6 10 acid catalyzed hydration of alkenes
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6.10 Acid-Catalyzed Hydration of Alkenes. C. C. C. H. C. OH. Acid-Catalyzed Hydration of Alkenes. reaction is acid catalyzed; typical hydration medium is 50% H 2 SO 4 -50% H 2 O. +. H—OH. CH 3. H 3 C. H. 50% H 2 SO 4. CH 2 CH 3. CH 3. C. C. C. 50% H 2 O. H 3 C. CH 3. OH.

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6.10 Acid-Catalyzed Hydration of Alkenes

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6 10 acid catalyzed hydration of alkenes

6.10Acid-Catalyzed Hydration of Alkenes


Acid catalyzed hydration of alkenes

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


Follows markovnikov s rule

CH3

H3C

H

50% H2SO4

CH2CH3

CH3

C

C

C

50% H2O

H3C

CH3

OH

Follows Markovnikov's Rule

(90%)


Follows markovnikov s rule1

CH3

50% H2SO4

CH2

50% H2O

OH

Follows Markovnikov's Rule

(80%)


Mechanism

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


Mechanism1

H

H3C

+

:

H

O

C

CH2

H3C

H

H

:

:

O

Mechanism

Step (1) Protonation of double bond

+

slow

H3C

+

+

C

CH3

H3C

H


Mechanism2

H

H3C

+

:

:

+

O

C

CH3

H3C

H

fast

CH3

H

+

:

C

CH3

O

H

CH3

Mechanism

Step (2) Capture of carbocation by water


Mechanism3

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

+

+

+


Relative rates

Relative Rates

Acid-catalyzed hydration

  • ethyleneCH2=CH21.0

  • propeneCH3CH=CH21.6 x 106

  • 2-methylpropene (CH3)2C=CH22.5 x 1011

  • The more stable the carbocation, the fasterit is formed, and the faster the reaction rate.


Principle of microscopic reversibility

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


6 11 hydroboration oxidation of alkenes

6.11Hydroboration-Oxidation of Alkenes


Synthesis

OH

Synthesis

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


Synthesis1

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.


Synthesis2

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.


Hydroboration step

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.


Hydroboration step1

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


Hydroboration step2

H

BH2

C

C

C

C

••

+

O

BH3

Hydroboration Step

+ H—BH2

Hydroboration reagents:

Borane-tetrahydrofurancomplex (H3B-THF)


Oxidation step

H

BH2

H

OH

C

C

C

C

Oxidation Step

H2O2, HO–

Organoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.


Example

1. B2H6, diglyme

2. H2O2, HO–

OH

Example

(93%)


Example1

H

OH

1. H3B-THF

CH3

CH3

C

C

C

C

2. H2O2, HO–

CH3

H

Example

H3C

CH3

H3C

H

(98%)


Features of hydroboration oxidation

Features of Hydroboration-Oxidation

  • hydration of alkenes

  • regioselectivity opposite to Markovnikov's rule

  • no rearrangement

  • stereospecific syn addition


Example2

OH

Example

1. B2H6, diglyme

2. H2O2, HO–

(82%)


6 12 stereochemistry of hydroboration oxidation

6.12Stereochemistry of Hydroboration-Oxidation


Features of hydroboration oxidation1

Features of Hydroboration-Oxidation

  • hydration of alkenes

  • regioselectivity opposite to Markovnikov's rule

  • no rearrangement

  • stereospecific syn addition


Syn addition

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


6 13 mechanism of hydroboration oxidation

6.13Mechanism of Hydroboration-Oxidation


1 methylcyclopentene bh 3

1-Methylcyclopentene + BH3

  • syn addition of Hand B to double bond

  • B adds to less substituted carbon


Organoborane intermediate

Organoborane intermediate


Add hydrogen peroxide

Add hydrogen peroxide

  • OH replaces B on same side


Trans 2 methylcyclopentanol

trans-2-Methylcyclopentanol


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