Acid-Catalyzed Hydration of Alkenes
C
6.10
Acid-Catalyzed Hydration of Alkenes
C
+
H—OH
C
H
C
OH
reaction is acid catalyzed; typical hydration
medium is 50% H2SO4-50% H2O
Follows Markovnikov's Rule
H 3C
H
C
H 3C
C
CH3
50% H2SO4
50% H2O
Follows Markovnikov's Rule
CH3
CH3
C
CH2CH3
CH2
50% H2SO4
CH3
50% H2O
OH
OH
(80%)
(90%)
Mechanism
Mechanism
Step (1) Protonation of double bond
involves a carbocation intermediate
is the reverse of acid-catalyzed dehydration
of alcohols to alkenes
H 3C
C
H 3C
CH2 + H2O
H+
C
C
CH2
+
H
H 3C
CH3
CH3
H
+
O:
H 3C
H
slow
CH3
H 3C
OH
H 3C
H
+
C
CH3 +
:O :
H
Mechanism
Mechanism
Step (2) Capture of carbocation by water
H 3C
Step (3) Deprotonation of oxonium ion
H
+
C
:O :
CH3 +
H 3C
CH3
fast
CH3
H
C
H
+O :
CH3
H
+
:O :
H
H
fast
CH3
CH3
C
CH3
H
CH3
+
O:
CH3
H
CH3
Relative Rates
propene
2-methylpropene
CH2=CH2
CH3CH=CH 2
(CH3)2C=CH2
+
H
H
+
O:
H
H
Principle of Microscopic Reversibility
Acid-catalyzed hydration
ethylene
C
..
O:
H 3C
1.0
C
1.6 x
106
2.5 x
1011
The more stable the carbocation, the faster
it is formed, and the faster the reaction rate.
CH2 + H2O
CH3
H+
CH3
H 3C
C
CH3
OH
In an equilibrium process, the same intermediates
and transition states are encountered in the
forward direction and the reverse, but in the
opposite order.
Hydration-Dehydration
Equilibrium
CCHHHH+ H2OH+CCOHH
6.11
Thermodynamics of Addition-Elimination
Equlibria
How do we control the position of the
equilibrium and maximize the product?
Le Chatelier’
Chatelier’s Principle
Le Chatelier’
Chatelier’s Principle
A system at equilibrium adjusts so to
minimize any stress applied to it.
At constant temperature and pressure a
reaction proceeds in a direction which is
spontaneous or decreases free energy (G).
For the hydration-dehydration equilibria, the
key stress is water.
The sign of G is always positive, but ΔG can
be positive or negative.
Adding water pushes the equilibrium toward
more product (alcohol).
ΔG = Gproducts – Greactants
Spontaneous when ΔG < 0
Removing water pushes the equilibrium
toward more reactant (alkene).
Le Chatelier’
Chatelier’s Principle
Le Chatelier’
Chatelier’s Principle
At equilibrium ΔG = 0 and the following
becomes true:
For a reversible reaction:
aA + bB ↔ cC + dD
The relationship between ΔG and ΔGo is:
Substituting Keq into the previous equation gives:
ΔGo = - RT lnKeq
Reactions for ΔGo positive are endergonic and for
R = 8.314
J/(mol.K)
and T is the temperature in K
[
C
ΔGo negative are exergonic.
exergonic.
K
=
e
[
q
o
?
G
=
?
G
+
R
Synthesis
T
l
n
[
A
Suppose you wanted to prepare 1-decanol
from 1-decene?
6.12
Hydroboration-Oxidation of Alkenes
[
OH
Needed: a method for hydration of
alkenes with a regioselectivity opposite to
Markovnikov's rule.
Synthesis
Hydroboration Step
Two-step reaction sequence called hydroborationhydroborationoxidation converts alkenes to alcohols with a
regiochemistry opposite to Markovnikov's rule.
C
C
+
H—
H—BH2
H
C
C
BH2
1. hydroboration
2. oxidation
Hydroboration can be viewed as the addition of
borane (BH3) to the double bond. But BH3 is
not the reagent actually used.
OH
Hydroboration Step
C
C
+
H—
H—BH2
H
Hydroboration Step
C
C
BH2
C
C
+
H—
H—BH2
H
C
C
Hydroboration reagents:
Hydroboration reagents:
H
H 2B
Diborane (B2H6)
normally used in an
ether-like solvent
called "diglyme"
BH2
H
Oxidation Step
Borane-tetrahydrofuran
complex (H3B-THF)
••
+O
– BH
3
Example
H2O2, HO–
H
C
C
BH2
H
C
C
OH
1. B2H6, diglyme
2. H2O2, HO–
Organoborane formed in the hydroboration
step is oxidized with hydrogen peroxide.
OH
(93%)
BH2
Example
H 3C
CH3
C
H 3C
C
H
Features of Hydroboration-Oxidation
1. H3B-THF
2. H2O2, HO–
CH3
H
OH
C
C
CH3 H
hydration of alkenes
CH3
regioselectivity opposite to Markovnikov's rule
no rearrangement
stereospecific syn addition
(98%)
Example
OH
1. B2H6, diglyme
2. H2O2, HO–
6.13
Stereochemistry of Hydroboration-Oxidation
(82%)
syn Addition
Features of Hydroboration-Oxidation
H and OH become attached to same
face of double bond
hydration of alkenes
H
regioselectivity opposite to Markovnikov's rule
CH3
no rearrangement
stereospecific syn addition
1. B2H6
2. H2O2, NaOH
H
CH3
HO
H
only product is trans-2-methylcyclopentanol
trans-2-methylcyclopentanol
(86%) yield
1-Methylcyclopentene + BH3
6.14
Mechanism of Hydroboration-Oxidation
syn addition of H and
B to double bond
B adds to less
substituted carbon
Organoborane Intermediate
Add Hydrogen Peroxide
OH replaces B on
same side
trans-2-Methylcyclopentanol
trans-2-Methylcyclopentanol
6.15
Addition of Halogens
to Alkenes
General features
C
C
+ X2
X
Example
C
C
X
CH3CH
CHCH(CH
CHCH(CH 3)2
Br2
CHCl 3
0° C
CH3CHCHCH(CH
CHCHCH(CH3)2
Br Br
(100%)
electrophilic addition to double bond
forms a vicinal dihalide
Scope
limited to Cl2 and Br2
6.16
Stereochemistry of Halogen Addition
F2 addition proceeds with explosive violence
I2 addition is endothermic: vicinal diiodides
dissociate to an alkene and I 2
anti addition
Example
Example
H
H
H
H
Br2
Br
Br
H
Cl
Cl2
H
H
trans-1,2Dibromocyclopentane
trans-1,2-Dibromocyclopentane
80% yield; only product
H
Cl
trans-1,2Dichlorocyclooctane
trans-1,2-Dichlorocyclooctane
73% yield; only product
Mechanism is electrophilic addition
Br2 is not polar, but it is polarizable
6.17
Mechanism of Halogen Addition to
Alkenes: Halonium Ions
two steps
(1) formation of bromonium ion
(2) nucleophilic attack on bromonium
ion by bromide
Mechanism?
Relative Rates of Bromination
H 2C
ethylene
H 2C=CH 2
propene
CH 3CH=CH 2
2-methylpropene
2-methylpropene
(CH 3)2C=CH 2
2,3-dimethyl
-2-butene
butene
2,3-dimethyl-2-
(CH 3)2C=C(CH 3)2
61
Br2
BrCH
BrCH2CH2Br
?
5400
920,000
More highly substituted double bonds react faster.
Alkyl groups on the double bond make it
more “electron rich.”
rich.”
Mechanism
H 2C
CH2 +
1
C
.. –
+ : Br :
..
+
C
: Br :
..
No obvious explanation for anti addition
provided by this mechanism.
Formation of Bromonium Ion
CH2 +
Br2
BrCH
BrCH2CH2Br
Br
Mutual polarization
of electron distributions
of Br2 and alkene
C
C
.. –
+ : Br :
..
: Br :
+
Cyclic bromonium ion
Br
Formation of Bromonium Ion
Formation of Bromonium Ion
–
Br
Electrons flow
from alkene
toward Br2
Br
π electrons of
alkene displace
Br– from Br
Br
δ+
.. –
: Br :
..
δ–
+
Br
δ+
Example
Stereochemistry
..
Br +
..
H
..
: Br :
..
attack of Br – from side opposite
C—Br bond of bromonium ion gives
anti addition
H
Br2
Br
Br
H
H
trans-1,2Dibromocyclopentane
trans-1,2-Dibromocyclopentane
80% yield; only product
..
Br :
..
Cyclopentene +Br2
Bromonium ion
–
–
–
Bromide ion attacks
the bromonium ion
from side opposite
carbon-bromine bond
trans-Stereochemistry in
vicinal dibromide
C
6.18
Conversion of Alkenes to Vicinal
Halohydrins
C
+ X2
X
C
C
X
alkenes react with X2 to form vicinal dihalides
Examples
C
C
+ X2
X
C
C
X
H 2C
CH2 +
Br2
H 2O
BrCH
BrCH2CH2OH
(70%)
alkenes react with X2 to form vicinal dihalides
alkenes react with X2 in water to give vicinal
halohydrins
H
H
Cl2
OH
H 2O
C
C
+ X2 + H2O
X
C
C
OH
+ H—X
H
Cl
H
anti addition: only product
Examples
Mechanism
O:
..
H 2C
CH2 +
Br2
H 2O
BrCH
BrCH2CH2OH
(70%)
..
Br +
..
O
..
bromonium ion is
intermediate
water is nucleophile that
attacks bromonium ion
+
H
H
Cl2
OH
H 2O
H
..
Br :
..
H
anti addition: only product
Cyclopentene + Cl2
Chloronium ion
trans-Stereochemistry
in oxonium ion
Water attacks
chloronium ion
from side
opposite
carbon-chlorine
bond
Cl
Regioselectivity
H 3C
C
H 3C
CH2
Br2
H 2O
CH3
CH3
C
CH2Br
OH
(77%)
Markovnikov's rule applied to halohydrin
formation: the halogen adds to the carbon
having the greater number of hydrogens.
hydrogens.
trans-2-Chlorocyclopentanol
trans-2-Chlorocyclopentanol
Explanation
H
H
H
..
O δ+
H 3C
H 3C
..
δ+ O
δ+
C
CH2
: Br :
δ+
H 3C
H 3C
δ+
CH2
C
: Br :
δ+
transition state for attack of water on bromonium ion has carbocation
character; more stable transition state (left) has positive charge on
more highly substituted carbon
H