Reactions of Alkenes: Addition Reactions

1. Reactions of Alkenes:Addition Reactions

2. Addition Reactions • Addition is the opposite of elimination. • Here are some of the many addition reactions that will be considered.

3. Addition of Halogensto Alkenes

4. C C C C General features • electrophilic addition to double bond • forms a vicinal dihalide +X2 X X

5. Example Br2 CH3CHCHCH(CH3)2 CH3CH CHCH(CH3)2 CHCl3 0°C Br Br (100%)

6. Scope limited to Cl2 and Br2 • F2 addition proceeds with explosive violence • I2 addition is endothermic: vicinal diiodidesdissociate to an alkene and I2

7. Halogenation

8. Stereochemistry of Halogen Addition anti addition

9. H H Br Br H H Example Br2 trans-1,2-Dibromocyclopentane80% yield; only product

10. H Cl H Cl Example H Cl2 H trans-1,2-Dichlorocyclooctane73% yield; only product

11. Mechanism of Halogen Addition to Alkenes: Halonium Ions anti addition

12. Mechanism is electrophilic addition • Br2 is not polar, but it is polarizable • two steps (1) formation of bromonium ion (2) nucleophilic attack on bromonium ion by bromide

13. Addition of Halogens to Alkenes

14. Relative Rates of Bromination • ethylene H2C=CH2 1 • propene CH3CH=CH2 61 • 2-methylpropene (CH3)2C=CH2 5400 • 2,3-dimethyl-2-butene (CH3)2C=C(CH3)2 920,000 • More highly substituted double bonds react faster.Alkyl groups on the double bond make itmore “electron rich.”

15. Question • Arrange the alkenes in order of decreasing rate of reaction toward bromine addition: • 2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-butene • A) 2-methyl-1-butene > 3-methyl-1-butene > 2-methyl-2-butene • B) 3-methyl-1-butene > 2-methyl-1-butene > 2-methyl-2-butene • C) 2-methyl-2-butene > 2-methyl-1-butene > 3-methyl-1-butene • D) 2-methyl-2-butene > 3-methyl-1-butene > 2-methyl-1-butene

16. H H Br Br H H Example Br2 trans-1,2-dibromocyclopentane80% yield; 2 asymmetric carbon atoms, Chiral product,Optically inactive, Racemic mixture, only products formed

17. Diastereomers • In general, diastereomers fall into two categories: • geometric isomers • cis-trans • stereoisomers containing two or more asymmetric atoms; (that are not enantiomers) (S) (R) diastereomers (R) (R)

18. Diastereomers • Diastereomers have different physical properties: • BP, MP, density, refractive index, solubility • Can be separated through conventional means (distillation, recrystallization, chromatography) MP = 158oC MP = 256oC

19. Diastereomers • A compound with “n” asymmetric carbon atoms can have a maximum of 2nstereoisomers. 2,3-dibromobutane (S) (R) (R) (S) s (S) (R) (S) (R) Enantiomers Same compound (meso) diastereomers Only 3stereoisomers for 2,3-dibromobutane.

20. CH3 CH3 CH3 H Br Br Br H H Br Br H Br H H CH3 CH3 CH3 Three stereoisomers of 2,3-dibromobutane 2R,3R 2S,3S 2R,3S chiral chiral achiral

21. Question • The addition of bromine to cis-2-butene produces: • A single enantiomer • A product with one asymmetric carbon atom • An optically inactive meso product • A racemic mixture • 4 different stereoisomers each with 2 chiral carbon atoms

22. Question • The addition of bromine to trans-2-butene produces: • A single enantiomer • A product with one asymmetric carbon atom • An optically inactive meso product • A racemic mixture • 4 different stereoisomers each with 2 chiral carbon atoms

23. Diastereomers • When naming compounds containing multiple chiral atoms, you must give the configuration around each chiral atom: • position number and configuration of each chiral atom, separated by commas, all in ( ) at the start of the compound name (S) (2S, 3S)-2-bromo-3-chlorobutane (S)

24. Diastereomers • Which ones are chiral? Name each one. Give the stereochemical relationship between them. All of them are chiral.

25. Diastereomers (2S, 3R)-2,3-dichloropentane (2R, 3S)-2,3-dichloropentane (2S, 3S)-2,3-dichloropentane (2R, 3R)-2,3-dichloropentane

26. Diastereomers A B C D A and B: enantiomers C and D: enantiomers A and C: diastereomers A and D: diastereomers B and C: diastereomers B and D: diastereomers

27. Hydrogenation of Alkenes Heterogeneous Catalysis

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

29. H H H H C C H H C C H H H H Hydrogenation of Ethylene • exergonic H° = –136 kJ/mol • catalyzed by finely divided (heterogeneous) metals • (Insoluble) Pt, Pd, Rh, Ni     + H—H

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

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

32. Question • Which one of the following terms best applies to the hydrogenation of an alkene in the • presence of finely divided platinum? (in ethanol as solvent) • A) anti addition • B) concerted reaction • C) heterogeneous catalysis • D) endothermic reaction

33. Catalytic Hydrogenation • If catalysis takes place on the surface of a solid surrounded by solution, the catalyst is HETEROGENEOUS. • HOMOGENEOUS catalysts also exist. • What advantage might a homogeneous catalyst have?

34. Hydrogenation of Alkenes Homogeneous Catalysis

35. Asymmetric Hydrogenation • In 1968, Knowles modified Wilkinson’s catalyst by using a chiral phosphine ligand. • A chiral catalyst can produce one desired enantiomer over another.

36. Asymmetric Hydrogenation • A chiral catalyst allows one enantiomer to be formed. • Some chiral catalysts give better enantioselectivity than others.

37. Asymmetric Hydrogenation • BINAP is a chiral ligand that gives very pronounced enantioselectivity. • For any reaction, stereoselectivity can only be occur if at least one reagent (reactant or catalyst) ischiral.

38. Question • Which of the alkenes below will produce 2-methylbutane on catalytic hydrogenation? • A) 1 and 3 • B) 1, 2 and 3 • C) 2 and 4 • D) 2, 3 and 4

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

40. Heats of Hydrogenation of Isomers 126 119 115 CH3CH2CH2CH3

41. 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

42. Question • Rank the following alkenes in order of decreasing heat of hydrogenation. • A) 1 > 3 > 2 • B) 3 > 2 > 1 • C) 2 > 3 > 1 • D) 1 > 2 > 3

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

44. Question • Which alkene has the lowest heat of hydrogenation? • A) B) • C) D)

45. Stereochemistry of Alkene Hydrogenation

46. Catalytic Hydrogenation of an Alkene

47. Two Spatial (stereochemical) Aspects ofAlkene Hydrogenation • (1) syn addition of both H atoms to double bond (adds from the same side) • (2) hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond

48. Two Spatial (stereochemical) Aspects ofAlkene Hydrogenation • (1) syn addition of both H atoms to double bond

49. syn Addition versus anti Addition syn addition anti addition

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