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6.9 Addition of Sulfuric Acid to Alkenes

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6.9 Addition of Sulfuric Acid to Alkenes. CH 3 CHCH 3. CH 3 CH. CH 2. OSO 2 OH. Addition of H 2 SO 4. HOSO 2 OH. follows Markovnikov's rule: yields an alkyl hydrogen sulfate. Isopropyl hydrogen sulfate. CH 3 CH. CH 2. +. CH 3 CH. CH 3. CH 3 CHCH 3. OSO 2 OH. Mechanism. .

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slide2
CH3CHCH3

CH3CH

CH2

OSO2OH

Addition of H2SO4

HOSO2OH

follows Markovnikov's rule:

yields an alkyl hydrogen sulfate

Isopropylhydrogen sulfate

slide3
CH3CH

CH2

+

CH3CH

CH3

CH3CHCH3

OSO2OH

Mechanism

..

SO2OH

+

O

H

..

slow

..

:

+

SO2OH

O

..

fast

:

..

slide4
CH3CHCH3

SO2OH

O

heat

CH3CHCH3

O

H

Alkyl hydrogen sulfates undergo hydrolysis in hot water

+

H—OH

+

HO—SO2OH

slide5
OH

Application: Coversion of alkenes to alcohols

1. H2SO4

2. H2O, heat

(75%)

slide6
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)

slide8
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

slide9
CH3

H3C

H

50% H2SO4

CH2CH3

CH3

C

C

C

50% H2O

H3C

CH3

OH

Follows Markovnikov's Rule

(90%)

slide10
CH3

50% H2SO4

CH2

50% H2O

OH

Follows Markovnikov's Rule

(80%)

slide11
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

slide12
H

H3C

+

:

H

C

O

CH2

H3C

H

H

:

:

O

Mechanism

Step (1) Protonation of double bond

+

slow

H3C

+

C

+

CH3

H3C

H

slide13
H

H3C

+

:

:

C

+

O

CH3

H3C

H

fast

CH3

H

+

:

C

CH3

O

H

CH3

Mechanism

Step (2) Capture of carbocation by water

slide14
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

+

+

+

slide15
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.

slide16
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

hydration dehydration equilibrium
Hydration-Dehydration Equilibrium

How do we control the position of the equilibrium and maximize the product?

le chatelier s principle
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).

le chatelier s principle20
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

le chatelier s principle21
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

le chatelier s principle22
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.

slide24
Synthesis

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

OH

slide25
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

slide26
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

slide27
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.

slide28
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

slide29
H

BH2

C

C

C

C

Hydroboration step

+ H—BH2

Hydroboration reagents:

Borane-tetrahydrofurancomplex (H3B-THF)

..

+

O

BH3

slide30
H

BH2

H

OH

C

C

C

C

Oxidation step

H2O2, HO–

Organoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.

slide31
1. B2H6, diglyme

2. H2O2, HO–

Example

OH

(93%)

slide32
H

OH

1. H3B-THF

CH3

CH3

C

C

C

C

2. H2O2, HO–

CH3

H

Example

H3C

CH3

H3C

H

(98%)

slide33
Example

OH

1. B2H6, diglyme

2. H2O2, HO–

(no rearrangement)

(82%)

features of hydroboration oxidation
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

hydroboration oxidation
Hydroboration-oxidation

 electrons from alkene flow into 2P orbital of boron

hydroboration oxidation39
Hydroboration-oxidation

H atom with 2 electrons migrates to C with

greatest (+) charge (most highly substituted). Syn add.

hydroboration oxidation40
Hydroboration-oxidation

Anion of hydrogen peroxide attacks boron

hydroboration oxidation41
Hydroboration-oxidation

Carbon with pair of electrons migrates to oxygen,

displacing hydroxide. Oxygen on same side as boron

hydroboration oxidation42
Hydroboration-oxidation

Hydrolysis cleaves boron-oxygen bond resulting

in anti-Markovnikov addition

1 methylcyclopentene bh 3
1-Methylcyclopentene + BH3
  • syn addition of Hand B to double bond
  • B adds to less substituted carbon
add hydrogen peroxide
Add hydrogen peroxide
  • OH replaces B on same side
slide48
C

C

C

C

+ X2

electrophilic addition to double bond

forms a vicinal dihalide

X

X

slide49
Example

Br2

CH3CHCHCH(CH3)2

CH3CH

CHCH(CH3)2

CHCl3

0°C

Br

Br

(100%)

slide50
Scope

Limited to Cl2 and Br2

F2 addition proceeds with explosive violence

I2 addition is endothermic: vicinal diiodides dissociate to an alkene and I2

slide51
Anti-addition

6.16Stereochemistry of Halogen Addition

slide52
H

Br

H

H

Br

H

Example

Br2

trans-1,2-Dibromocyclopentane80% yield; only product

slide53
H

H

Example

H

Cl

Cl2

H

Cl

trans-1,2-Dichlorocyclooctane73% yield; only product

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