1. © E.V. Blackburn, 2011 Alkenes and Alkynes Addition Reactions

2. © E.V. Blackburn, 2011 Reactions of alkenes - catalytic hydrogenation Heat of hydrogenation - the heat liberated during this reaction.  H is ~125 kJ/mol for each double bond in the compound.

3. © E.V. Blackburn, 2011 Mechanism of alkene hydrogenation catalyst H 2 adsorbed at surface complexation of alkene to catalyst hydrogen insertion catalyst alkane

4. © E.V. Blackburn, 2011 Mechanism of alkene hydrogenation Hydrogenation is stereospecific. The two hydrogens add to the same side of the double bond - a syn addition.

5. © E.V. Blackburn, 2011 The  electrons are less tightly held than the  electrons. The double bond therefore acts as a source of electrons - a base – a nucleophile. It reacts with electron deficient compounds - acids - electrophiles. Reactions of alkenes

6. © E.V. Blackburn, 2011 Electrophilic addition The typical reaction of alkenes is one of electrophilic addition - an acid - base reaction. Don’t forget that free radicals are electron deficient. They undergo addition reactions with alkenes. Electron seeking reagents are called electrophilic reagents.

7. © E.V. Blackburn, 2011 Addition of hydrogen halides

8. © E.V. Blackburn, 2011 Addition of hydrogen halides Only 2-chloropropane is formed

9. © E.V. Blackburn, 2011 Markovnikov’s rule In 1869, Markovnikov proposed that in the addition of an acid to an alkene, the hydrogen of the acid bonds to the carbon which is already bonded to the greater number of hydrogens.

10. © E.V. Blackburn, 2011 Each carbon of the double bond is bonded to one H therefore both isomers are formed. Markovnikov’s rule

11. © E.V. Blackburn, 2011 Markovnikov addition - a regioselective reaction These reactions are said to be regioselective because only one of the two possible directions of addition occurs. Regioselectivity - the preferential formation of one isomer in those situations where a choice is possible.

12. © E.V. Blackburn, 2011 HBr - the peroxide effect 1933, Kharasch and Mayo

13. © E.V. Blackburn, 2011 Addition of sulfuric acid

14. © E.V. Blackburn, 2011 Hydration a Markovnikov addition

15. © E.V. Blackburn, 2011 The mechanism of the addition

16. © E.V. Blackburn, 2011 An example

17. © E.V. Blackburn, 2011 Orientation

18. © E.V. Blackburn, 2011 Orientation

19. © E.V. Blackburn, 2011 A more general “rule” Why? Let’s look at the transition state:- Electrophilic addition to a carbon - carbon double bond involves the intermediate formation of the most stable carbocation.

20. © E.V. Blackburn, 2011 A more general “rule”

21. © E.V. Blackburn, 2011 Carbocation rearrangements

22. © E.V. Blackburn, 2011 Oxymercuration

23. © E.V. Blackburn, 2011 Oxymercuration Dissociation: Electrophilic attack: Nucleophilic opening: An anti addition via a mercurinium ion:

24. © E.V. Blackburn, 2011 Oxymercuration Why do we observe Markovnikov addition? In the mercurinium ion, the positive charge is shared between the more substituted carbon and the mercury atom. Only a small portion of the charge resides on this carbon but it is sufficient to account for the orientation of the addition but is insufficient to allow a rearrangement to occur.

25. © E.V. Blackburn, 2011 Hydroboration H.C. Brown and G. Zweifel, J. Am. Chem. Soc., 83, 2544 (1961) Brown was co-winner of the 1979 Nobel Prize in Chemistry.

26. © E.V. Blackburn, 2011 Hydroboration syn addition no rearrangement no carbocation!

27. © E.V. Blackburn, 2011 Hydroboration

28. © E.V. Blackburn, 2011 Hydroboration - the mechanism

29. © E.V. Blackburn, 2011 Hydroboration - the mechanism

30. © E.V. Blackburn, 2011 Hydroboration - the mechanism

31. © E.V. Blackburn, 2011 Addition of halogens 1,2-dibromopropane X 2 = Cl 2, Br 2 usually iodine does not react

32. © E.V. Blackburn, 2011 Mechanism of X 2 addition

33. © E.V. Blackburn, 2011 Mechanism of X 2 addition

34. © E.V. Blackburn, 2011 Stereospecific reactions (Z)-2-butene gives racemic 2,3-dibromobutane and no meso compound is formed. (E)-2-butene gives only meso-2,3-dibromobutane. A reaction is stereospecific if a particular stereoisomer of the reactant produces a specific stereoisomer of the product.

35. © E.V. Blackburn, 2011 syn and anti addition

36. © E.V. Blackburn, 2011 Bromine addition - an anti addition I. Roberts and G.E. Kimball, J. Am. Chem. Soc., 59, 947 (1937) bromonium ion

37. © E.V. Blackburn, 2011 The bromonium ion (S,S)

38. © E.V. Blackburn, 2011 Halohydrin formation

39. © E.V. Blackburn, 2011 Halohydrin formation

40. © E.V. Blackburn, 2011 Unsymmetric electrophiles In the electrophilic addition of an unsymmetric reagent, the electrophilic part adds to the less substituted carbon of the alkene unit:

41. © E.V. Blackburn, 2011 Free radical addition reactions

42. © E.V. Blackburn, 2011 ionic vs radical addition

43. © E.V. Blackburn, 2011 Polymerization A polymer is a long chain molecule made up of structural units (monomers) joined together.

44. © E.V. Blackburn, 2011 Free radical polymerization of alkenes Initiation

45. © E.V. Blackburn, 2011 Free radical polymerization of alkenes chain propagation etc

46. © E.V. Blackburn, 2011 Free radical polymerization of alkenes Chain termination combination 2

47. © E.V. Blackburn, 2011 Free radical polymerization of alkenes Chain termination disproportionation +

48. © E.V. Blackburn, 2011 Examples Gmonomerpolymer Cl CH 2 =CHCl -CH 2 CHCl-CH 2 CHCl-CH 2 - vinyl chloride polyvinyl chloride, PVC CN CH 2 =CHCN -CH 2 CHCN-CH 2 CHCN- acrylonitrile polyacrylonitrile Orlon, Acrilon

49. © E.V. Blackburn, 2011 Examples Gmonomerpolymer Ph CH 2 =CHPh -CH 2 CHPh-CH 2 CHPh- styrene polystyrene methyl methacrylatepoly(methyl methacrylate) Plexiglas, Lucite

50. © E.V. Blackburn, 2011 The addition of other compounds can modify the polymerization: Polymerization

51. © E.V. Blackburn, 2011 Carbenes Carbenes (and carbenoids) add to alkenes in a stereospecific manner to form cyclopropanes.

52. © E.V. Blackburn, 2011 Carbenes

53. © E.V. Blackburn, 2011 Hydroxylation

54. © E.V. Blackburn, 2011 Ozonolysis aldehydes and ketones ozonide

55. © E.V. Blackburn, 2011 Ozonolysis

56. © E.V. Blackburn, 2011 KMnO 4 oxidation

57. © E.V. Blackburn, 2011 Addition of halogen to alkynes

58. © E.V. Blackburn, 2011 Addition of HX to alkynes Does this seem reasonable? This carbocation should be destabilized by the inductive effect of the Cl! The cation is stablized by resonance! Remember this in CHEM 263

59. © E.V. Blackburn, 2011