Download Acid-Catalyzed Hydration of Alkenes

Acid-Catalyzed Hydration of Alkenes
6.10
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
Follows Markovnikov's Rule
H 3C
H
C
H 3C
C
CH3
50% H2SO4
50% H2O
CH3
CH3
C
CH2CH3
OH
(90%)
Mechanism
Follows Markovnikov's Rule
involves a carbocation intermediate
CH2
50% H2SO4
CH3
50% H2O
OH
is the reverse of acid-catalyzed dehydration
of alcohols to alkenes
H 3C
(80%)
C
H 3C
CH2 + H2O
H+
CH3
CH3
C
OH
CH3
Addition of Water to Alkene
Addition of Water to Alkenes
(alcohols)
(alcohols)
Question
Principle of Microscopic Reversibility
Which alkene will undergo acid-catalyzed
hydrolysis at the fastest rate?
A)
C)
H 3C
C
CH2 + H2O
CH3
H+
CH3
C
CH3
B)
H 3C
D)
In an equilibrium process, the same intermediates
and transition states are encountered in the
forward direction and the reverse, but in the
opposite order.
H2O addition: Markovnikov's Rule
OH
Question
The product isolated from the acid-catalyzed
hydration of (Z)
-3-methyl-2-pentene is:
(Z)-3-methyl-2-pentene
A) 2-ethyl-2-butanol
B) 2-ethyl-1-butanol
C) 3-methyl-2-pentanol
D) 3-methyl-3-pentanol
Addition of Alcohol to Alkenes
(ethers)
Question
The product isolated from the acid-catalyzed
hydration of (E)
-3-methyl-2-pentene is:
(E)-3-methyl-2-pentene
A) chiral
B) achiral
Stereochemistry of Addition to Alkenes
C
C
+
E—Y
E
C
C
Stereochemistry of Addition to Alkenes
Y
Regiochemistry
Regiochemistry withstanding, in order to understand the
stereochemistry of the product, you must consider two things:
(1) Stereochemistry of the starting alkene (cis or trans;
Z or E)
(2) Stereochemistry of the addition (syn or anti)
C
C
+
E—Y
E
C
Y
Optically inactive reactants produce optically inactive products.
(Racemic mixtures are optically inactive)
The correlary is that an optically active starting material
MAY produce an
optically active product depending on the mechanism.
Question
The product isolated from the acid-catalyzed
hydration of (E)(E)- or (Z)-3-methyl-2-pentene
(Z)-3-methyl-2-pentene is:
A) optically active
B) an optically inactive racemic mixture
C) an optically inactive enantiomer
C
6.12
Hydroboration-Oxidation of Alkenes
Synthesis
Synthesis
Suppose you wanted to prepare 1-decanol
from 1-decene?
Two-step reaction sequence called hydroborationhydroborationoxidation converts alkenes to alcohols with a
regiochemistry opposite to Markovnikov's rule.
1. hydroboration
2. oxidation
OH
OH
Needed: a method for hydration of
alkenes with a regioselectivity opposite to
Markovnikov's rule.
Hydroboration Step
C
C
+
H—
H—BH2
H
Hydroboration Step
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.
C
C
+
H—
H—BH2
H
C
C
BH2
Hydroboration reagents:
H
H 2B
Diborane (B2H6)
normally used in an
ether-like solvent
called "diglyme"
BH2
H
Hydroboration Step
Oxidation Step
H2O2, HO–
C
C
+
H—
H—BH2
H
C
C
BH2
H
C
C
BH2
H
C
C
Hydroboration reagents:
BoraneBorane-tetrahydrofuran
complex (H3B-THF)
••
+O
– BH
3
Organoborane formed in the hydroboration
step is oxidized with hydrogen peroxide.
OH
Example
Example
1. B2H6, diglyme
2. H2O2, HO–
H 3C
CH3
C
H 3C
C
H
1. H3B-THF
2. H2O2, HO–
CH3
H
OH
C
C
CH3 H
(98%)
OH
(93%)
Features of Hydroboration-Oxidation
Example
hydration of alkenes
regioselectivity opposite to Markovnikov's rule
1. B2H6, diglyme
no rearrangement
2. H2O2, HO–
stereospecific syn addition
OH
(82%)
Features of Hydroboration-Oxidation
hydration of alkenes
6.13
Stereochemistry of Hydroboration-Oxidation
regioselectivity opposite to Markovnikov's rule
no rearrangement
stereospecific syn addition
CH3
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
trans-2-methylcyclopentanol
(86%) yield
Question
Hydroboration-oxidation
Hydroboration-oxidation of which one of the
following yields a primary alcohol as the major
product?
A)
B)
C)
D)
Question
H 3C
CH3
C
H 3C
C
H
1. H3B-THF
2. H2O2, HO–
CH3
H
OH
C
C
CH3 H
A) The product is achiral
B) The product is optically active
C) The product is a racemic mixture
D) The product is a single enantiomer
CH3
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
CH3CHCHCH(CH
CHCHCH(CH3)2
0° C
Br Br
(100%)
electrophilic addition to double bond
forms a vicinal dihalide
Scope
Halogenation
limited to Cl2 and Br2
F2 addition proceeds with explosive violence
I2 addition is endothermic: vicinal diiodides
dissociate to an alkene and I 2
Example
6.16
Stereochemistry of Halogen Addition
H
H
H
Br2
Br
Br
H
anti addition
trans-1,2Dibromocyclopentane
trans-1,2-Dibromocyclopentane
80% yield; only product
Example
H
H
Cl
Cl2
H
6.17
Mechanism of Halogen Addition to
Alkenes: Halonium Ions
H
Cl
trans-1,2Dichlorocyclooctane
trans-1,2-Dichlorocyclooctane
73% yield; only product
Addition of Halogens to Alkenes
Mechanism is electrophilic addition
Br2 is not polar, but it is polarizable
two steps
(1) formation of bromonium ion
(2) nucleophilic attack on bromonium
ion by bromide
Relative Rates of Bromination
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
More highly substituted double bonds react faster.
Alkyl groups on the double bond make it
more “electron rich.”
rich.”
Question
1
61
5400
920,000
Arrange the alkenes in order of decreasing rate
of reaction toward bromine addition:
2-methyl-1-butene, 2-methyl-2-butene, and 3methyl-1-butene
A) 2-methyl-1-butene > 3-methyl-1-butene >
2-methyl-2-butene
B) 3-methyl-1-butene > 2-methyl-1-butene >
2-methyl-2-butene
C) 2-methyl-2-butene > 2-methyl-1-butene >
3-methyl-1-butene
D) 2-methyl-2-butene > 3-methyl-1-butene >
2-methyl-1-butene
Example
H
CH3
CH3
H
Diastereomers
C C
Br2
H
In general, diastereomers fall into two categories:
geometric isomers
H
H
H
CH3
C C
C C
cis-trans
H
Br
CH3
Br
H
H
CH3
H Br
Cl
(R) H
CH3
(S)
Diastereomers
CO2H CO2H CO2H CO2H
H BrH BrH
BrH
Br
Br H Br HH
BrH
Br
CO2H CO2H Co2H CO2H
CH3
Br C
H C
CH3
H C
H C
CH3
MP = 256 oC
CH3
H
Br
H
H
Br
C C
CH3 CH3(R)
Br H
H
Cl (R)
CH3
CH3
A compound with
carbon atoms
CH3 “n” asymmetric
CH3
CH3
can have Br
a maximum
of 2Hn C
stereoisomers.
.
C H
stereoisomers
Br
Br
H
Br
H
H
Br
CH3
CH3
2R,3R
,3R
2S,3S
,3S
2R,3S
,3S
chiral
chiral
achiral
Br
Br
The addition of bromine to cis-2-butene
cis-2-butene
produces:
CH3
CH3
CH3
Br C H
H C Br
CH3
3
CH3
CH3 (R)
CH3
CH33
(R) CH
(S)
H C Br
H CH Br
H
C
Br
H
Br
C
C BrBr C H
H C Br
Br C H
H H C Br
H C Br
C H
Br C
Br
(S)
(R)
(S)
CH3
(R)
CH3
CH3
CH3
CH33
CH
CH3
CH
Enantiomers
H 3H
C Br
CH3 compound
Same
Br C
Br C H(meso)
H H
C Br
Br C
diastereomers
Br C H
CH3
Only
for 2,3-dibromobutane.
CH33 stereoisomers
CH3
(S) CH3
H
Br
H C Br
Br C H
CH3
CH
2,3-dibromobutane
Question
Three stereoisomers of 2,3-dibromobutane
2,3-dibromobutane
Br
CH
CH
3 3
Br
H
BrH
diastereomers
HH ClCl
CH
CH
3 3
Diastereomers
Diastereomers have different physical
properties:
BP, MP, density, refractive index, solubility
Can be separated through conventional
means (distillation, recrystallization,
recrystallization,
chromatography)
CH3
H
CH3
stereoisomers containing two or more
H
asymmetric atoms; (that are not enantiomers)
) H
enantiomers
trans-1,2dibromocyclopentane
trans-1,2-dibromocyclopentane
80% yield; 2 asymmetric carbon atoms,
Chiral product,Optically inactive,
Racemic mixture, only products formed
MP = 158 oC
CH3
A) A single enantiomer
B) A product with one asymmetric carbon atom
C) An optically inactive meso product
D) A racemic mixture
E) 4 different stereoisomers each with 2 chiral
carbon atoms
σ
CH3
H C Br
Br C H
CH3
CH3
Br C
Br C
CH3
H
H
Question
Diastereomers
The addition of bromine to trans-2-butene
trans-2-butene
produces:
When naming compounds containing multiple
chiral atoms, youCH
must
give the configuration
3
around each chiral
H atom:
Cl
Br and
H configuration of each
position number
Cl
chiral atom, H
separated
by commas, all in ( )
CHcompound
3
at the start of the
name
(S) CH3
A) A single enantiomer
B) A product with one asymmetric carbon atom
C) An optically inactive meso product
D) A racemic mixture
E) 4 different stereoisomers each with 2 chiral
carbon atoms
H C Br
Cl C H
(S)
CH3
Diastereomers
Diastereomers
CH
CH33
H
H Cl
Cl
Which ones are chiral?
chiral? Name each one. Give
H Cl
Cl
the stereochemical relationship between H
them.
CH
CH22CH
CH33
CH
CH
3 3
H H ClCl
H H ClCl
CH
CH
2CH
2CH
3 3
CH
CH
3 3
H H ClCl
ClCl H H
CH
CH
2CH
2CH
3 3
CH
CH
3 3
ClCl H H
H H ClCl
CH
CH
2CH
2CH
3 3
CH
CH
3 3
ClCl H H
ClCl H H
CH
CH
2CH
2CH
3 3
H
H
Cl
Cl
CH
CH33
Cl
Cl
H
H
CH
CH22CH
CH33
Cl
Cl
H
H
CH
CH33
H
H
Cl
Cl
CH
CH22CH
CH33
H
Cl
CH33
Cl
H
CH22CH33
CH3
CH
CH33
CH3
CH
Cl
HCl
Cl
CH
3
H
Cl3 CHH
CH3
3
H
Cl
Cl
HCl Cl
Cl H
H
ClH Cl
H
Cl H
CH2CH3 H ClCH
CH2CH
Cl H
33 Cl HHCHCl
2CH
2CH3
CH2CH3 (2R, 3S)-2,3-dichloropentane
(2S, 3R)-2,3-dichloropentane
CH2CH32CH3
CH2CH3
CH
CH33
Cl
Cl H
H
Cl
Cl H
H
CH
CH22CH
CH33
H
Cl
Diastereomers
CH33
H Cl
Cl H
CH22CH33
C
CH33
Cl H
Cl H
CH22CH33
B
enantiomers
enantiomers
diastereomers
diastereomers
diastereomers
diastereomers
H
H
CH3
CH33
CH
CH3
CH
CH
Cl
H3 HCl
Cl
Cl3 CH
H3
H HClCl Cl
H
H Cl HH ClCl
H ClCH2CH
CH2CH
CH2CH3 Cl HCH
H3
2CH
33
CH2CH3
CH2CH
CH
3 2CH3
(2S, 3S)-2,3-dichloropentane
All of them are chiral.
H
H
CH33
H Cl
H Cl
CH22CH33
A
CHand
3
A
B:
3
Cl H
C and D:
H A and
Cl C:
CH22CH33
A and
D:
B and C:
B and D:
(2S, 3S)-2-bromo-3-chlorobutane
Cl
H
CH33
Cl
Cl
CH22CH33
CH33
H
Cl
CH22CH33
D
Cl
Cl
Cl
H
CH3
H
Cl
CH2CH3
(2R, 3R)-2,3-dichloropentane
CH33
H
H
CH22CH33
6.18
Conversion of Alkenes to Vicinal
Halohydrins
Addition of Halogens
in the Presence of Water
(halohydrins)
C
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
Examples
H 2C
CH2 +
Br2
H 2O
BrCH
BrCH2CH2OH
(70%)
H
H
Cl2
OH
H 2O
H
Cl
H
anti addition: only product
H
Cl
H
anti addition: only product
Perspective formula
Fischer projection
Explanation
Regioselectivity
H 3C
C
H 3C
CH2
Br2
H 2O
CH3
CH3
C
H
H
H
CH2Br
OH
(77%)
Markovnikov's rule applied to halohydrin
formation: the halogen adds to the carbon
having the greater number of hydrogens.
hydrogens.
..
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