1. Chapter 11
Radical Reactions
Suggested Problems -

2. 11.1 Free Radicals Free radicals form when bonds break homolytically
Note the single-barbed or fishhook arrow used to show the electron movement

3. 11.1 Free Radicals
Recall the orbital hybridization in carbocations and carbanions

4. 11.1 Free Radicals
Free radicals can be thought of as sp2 hybridized or quickly interconverting sp3 hybridized
sp2 hybridized

5. 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

6. 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

7. 11.1 Free Radical Resonance
Drawing resonance for free radicals using fishhook arrows 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?

8. 11.1 Free Radical Resonance
The benzylic radical is a hybrid that consists of 4 contributors
Draw the remaining contributors

9. 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

10. 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?
No resonance stabilization
Practice with SkillBuilder 11.2

11. 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

12. 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

13. 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

14. 11.2 Radical Electron Movement
Note the reversibility of radical processes
Practice with SkillBuilder 11.3

15. 11.2 Radical Electron Movement
Radical electron movement is generally classified as either initiation, termination, or propagation
Initiation
Propagation
Initiation occurs when radicals are created
Termination occurs when radicals are destroyed

16. 11.2 Radical Electron Movement
Propagation occurs when radicals are moved from one location to another
Propagation

17. 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

18. 11.3 Chlorination of Methane

19. 11.3 Chlorination of Methane

20. 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

21. 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.
Practice with SkillBuilder 11.4

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

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?

24. 11.3 Radical Inhibitors
Hydroquinone is also often used as a radical inhibitor

25. 11.5 Halogenation Regioselectivity
With substrates more complex than ethane, multiple monohalogenation products are possible
If the halogen were indiscriminant, predict the product ratio?

26. 11.5 Halogenation Regioselectivity
For the CHLORINATION process, the actual product distribution favors 2-chloropropane over 1-chloropropane
Which step in the mechanism determines the regioselectivity?

27. 11.5 Halogenation Regioselectivity
In one reaction, a 1° free radical forms, and in the other a 2° radical forms
Is the chlorination process thermodynamically or kinetically controlled?

28. 11.5 Halogenation Regioselectivity
For the BROMINATION process, the product distribution vastly favors 2-bromopropane over 1-bromopropane
Which step in the mechanism determines regioselectivity?

29. 11.5 Halogenation Regioselectivity
Which process is more regioselective? WHY?

30. 11.5 Halogenation Regioselectivity
Bromination at the 3° position happens 1600 times more often than at the 1° position

31. 11.5 Halogenation Regioselectivity
Which process is least regioselective?
What is the general relationship between reactivity and selectivity? WHY?
Practice with SkillBuilder 11.5

32. 11.5 Halogenation Regioselectivity
Ignoring possible addition products for now, draw the structure for EVERY possible monobromination product for the reaction below
Rank the products in order from most major to most minor

33. 11.5 Halogenation Regioselectivity
Second least abundant product
Most abundant product
Second most abundant product
Least abundant product

34. 11.6 Halogenation Stereochemistry
The halogenation of butane or more complex alkanes forms a new chirality center
2-chlorobutane will form as a racemic mixture

35. 11.6 Halogenation Stereochemistry
Whether the free radical carbon is sp2 or a rapidly interconverting sp3, the halogen abstraction will occur on either side of the plane with equal probability
sp2 hybridized

36. 11.6 Halogenation Stereochemistry
Three monosubstituted products form in the halogenation of butane
Draw all of the monosubstituted products that would form in the halogenation of 2-methylbutane including all stereoisomers

37. 11.6 Halogenation Stereochemistry
In the halogenation of (S)-3-methylhexane, the chirality center is the most reactive carbon in the molecule. WHY?
Name the product and predict the stereochemical outcome
Practice with SkillBuilder 11.6

38. 11.7 Allylic Halogenation
When an C=C double bond is present it affects the regioselectivity of the halogenation reaction
Given the bond dissociation energies below, which position of cyclohexene will be most reactive toward halogenation?

39. 11.7 Allylic Halogenation
When an allylic hydrogen is abstracted, it leaves behind an allylic free radical that is stabilized by resonance
Based on the high selectivity of bromination that we discussed, you might expect bromination to occur as shown below
What other set of side-products is likely to form in this reaction? Hint: addition reaction

40. 11.7 Allylic Halogenation with NBS
To avoid the competing halogenation addition reaction, NBS can be used to supply Br• radicals
Show how resonance stabilizes the succinimide radical
Heat or light initiates the process

41. 11.7 Allylic Halogenation with NBS
Propagation produces new Br• radicals to continue the chain reaction
Where does the Br-Br above come from? The amount of Br-Br in solution is minimal, so the competing addition reaction is minimized

42. 11.7 Allylic Halogenation with NBS
Give a mechanism that explains the following product distribution. Hint: resonance
Practice with SkillBuilder 11.7

43. 11.8 Atmospheric Chemistry and O3
Ozone is both created and destroyed in the upper atmosphere
O3 molecules absorb harmful UV radiation
O3 molecules are recycled as heat energy is released

44. 11.8 Atmospheric Chemistry and O3
O3 depletion (about 6% each year) remains a serious health and environmental issue
Compounds that are most destructive to the ozone layer
Are stable enough to reach the upper atmosphere
Form free radicals that interfere with the O3 recycling process
Chlorofluorocarbons (CFCs) fit both criteria
Atmosphere O3 is vital for protection, but what effect does O3 have at the earth’s surface?

45. 11.8 Atmospheric Chemistry and O3
CFC substitutes that generally decompose before reaching the O3 layer include hydrochlorofluorocarbons
Hydrofluorocarbons don’t even form the chlorine radicals that interfere with the O3 cycle

46. 11.8 Combustion and Firefighting
Water deprives the fire of the Eact needed by absorbing the energy
CO2 and Argon gas deprive the fire of needed oxygen
Halons are very effective fire-suppression agents
Halons suppress combustion in three main ways

47. 11.8 Combustion and Firefighting
Halons suppress combustion in three main ways
As a gas, they can smother the fire and deprive it of O2
They absorb some of the Eact for the fire by undergoing homolytic cleavage
The free radicals produced can combine with C• and H• free radicals to terminate the combustion chain reaction

48. 11.9 Autooxidation
Autooxidation is the process by which compounds react with molecular oxygen
The process is generally very slow

49. 11.9 Autooxidation
Some compounds such as ethers are particularly susceptible to autooxidation
Because hydroperoxides can be explosive, ethers like diethyl ether must not be stored for long periods of time
They should be dated and used in a timely fashion

50. 11.9 Autooxidation Light accelerates the autooxidation process
Dark containers are often used to store many chemicals such as vitamins
In the absence of light, autooxidation is usually a slow process
Compounds that can form a relatively stable C• radical upon H abstraction are especially susceptible to autooxidation. WHY?
Consider the autooxidation of compounds with allylic or benzylic hydrogen atoms

51. 11.9 Antioxidants Triglycerides are important to a healthy diet
Autooxidation can occur at the allylic positions causing the food to become rancid and toxic

52. 11.9 Antioxidants
Foods with unsaturated fatty acids have a short shelf life unless preservatives are used

53. 11.9 Antioxidants
Preservatives can undergo H abstraction to quench the C• radicals that form in the first step of autooxidation
One molecule of BHT can prevent thousands of autooxidation reactions by stopping the chain reaction
How does BHT’s structure make it good at taking on a free radical? Consider resonance and sterics

54. 11.9 Natural Antioxidants Vitamins C is hydrophilic
Vitamin E is hydrophobic
What parts of the body do these vitamins protect?
For each vitamin, show its oxidation mechanism, and explain how that protects the body from autooxidation

55. 11.10 Anti-Markovnikov Addition
We learned in chapter 9 that H-X will add across a C=C double bond with anti-Markovnikov regioselectivity when peroxides are present
Now that we have discussed free radicals, we can explain the mechanism for the anti-Markovnikov addition

56. 11.10 Anti-Markovnikov Addition
The O-O single bond can break homolytically with a relatively low Eact

57. 11.10 Anti-Markovnikov Addition
The Br• radical reacts to give the more stable C• radical
A 3° radical forms through a lower TS than a 2° radical

58. 11.11 Radical Polymerization
In chapter 9, we learned how some ionic polymerizations occur
Free radical conditions are also frequently used to form polymers
Recall that a polymerization process joins together many small units called monomers in a long chain

59. 11.11 Radical Polymerization
Radical polymerizations generally proceed through a chain reaction mechanism

60. 11.11 Radical Polymerization
Radical polymerizations generally proceed through a chain reaction mechanism
Note how the sum of the propagation steps yields the overall reaction

61. 11.11 Radical Polymerization
Radical polymerizations generally proceed through a chain reaction mechanism

62. 11.11 Radical Polymerization
Branching is common in some radical polymerizations
Branching makes polymer materials more flexible such as a polyethylene squeeze bottle
When catalysts are used to minimize branching, more rigid materials are produced such as the squeeze bottle cap
Why does branching affect rigidity?

63. 11.11 Radical Polymerization
Many derivatives of ethylene are also polymerized

64. 11.11 Radical Polymerization

65. 11.12 Radical Petroleum Processes
Recall the cracking and reforming processes from chapters 4 and 8
Crude oil is cracked to produce smaller alkanes and alkenes
Reforming increases branching. Propose a mechanism

66. 11.13 Synthetic Utility of Halogenation
Radical chlorination and bromination are both useful processes
Recall that bromination is more selective.
Temperature can be used to help avoid polysubstitution. HOW?

67. 11.13 Synthetic Utility of Halogenation
Chlorination can be useful with highly symmetrical substrates
It is difficult to avoid polysubstitution. WHY?
The synthetic utility of halogenation is limited
Chlorination is difficult to control
Bromination requires a substrate with 1 site that is significantly more reactive than all others

68. 11.13 Synthetic Utility of Halogenation
Synthesizing a target molecule from an alkane is challenging because of its limited reactivity
Often halogenation is the best option

69. Additional Practice Problems
Give the major product(s) for the reaction below. Carefully consider regiochemistry and stereochemistry.

70. Additional Practice Problems
Give the major product(s) for the reaction below. Carefully consider regiochemistry and stereochemistry.
There is only 1 tertiary center, so it should be where the majority of the reaction occurs, and although the text doesn’t discuss, Br is less sterically hindering than CH3, so it should occupy the axial position. If students place the Br in the equatorial position, that may also be an acceptable answer given the scope of the text content.

71. Additional Practice Problems
Predict the major product for the reaction below and explain why NBS is preferred over Br2.

72. Additional Practice Problems
Predict the major product for the reaction below and explain why NBS is preferred over Br2. Products form from both radical contributors once the free radical resonance hybrid forms. Equatorial substitution should be favored over axial. The primary free radical can also form because it is stabilized by resonance. NBS is preferred, because it will give much less addition side products.

73. Additional Practice Problems
Draw the monomer necessary to synthesize the given polymer using a free radical mechanism.