6.10
Acid-Catalyzed Hydration of Alkenes
Acid-Catalyzed Hydration of Alkenes
C
C
+
H—OH
H
C
C
OH
reaction is acid catalyzed; typical hydration
medium is 50% H2SO4-50% H2O
Follows Markovnikov's Rule
H3C
C
H3C
CH3
H
50% H2SO4
C
CH3
50% H2O
CH3
C
CH2CH3
OH
(90%)
Follows Markovnikov's Rule
CH2
50% H2SO4
CH3
50% H2O
OH
(80%)
Mechanism
involves a carbocation intermediate
is the reverse of acid-catalyzed dehydration
of alcohols to alkenes
H3C
H+
C
H3C
CH3
CH2 + H2O
CH3
C
OH
CH3
Mechanism
Step (1) Protonation of double bond
H
H3C
C
CH2
+
+
O:
H
H3C
H
slow
H3C
H3C
H
+
C
CH3 +
:O:
H
Mechanism
Step (2) Capture of carbocation by water
H3C
H
+
C
:O:
CH3 +
H3C
fast
CH3
CH3
C
CH3
H
+
O:
H
H
Mechanism
Step (3) Deprotonation of oxonium ion
CH3
CH3
C
H
:
+O
CH3
H
+
:O:
H
H
fast
H
CH3
CH3
C
CH3
..
O:
H
+
H
+
O:
H
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.
Principle of Microscopic Reversibility
H3C
H+
C
H3C
CH3
CH2 + H2O
CH3
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.
6.11
Hydroboration-Oxidation of Alkenes
Synthesis
Suppose you wanted to prepare 1-decanol
from 1-decene?
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.
OH
Synthesis
Two-step reaction sequence called hydroborationoxidation converts alkenes to alcohols with a
regiochemistry opposite to Markovnikov's rule.
1. hydroboration
2. oxidation
OH
Hydroboration Step
C
C
+
H—BH2
H
C
C
BH2
Hydroboration can be viewed as the addition of
borane (BH3) to the double bond. But BH3 is
not the reagent actually used.
Hydroboration Step
C
C
+
H—BH2
H
C
C
BH2
Hydroboration reagents:
H
H2B
BH2
H
Diborane (B2H6)
normally used in an
ether- like solvent
called "diglyme"
Hydroboration Step
C
+
C
H
H—BH2
C
C
Hydroboration reagents:
••
+O
–
BH3
Borane-tetrahydrofuran
complex (H3B-THF)
BH2
Oxidation Step
H2O2, HO–
H
C
C
BH2
H
C
C
OH
Organoborane formed in the hydroboration
step is oxidized with hydrogen peroxide.
Example
1. B2H6, diglyme
2. H2O2, HO–
OH
(93%)
Example
H3C
CH3
C
H3C
C
H
1. H3B-THF
2. H2O2, HO–
CH3
H
OH
C
C
CH3 H
(98%)
CH3
Features of Hydroboration-Oxidation
hydration of alkenes
regioselectivity opposite to Markovnikov's rule
no rearrangement
stereospecific syn addition
Example
1. B2H6, diglyme
OH
2. H2O2, HO–
(82%)
6.12
Stereochemistry of Hydroboration-Oxidation
Features of Hydroboration-Oxidation
hydration of alkenes
regioselectivity opposite to Markovnikov's rule
no rearrangement
stereospecific syn addition
syn-Addition
H and OH become attached to same
face of double bond
H
CH3
1. B2H6
2. H2O2, NaOH
H
CH3
HO
H
only product is trans-2-methylcyclopentanol
(86%) yield
6.13
Mechanism of Hydroboration-Oxidation
1-Methylcyclopentene + BH3
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