1. Free Radical Reactions Halogenation of Alkanes RH + X 2  RX + HX

2. Orbital hybridization in carbocations and carbanions: Reactive Intermediates

3. Free radicals can be thought of as sp 2 hybridized or quickly interconverting sp 3 hybridized. Reactive Intermediates: Free Radicals

4. RH + X 2  RX + HX explosive for F 2 exothermic for Cl 2 and Br 2 endothermic for I 2 Energetics

5. Halogenation Thermodynamics

6. Chlorination of Methane

7. carried out at high temperature (400 °C) CH 4 + Cl 2  CH 3 Cl + HCl CH 3 Cl + Cl 2  CH 2 Cl 2 + HCl CH 2 Cl 2 + Cl 2  CHCl 3 + HCl CHCl 3 + Cl 2  CCl 4 + HCl Chlorination of Methane

8. Mechanism of Chlorination of Methane

9. The initiation step "gets the reaction going" by producing free radicals—chlorine atoms from chlorine molecules in this case. Initiation step is followed by propagation steps. Each propagation step consumes one free radical but generates another one. :.. Cl.. : Cl..:.... Cl..:. : Cl..... + Free-radical chain mechanism. Initiation step: (Light or Heat is Necessary) Mechanism of Chlorination of Methane

10. Which of the above initiates reactions most readily? Question A) B) C)

11. Cl:..... H 3 C : H H : Cl:.... H3CH3CH3CH3C.+ + First propagation step: Second propagation step: + + Cl:..... H3CH3CH3CH3C. :.. Cl.. : Cl.. :.. H3CH3CH3CH3C.... : Cl : Mechanism of Chlorination of Methane

12. Cl:..... H 3 C : H H : Cl:.... H3CH3CH3CH3C.+ + Second propagation step: + + ++ First propagation step: Cl:..... H3CH3CH3CH3C. :.. Cl.. : Cl..:.. H 3 C : H H : Cl:.... :.. Cl.. : Cl..:.. H3CH3CH3CH3C.... : Cl : H3CH3CH3CH3C.... : Cl : Mechanism of Chlorination of Methane

13. Almost all of the product is formed by repetitive cycles of the two propagation steps. Cl:..... H 3 C : H H : Cl:.... H3CH3CH3CH3C.+ + Second propagation step: + + ++ First propagation step: Cl:..... H3CH3CH3CH3C. :.. Cl.. : Cl..:.. H 3 C : H H : Cl:.... :.. Cl.. : Cl..:.. H3CH3CH3CH3C.... : Cl : H3CH3CH3CH3C.... : Cl :

14. stop chain reaction by consuming free radicals hardly any product is formed by termination step because concentration of free radicals at any instant is extremely low hardly any product is formed by termination step because concentration of free radicals at any instant is extremely low + H3CH3CH3CH3C. Cl:..... H3CH3CH3CH3C.... : Cl : Termination Steps

15. Question The step shown below is a _____________ step of the free-radical chlorination of chloromethane. A)initiation B)propagation C)chain-terminating D)bond cleavage

16. Question For the free-radical reaction below, light is involved in which of the following reaction steps? A)Initiation only B)Propagation only C)Termination only D)Initiation and propagation

17. Halogenation of Higher Alkanes

18. CH 3 CH 3 + Cl 2 CH 3 CH 2 Cl + HCl 420°C (78%) Cl + Cl 2 + Cl 2 + HCl + HCl h (73%) can be used to prepare alkyl chlorides from alkanes in which all of the hydrogens are equivalent to one another Chlorination of Alkanes

19. Major limitation: Chlorination gives every possible monochloride derived from original carbon skeleton. Not much difference in reactivity of different hydrogens in molecule. Chlorination of Alkanes

20. CH 3 CH 2 CH 2 CH 3 Cl 2 h Chlorination of butane gives a mixture of 1-chlorobutane and 2-chlorobutane. CH 3 CH 2 CH 2 CH 2 Cl CH 3 CHCH 2 CH 3 Cl (28%) (72%) Example

21. Percentage of Product that Results from Substitution of Indicated Hydrogen if Every Collision with Chlorine Atoms is Productive 10% 10% 10% 10% 10% 10% 10% 10% 10%10%

22. Percentage of Product that Actually Results from Replacement of Indicated Hydrogen 4.6% 4.6% 4.6% 18% 18% 18% 4.6% 4.6% 4.6% 18%

23. divide by 4.6 4.64.6 = 1 184.6 = 3.9 A secondary hydrogen is abstracted 3.9 times faster than a primary hydrogen by a chlorine atom. 4.6% 18% Relative Rates of Hydrogen Atom Abstraction

24. Similarly, chlorination of 2-methylbutane gives a mixture of isobutyl chloride and tert-butyl chloride H CH 3 CCH 3 CH 3 CH 3 CCH 2 Cl CH 3 H Cl CH 3 CCH 3 CH 3 h Cl 2 (63%)(37%)

25. Question How many monochlorination products do you expect to obtain from the chlorination of 2-methylbutane? A)two B)three C)four D)five

26. Percentage of Product that Results from Replacement of Indicated Hydrogen 7.0% 37%

27. 37 7.0 divide by 7 7 = 1 7 = 5.3 A tertiary hydrogen is abstracted 5.3 times faster than a primary hydrogen by a chlorine atom. Relative Rates of Hydrogen Atom Abstraction

28. Radicals - Resonance ?

29. Hyperconjugation contributes more to thermodynamic stability than resonance. Question True (A) / False (B)

30. Vinyl free radicals are more thermodynamically stable than benzylic and allylic free radicals. Question True (A) / False (B)

31. R 3 CH>R 2 CH 2 >RCH 3 chlorination: 5 41 bromination:1640821 Chlorination of an alkane gives a mixture of every possible isomer having the same skeleton as the starting alkane. Useful for synthesis only when all hydrogens in a molecule are equivalent. Bromination is highly regioselective for substitution of tertiary hydrogens. Major synthetic application is in synthesis of tertiary alkyl bromides. Selectivity of Free-radical Halogenation

32. Radicals - Bromination

33. How many mono-bromination products are expected from the following reaction? A. 5B. 4 C. 3D. 2E. 1 Question

34. Chlorination is useful for synthesis only when all of the hydrogens in a molecule are equivalent. (64%) Cl 2 h Synthetic Application of Chlorination of an Alkane Cl

35. Question An alkane with a molecular formula of C 8 H 18 reacts with Cl 2 in the presence of light and heat to give a single monochloride C 8 H 17 Cl. What is the most reasonable structure for the starting alkane? A)CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 B)(CH 3 CH 2 ) 2 CHCH 2 CH 2 CH 3 C)(CH 3 ) 2 CHCH 2 CH 2 CH(CH 3 ) 2 D)(CH 3 ) 3 CC(CH 3 ) 3

36. Bromination is highly selective for substitution of tertiary hydrogens. Major synthetic application is in synthesis of tertiary alkyl bromides. Major synthetic application is in synthesis of tertiary alkyl bromides. (76%) Br 2 hH CH 3 CCH 2 CH 2 CH 3 CH 3 Br CH 3 CCH 2 CH 2 CH 3 CH 3 Synthetic Application of Bromination of an Alkane

37. Question Which of the following best describes a mechanistic feature of the free-radical bromination (Br 2, light) of 2- methylpropane? A)The initiation step involves cleavage of a C-H bond. B)The free-radical (CH 3 ) 3 C· is produced in one propagation step and reacts with Br 2 in another. C)The reaction is characterized by the homolytic cleavage of the C-Br bond. D)The reaction is concerted; i.e., it occurs in a single step.

38. Which reaction has a faster rate? Which product is kinetically favored? A. I) and I) B. II) and II) C. I) and II)D. II) and I) Question I)II)

39. Which reaction has a propagation step that is endothermic? Which reaction is more regioselective? A. I) and I) B. II) and II) C. I) and II)D. II) and I) Question I)II)

40. Three mono-substituted isomers form in the halogenation of butane. The products are optically inactive. Stereochemistry

41. Bromination of optically active 3-methylhexane produces only (S)-3-bromo-3-methylhexane. Question True (A) / False (B)

42. How many products (including stereoisomers) are expected in the following halogenation? A. 5B. 4 C. 3D. 2E. 1 Question

43. Resonance and regioselectivity: Allylic/Benzylic Bromination A mixture is obtained that includes the di-brominated product. Br- Di-bromination can be avoided using a specialized reagent: N-bromosuccinimide (NBS)

44. Allylic/Benzylic Bromination

45. What is the major product of the following reaction? Question

46. (A) Barbamide, a cyanobacterial peptide containing a trichloromethyl group and (B) dysidenin, a barbamide-related compound isolated from a sponge-cyanobacterial association Halogenated Marine Natural Products

47. Polymers Polymers

48. Free radical conditions are frequently used to form polymers. Recall that a polymerization process joins together many small units called monomers in a long chain. Radical Polymerization

49. Radical polymerizations commonly proceed through a chain reaction mechanism.

50. Radical Polymerization

52. Radical polymerizations generally produce chains of monomers with a wide distribution of lengths. Because the mechanism proceeds through 1° carbon free radical intermediates, it is usually difficult. Radical Polymerization

53. Branching is common in radical polymerizations. Branching makes polymer materials more flexible, such as a polyethylene used for squeeze bottles. When catalysts are used to minimize branching, more rigid materials are produced, such as the material used for squeeze bottle caps. Radical Polymerization

54. Many derivatives of ethylene are polymerized. Radical Polymerization

56. Stereoregular Polymers atacticisotacticsyndiotactic

57. Atactic Polypropylene Random stereochemistry of methyl groups attached to main chain (stereorandom) Properties not very useful for fibers etc. Formed by free-radical polymerization

58. Isotactic Polypropylene Stereoregular polymer; all methyl groups on same side of main chain Useful properties Prepared by coordination polymerization under Ziegler-Natta conditions

59. Syndiotactic Polypropylene Stereoregular polymer; methyl groups alternate side-to-side on main chain Useful properties Prepared by coordination polymerization under Ziegler-Natta conditions

60. Properties of Polymers Isotactic and syndiotactic polymers are stronger and stiffer due to their regular packing arrangement. Anionic polymerization usually gives isotactic or syndiotactic polymers. Free radical polymerization is nearly random, giving branched atactic polymers.

61. Free Radical Polymerization

62. Polystyrene & Others