1. 6.10 Acid-Catalyzed Hydration of Alkenes19

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

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

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

5. involves a carbocation intermediate
Mechanism
involves a carbocation intermediate
is the reverse of acid-catalyzed dehydration of alcohols to alkenes OH C
CH3
H3C C
CH2 H+
+ H2O 5

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

7. Step (2) Capture of carbocation by water
Mechanism
Step (2) Capture of carbocation by water +
H3C C
CH3 H O :
fast + H O : C
CH3 6

8. Step (3) Deprotonation of oxonium ion
Mechanism
Step (3) Deprotonation of oxonium ion H O : C
CH3 O H : + +
fast H O : C
CH3 .. O + H : + 6

9. Acid-catalyzed hydration
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 faster it is formed, and the faster the reaction rate. 9

10. Principle of Microscopic ReversibilityOH C
CH3
H3C C
CH2 H+
+ H2O
In an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in the opposite order. 11

11. 6.11 Hydroboration-Oxidation of Alkenes19

12. Suppose you wanted to prepare 1-decanol from 1-decene?
Synthesis
Suppose you wanted to prepare 1-decanol from 1-decene? OH 13

13. Suppose you wanted to prepare 1-decanol from 1-decene?
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 13

14. Synthesis
Two-step reaction sequence called hydroboration- oxidation converts alkenes to alcohols with a regiochemistry opposite to Markovnikov's rule.
1. hydroboration
2. oxidation OH 13

15. Hydroboration Step C H BH2 C + H—BH2
Hydroboration can be viewed as the addition of borane (BH3) to the double bond. But BH3 is not the reagent actually used. 14

16. Hydroboration reagents:
Hydroboration Step C H
BH2 C
+ H—BH2
Hydroboration reagents: H
Diborane (B2H6) normally used in an ether- like solvent called "diglyme"
H2B
BH2 H 14

17. Hydroboration reagents:
Hydroboration Step C H
BH2 C
+ H—BH2
Hydroboration reagents: + O
BH3 – ••
Borane-tetrahydrofuran complex (H3B-THF) 14

18. Oxidation Step H2O2, HO– H BH2 C H OH C
Organoborane formed in the hydroboration step is oxidized with hydrogen peroxide. 17

19. Example
1. B2H6, diglyme
2. H2O2, HO– OH
(93%) 13

20. Example H C
CH3 OH
H3C
CH3
1. H3B-THF
2. H2O2, HO– C
H3C H
(98%) 13

21. Features of Hydroboration-Oxidation
hydration of alkenes
regioselectivity opposite to Markovnikov's rule
no rearrangement
stereospecific syn addition 18

22. Example OH
1. B2H6, diglyme
2. H2O2, HO–
(82%) 13

23. 6.12 Stereochemistry of Hydroboration-Oxidation19

24. Features of Hydroboration-Oxidation
hydration of alkenes
regioselectivity opposite to Markovnikov's rule
no rearrangement
stereospecific syn addition 18

25. only product is trans-2-methylcyclopentanol (86%) yield
syn-Addition
H and OH become attached to same face of double bond H
CH3 HO
CH3 H
1. B2H6
2. H2O2, NaOH
only product is trans-2-methylcyclopentanol (86%) yield 20

26. 6.13 Mechanism of Hydroboration-Oxidation19

27. 1-Methylcyclopentene + BH3
syn addition of H and B to double bond
B adds to less substituted carbon 22

28. Organoborane intermediate23

29. OH replaces B on same side
Add hydrogen peroxide
OH replaces B on same side 23

30. trans-2-Methylcyclopentanol24