in radical reactions
Be able to describe the geometry and hybridization of free radical carbons
Be able to determine relative stabilities of radicals based on resonance and hyperconjugation
Be able to draw radical resonance contributors and a resonance hybrid
Be able to explain why vinylic free radicals are especially unstable
Be able to draw and label initiation, propagation, and termination steps in a radical mechanism
Be able to predict major products and give a mechanism for the free radical halogenations of a hydrocarbon
Be able to draw the structure of a radical inhibitor and explain mechanistically how an inhibitor acts to prevent radical reaction
Be able to give a complete description of the thermodynamic analysis of radical halogenations processes with either Cl, Br, or I.

32. Extra Practice Problems for Sections 11.1-11.4
Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period a d

33. Prep for Day 2 Must Watch videos: Other helpful videos:
(free radical resonance, Leah)
(free radical chlorination, Khan)
(radical halogenation, Dave)
(chlorination versus bromination kinetics, thermodynamics, and stereochem, FLC)
(allylic halogenation, FLC)
(autooxidation, JP McCormick)
Other helpful videos:
(radical mechanisms, UC-Irvine)
(radical reactions, UC-Irvine)
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