6.9 Addition of Sulfuric Acid to Alkenes

1. 6.9Addition of Sulfuric Acid to Alkenes

2. CH3CHCH3 CH3CH CH2 OSO2OH Addition of H2SO4 HOSO2OH follows Markovnikov's rule: yields an alkyl hydrogen sulfate Isopropylhydrogen sulfate

3. CH3CH CH2 + CH3CH CH3 CH3CHCH3 OSO2OH Mechanism .. SO2OH + O H .. slow – .. : + SO2OH O .. fast : ..

4. CH3CHCH3 SO2OH O heat CH3CHCH3 O H Alkyl hydrogen sulfates undergo hydrolysis in hot water + H—OH + HO—SO2OH

5. OH Application: Coversion of alkenes to alcohols 1. H2SO4 2. H2O, heat (75%)

6. But... not all alkenes yield alkyl hydrogen sulfateson reaction with sulfuric acid these do: H2C=CH2, RCH=CH2, and RCH=CHR' these don't: R2C=CH2, R2C=CHR, and R2C=CR2 (These form polymers, sec 6.21)

7. 6.10Acid-Catalyzed Hydration of Alkenes

8. C C C H C OH Acid-Catalyzed Hydration of Alkenes + H—OH reaction is acid catalyzed; typical hydration medium is 50% H2SO4-50% H2O

9. CH3 H3C H 50% H2SO4 CH2CH3 CH3 C C C 50% H2O H3C CH3 OH Follows Markovnikov's Rule (90%)

10. CH3 50% H2SO4 CH2 50% H2O OH Follows Markovnikov's Rule (80%)

11. CH3 H3C CH3 CH3 C C CH2 H3C OH Mechanism involves a carbocation intermediate is the reverse of acid-catalyzed dehydrationof alcohols to alkenes H+ + H2O

12. H H3C + : H C O CH2 H3C H H : : O Mechanism Step (1) Protonation of double bond + slow H3C + C + CH3 H3C H

13. H H3C + : : C + O CH3 H3C H fast CH3 H + : C CH3 O H CH3 Mechanism Step (2) Capture of carbocation by water

14. CH3 H H : C CH3 O : : O H H CH3 fast CH3 H .. + : : C H CH3 O O H H CH3 Mechanism Step (3) Deprotonation of oxonium ion + + +

15. Relative Rates Acid-catalyzed hydration ethylene CH2=CH2 1.0 propene CH3CH=CH2 1.6 x 106 2-methylpropene (CH3)2C=CH2 2.5 x 1011 The more stable the carbocation, the fasterit is formed, and the faster the reaction rate.

16. CH3 H3C CH3 CH3 C C CH2 H3C OH Principle of microscopic reversibility In an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in the opposite order. H+ + H2O

17. 6.11Thermodynamics of Addition-EliminationEquilibria

18. Hydration-Dehydration Equilibrium How do we control the position of the equilibrium and maximize the product?

19. Le Chatelier’s Principle A system at equilibrium adjusts so to minimize any stress applies to it. For the hydration-dehydration equilibria, the key stress is water. Adding water pushes the equilibrium toward more product (alcohol). Removing water pushes the equilibrium toward more reactant (alkene).

20. Le Chatelier’s Principle At constant temperature and pressure a reaction proceeds in a direction which is spontaneous or decreases free energy (G). The sign of G is always positive, but DG can be positive or negative. DG = Gproducts – Greactants Spontaneous when DG < 0

21. c d = + R T l n [ C ] [ D ] a b [ A ] [ B ] Le Chatelier’s Principle For a reversible reaction: aA + bB  cC + dD The relationship between DG and DGo is: DGo DG R = 8.314 J/(mol.K) and T is the temperature in K

22. Le Chatelier’s Principle At equilibrium G = 0 and the following becomes true: Substituting Keq into the previous equation gives: Go = - RT lnKeq Reactions for Go positive are endergonic and for Go negative are exergonic.

23. 6.12Hydroboration-Oxidation of Alkenes

24. Synthesis Suppose you wanted to prepare 1-decanol from 1-decene? OH

25. Synthesis Suppose you wanted to prepare 1-decanol from 1-decene? Needed: a method for hydration of alkenes with a regioselectivity opposite to Markovnikov's rule. OH

26. 1. hydroboration 2. oxidation Synthesis Two-step reaction sequence called hydroboration-oxidation converts alkenes to alcohols with a regiochemistry opposite to Markovnikov's rule. OH

27. H BH2 C C C C Hydroboration step + H—BH2 Hydroboration can be viewed as the addition ofborane (BH3) to the double bond. But BH3 is not the reagent actually used.

28. H BH2 C C C C Hydroboration step + H—BH2 Hydroboration reagents: H Diborane (B2H6)normally used in an ether- like solventcalled "diglyme" BH2 H2B H

29. H BH2 C C C C Hydroboration step + H—BH2 Hydroboration reagents: Borane-tetrahydrofurancomplex (H3B-THF) .. + O – BH3

30. H BH2 H OH C C C C Oxidation step H2O2, HO– Organoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.

31. 1. B2H6, diglyme 2. H2O2, HO– Example OH (93%)

32. H OH 1. H3B-THF CH3 CH3 C C C C 2. H2O2, HO– CH3 H Example H3C CH3 H3C H (98%)

33. Example OH 1. B2H6, diglyme 2. H2O2, HO– (no rearrangement) (82%)

34. 6.13Stereochemistry of Hydroboration-Oxidation

35. Features of Hydroboration-Oxidation • hydration of alkenes • regioselectivity opposite to Markovnikov's rule • no rearrangement • stereospecific syn addition

36. H CH3 CH3 HO H H syn-Addition • H and OH become attached to same face of double bond 1. B2H6 2. H2O2, NaOH only product is trans-2-methylcyclopentanol(86%) yield

37. 6.14Mechanism of Hydroboration-Oxidation

38. Hydroboration-oxidation  electrons from alkene flow into 2P orbital of boron

39. Hydroboration-oxidation H atom with 2 electrons migrates to C with greatest (+) charge (most highly substituted). Syn add.

40. Hydroboration-oxidation Anion of hydrogen peroxide attacks boron

41. Hydroboration-oxidation Carbon with pair of electrons migrates to oxygen, displacing hydroxide. Oxygen on same side as boron

42. Hydroboration-oxidation Hydrolysis cleaves boron-oxygen bond resulting in anti-Markovnikov addition

43. 1-Methylcyclopentene + BH3 • syn addition of Hand B to double bond • B adds to less substituted carbon

44. Organoborane intermediate

45. Add hydrogen peroxide • OH replaces B on same side

46. trans-2-Methylcyclopentanol

47. 6.15Addition of Halogensto Alkenes

48. C C C C + X2 electrophilic addition to double bond forms a vicinal dihalide X X

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

50. Scope Limited to Cl2 and Br2 F2 addition proceeds with explosive violence I2 addition is endothermic: vicinal diiodides dissociate to an alkene and I2