Chapter 6

1. Chapter 6 Reactions of Alkenes:Addition Reactions

2. B C A C C C Reactions of Alkenes The characteristic reaction of alkenes is addition to the double bond. + A—B

3. 6.1Hydrogenation of Alkenes

4. H H H H C C H H C C H H H H Hydrogenation of ethylene exothermic H° = –136 kJ/mol catalyzed by finely divided Pt, Pd, Rh, Ni     + H—H

5. H3C CH2 H3C CH3 H3C H H3C Example H2, Pt (73%)

6. Problem 6.1 What three alkenes yield 2-methylbutane on catalytic hydrogenation?

7. Problem 6.1 What three alkenes yield 2-methylbutane on catalytic hydrogenation? H2, Pt

8. H H B Y H H C C A X Mechanism of catalytic hydrogenation.Figure 6.1

9. H H H H Mechanism of catalytic hydrogenation.Figure 6.1 B Y C C A X

10. B Y X A H H H C C H Mechanism of catalytic hydrogenation.Figure 6.1

11. H H H H Mechanism of catalytic hydrogenation.Figure 6.1 B Y X A C C

12. B Y X A H H C C H H Mechanism of catalytic hydrogenation.Figure 6.1

13. B Y X A C C H H H H Mechanism of catalytic hydrogenation.Figure 6.1

14. 6.2Heats of Hydrogenation can be used to measure relative stability of isomeric alkenes correlation with structure is same as when heats of combustion are measured

15. 126 119 115 CH3CH2CH2CH3

16. Heats of Hydrogenation (kJ/mol) Ethylene 136 Monosubstituted 125-126 cis-Disubstituted 117-119 trans-Disubstituted 114-115 Terminally disubstituted 116-117 Trisubstituted 112 Tetrasubstituted 110

17. 126 kJ/mol 118 kJ/mol 112 kJ/mol Problem 6.2 Match each alkene of Problem 6.1 with its correctheat of hydrogenation.

18. 126 kJ/mol 118 kJ/mol 112 kJ/mol Problem 6.2 Match each alkene of Problem 6.1 with its correctheat of hydrogenation. highest heat ofhydrogenation;least stable isomer lowest heat ofhydrogenation;most stable isomer

19. 6.3Stereochemistry of Alkene Hydrogenation

20. Two spatial (stereochemical) aspects ofalkene hydrogenation: • syn addition of both H atoms to double bond • hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond

21. syn-Addition versus anti-Addition syn addition anti addition

22. CO2CH3 H2, Pt CO2CH3 Example of syn-Addition H CO2CH3 CO2CH3 H (100%)

23. Stereoselectivity A reaction in which a single starting materialcan give two or more stereoisomeric productsbut yields one of them in greater amounts thanthe other (or even to the exclusion of the other)is said to be stereoselective.

24. H3C CH3 H H3C H3C CH3 H3C CH3 H H H H H3C H CH3 H Example ofstereoselective reaction H2, cat Both productscorrespond tosyn additionof H2.

25. H3C CH3 H H3C H3C CH3 H H H3C H Example ofstereoselective reaction H2, cat But only thisone is formed.

26. H3C CH3 H H3C H3C CH3 H H H3C H Example ofstereoselective reaction H2, cat Top face of doublebond blocked bythis methyl group

27. H3C CH3 H H3C H3C CH3 H H H3C H Example ofstereoselective reaction H2, cat H2 adds to bottom face of double bond.

28. 6.4Electrophilic Addition of Hydrogen Halides to Alkenes

29. C C C E C Y General equation for electrophilic addition  – +E—Y

30. C C C H C X When EY is a hydrogen halide  – +H—X

31. C C Example CH2CH3 CH3CH2 HBr CHCl3, -30°C H H CH3CH2CH2CHCH2CH3 Br (76%)

32. Mechanism Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate. Electrons flow from the  system of thealkene (electron rich) toward the positivelypolarized proton of the hydrogen halide.

33. .. – : : X .. .. : .. C C Mechanism + C C H H X

34. .. – : : X .. .. : .. .. C : C C C H X .. Mechanism + C C H H X

35. 6.5Regioselectivity ofHydrogen Halide Addition:Markovnikov's Rule

36. Markovnikov's Rule When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.

37. CH3CH2CH CH3CH2CHCH3 CH2 Br Markovnikov's Rule HBr acetic acid (80%) Example 1

38. CH3 CH3 CH3 C C C Br Markovnikov's Rule CH3 H HBr acetic acid CH3 H (90%) Example 2

39. CH3 CH3 Cl Markovnikov's Rule HCl 0°C (100%) Example 3

40. 6.6Mechanistic BasisforMarkovnikov's Rule Protonation of double bond occurs in direction that gives more stable of two possible carbocations.

41. HBr CH3CH2CH CH3CH2CHCH3 CH2 acetic acid Br Mechanistic Basis for Markovnikov's Rule:Example 1

42. + CH3CH2CH—CH3 + Br – CH3CH2CH CH3CH2CHCH3 CH2 Br Mechanistic Basis for Markovnikov's Rule:Example 1 HBr

43. + CH3CH2CH—CH3 + Br – CH3CH2CH CH3CH2CHCH3 CH2 Br Mechanistic Basis for Markovnikov's Rule:Example 1 + CH3CH2CH2—CH2 primary carbocation is less stable: not formed HBr

44. H CH3 HCl CH3 Cl 0°C Mechanistic Basis for Markovnikov's Rule:Example 3

45. H CH3 HCl CH3 Cl 0°C Mechanistic Basis for Markovnikov's Rule:Example 3 H H Cl – CH3 + HCl

46. H CH3 HCl CH3 Cl 0°C Mechanistic Basis for Markovnikov's Rule:Example 3 secondary carbocation is less stable: not formed H H H + CH3 Cl – CH3 + H HCl

47. 6.7Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes

48. CHCH(CH3)2 H2C + CH3CHCH(CH3)2 CH3CH2C(CH3)2 CH3CHCH(CH3)2 Cl Cl Rearrangements sometimes occur HCl, 0°C H + CH3CHC(CH3)2 (40%) (60%)

49. 6.8Free-radical Addition of HBr to Alkenes The "peroxide effect"

50. CH3CH2CH CH3CH2CHCH3 CH2 Br Markovnikov's Rule HBr acetic acid (80%) Example 1