6.10 Acid-Catalyzed Hydration of Alkenes

1. 6.10Acid-Catalyzed Hydration of Alkenes

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

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

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

5. 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

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

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

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

9. 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.

10. 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

11. 6.11Hydroboration-Oxidation of Alkenes

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

13. OH 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.

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

15. 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.

16. 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

17. H BH2 C C C C •• + O – BH3 Hydroboration Step + H—BH2 Hydroboration reagents: Borane-tetrahydrofurancomplex (H3B-THF)

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

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

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

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

22. OH Example 1. B2H6, diglyme 2. H2O2, HO– (82%)

23. 6.12Stereochemistry of Hydroboration-Oxidation

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

25. 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

26. 6.13Mechanism of Hydroboration-Oxidation

27. 1-Methylcyclopentene + BH3 • syn addition of Hand B to double bond • B adds to less substituted carbon

28. Organoborane intermediate

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

30. trans-2-Methylcyclopentanol