Transcript Alkene-Addn-PartB-2012-ques

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
Hydration – Thermodynamics
Hydrohalogenation & hydration reactions are at
equilibrium.
Hydration – Thermodynamics
How could Le Châtelier’s principle be used to shift the
equilibrium to the right or left?
SEE: CONCEPTUAL CHECKPOINT 9.11.
Hydration – Thermodynamics
Similar to hydrohalogenation, the stereochemistry of
hydration reactions is controlled by the geometry of
the carbocation.
Draw the complete mechanism for the reaction above
to show WHY a racemic mixture is formed.
SEE: SKILLBUILDER 9.3.
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
Addition of Water to Alkene
(alcohols)
Addition of Water to Alkenes
(alcohols)
Question
Which alkene will undergo acid-catalyzed
hydrolysis at the fastest rate?
A)
B)
C)
D)
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.
H2O addition: Markovnikov's Rule
Question
The product isolated from the acid-catalyzed
hydration of (Z)-3-methyl-2-pentene is:
A) 2-ethyl-2-butanol
B) 2-ethyl-1-butanol
C) 3-methyl-2-pentanol
D) 3-methyl-3-pentanol
Question
The product isolated from the acid-catalyzed
hydration of (E)-3-methyl-2-pentene is:
A) chiral
B) achiral
Oxymercuration-Demercuration
Because rearrangements often produce a mixture of
products, the synthetic utility of Markovnikov
hydration reactions is somewhat limited.
OXYMERCURATION-DEMERCURATION is an
alternative process that can yield Markovnikov
products avoiding rearrangements
Oxymercuration-Demercuration
OXYMERCURATION begins with mercuric acetate.
How would you classify the mercuric cation?
As a nucleophile or an electrophile?
As a Lewis acid or Lewis base?
Oxymercuration-Demercuration
Similar to how we saw the alkene attack a proton
previously, it can also attack the mercuric cation.
Resonance stabilizes the mercurinium ion. Can you
draw a reasonable resonance hybrid?
Oxymercuration-Demercuration
The mercurinium ion is also a good electrophile, and
it can easily be attacked by a nucleophile, even a
weak nucleophile such as water.
NaBH4 is generally used to replace the –HgOAc
group with a –H group via a free radical
mechanism.
Addition of Alcohol to Alkenes
(ethers)
Stereochemistry of Addition to Alkenes
C
C
+
E—Y
E
C
C
Y
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)
Stereochemistry of Addition to Alkenes
C
C
+
E—Y
E
C
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)- or (Z)-3-methyl-2-pentene is:
A) optically active
B) an optically inactive racemic mixture
C) an optically inactive enantiomer
Hydroboration-Oxidation of Alkenes
Hydroboration-Oxidation
To achieve anti-Markovnikov hydration,
hydroboration-oxidation is often used.
Note that the process occurs in two steps.
Synthesis
Suppose you wanted to prepare 1-decanol
from 1-decene?
OH
Needed: a method for hydration of
alkenes with a regioselectivity opposite to
Markovnikov's rule.
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:
Borane-tetrahydrofuran
complex (H3B-THF)
••
+O
– BH
3
BH2
Oxidation Step
H2O2, HO–
H
C
C
BH2
H
C
C
Organoborane formed in the hydroboration
step is oxidized with hydrogen peroxide.
OH
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%)
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
Question
Hydroboration-oxidation of which one of the
following yields a primary alcohol as the major
product?
A)
B)
C)
D)
Question
H3C
CH3
C
H3C
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
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
C
C
+ X2
X
C
X
C
alkenes react with X2 to form vicinal dihalides
alkenes react with X2 in water to give vicinal
halohydrins
C
C
+ X2 + H2O
X
C
C
OH
+ H—X
Examples
H2O
H2C
CH2 +
Br2
BrCH2CH2OH
(70%)
H
H
Cl2
OH
H2O
H
Cl
H
anti addition: only product
Examples
H2O
H2C
CH2 +
Br2
BrCH2CH2OH
(70%)
H
H
Cl2
OH
H2O
H
Cl
H
anti addition: only product
Perspective formula
Fischer projection
Regioselectivity
CH3
H3C
C
H3C
CH2
Br2
H2O
CH3
C
CH2Br
OH
(77%)
Markovnikov's rule applied to halohydrin
formation: the halogen adds to the carbon
having the greater number of hydrogens.
Explanation
H
H
H
..
 O
..
O 
H3C
H3C

C
CH2
: Br :

H3C
H3C

CH2
C
: Br :

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
Question
What is the product (in addition to its enantiomer) of the following reaction?
B r2, H 2O
A.
Br
C.
OH
B.
OH
Br
Br
OH
D.
OH
Br
Conversion of Alkenes to Vicinal
Diols
Syn Dihydroxylation
SYN dihydroxylation adds across the C=C double
bond in ONE step.
Syn Dihydroxylation
MnO41- is similar to OsO4 but more reactive.
SYN dihydroxylation occurs with KMnO4 only under mild
conditions (cold temperatures).
Diols are commonly further oxidized by MnO41-, and in addition
MnO41- is reactive toward many other functional groups as
well. It is very useful in qualitative analysis due to a
pronounced color change.
SEE: CONCEPTUAL CHECKPOINT 9.33.