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Acid-Catalyzed Hydration of Alkenes. C. C. C. H. C. O H. Acid-Catalyzed Hydration of Alkenes. reaction is acid catalyzed; typical hydration medium is 50% H 2 SO 4 -50% H 2 O. +. H— O H. Follows Markovnikov's Rule. Hydration – Thermodynamics. - PowerPoint PPT Presentation
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Acid-Catalyzed Hydration of Alkenes
H—OHC C + OHC CH
Acid-Catalyzed Hydration of Alkenes
reaction is acid catalyzed; typical hydration medium is 50% H2SO4-50% H2O
Follows Markovnikov's Rule
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.
Hydration – Thermodynamics
(90%)
50% H2SO4
50% H2O
H3C
H3C CH3
H
C C
OH
C CH2CH3CH3
CH3
Markovnikov's Rule
50% H2SO4
50% H2O
Follows Markovnikov's Rule
(80%)
OH
CH3CH2
+ H2OH+
involves a carbocation intermediate
is the reverse of acid-catalyzed dehydrationof alcohols to alkenes
H3C
H3C
C CH2
OH
C CH3CH3
CH3
Mechanism
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)
In an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in the opposite order.
+ H2OH+
H3C
H3C
C CH2
OH
C CH3CH3
CH3
Principle of Microscopic Reversibility
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-butanolB) 2-ethyl-1-butanolC) 3-methyl-2-pentanolD) 3-methyl-3-pentanol
Question
The product isolated from the acid-catalyzed hydration of (E)-3-methyl-2-pentene is:A) chiralB) achiral
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.
Oxymercuration-Demercuration
Addition of Alcohol to Alkenes(ethers)
Regiochemistry withstanding, in order to understand thestereochemistry 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 C CE 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.
Stereochemistry of Addition to Alkenes
C C + E—Y C CE Y
Question
The product isolated from the acid-catalyzed hydration of (E)- or (Z)-3-methyl-2-pentene is:A) optically activeB) an optically inactive racemic mixture
C) an optically inactive enantiomer
Hydroboration-Oxidation of Alkenes
To achieve anti-Markovnikov hydration, hydroboration-oxidation is often used.
Note that the process occurs in two steps.
Hydroboration-Oxidation
Needed: a method for hydration of alkenes with a regioselectivity opposite to Markovnikov's rule.
Suppose you wanted to prepare 1-decanol from 1-decene?
Synthesis
OH
Two-step reaction sequence called hydroboration-oxidation converts alkenes to alcohols with a regiochemistry opposite to Markovnikov's rule.
1. hydroboration
2. oxidation
Synthesis
OH
+ H—BH2C C H BH2C C
Hydroboration can be viewed as the addition ofborane (BH3) to the double bond. But BH3 is not the reagent actually used.
Hydroboration Step
+ H—BH2C C H BH2C C
Hydroboration reagents:
H2B
H
H
BH2
Diborane (B2H6)normally used in an ether-like solventcalled "diglyme"
Hydroboration Step
+ H—BH2C C H BH2C C
Hydroboration reagents:
Borane-tetrahydrofurancomplex (H3B-THF)
Hydroboration Step
+O
BH3–
••
H2O2, HO–
H BH2C C H OHC C
Organoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.
Oxidation Step
1. B2H6, diglyme
2. H2O2, HO–
(93%)
Example
OH
(98%)
H3C
H3C
CH3
H
C C1. H3B-THF
2. H2O2, HO–
H
C CCH3
CH3
CH3
H OH
Example
hydration of alkenes
regioselectivity opposite to Markovnikov's rule
no rearrangement
stereospecific syn addition
Features of Hydroboration-Oxidation
1. B2H6, diglyme
2. H2O2, HO–
(82%)
Example
OH
Stereochemistry of Hydroboration-Oxidation
hydration of alkenes
regioselectivity opposite to Markovnikov's rule
no rearrangement
stereospecific syn addition
Features of Hydroboration-Oxidation
H and OH become attached to same face of double bond
1. B2H6
2. H2O2, NaOH
only product is trans-2-methylcyclopentanol(86%) yield
syn Addition
CH3
H H
CH3
H
HO
Question
Hydroboration-oxidation of which one of the following yields a primary alcohol as the major product?A) B)
C) D)
H3C
H3C
CH3
H
C C1. H3B-THF
2. H2O2, HO–
H
C CCH3
CH3
CH3
H OH
Question
A) The product is achiralB) The product is optically activeC) The product is a racemic mixtureD) The product is a single enantiomer
Conversion of Alkenes to Vicinal Halohydrins
Addition of Halogens in the Presence of Water
(halohydrins)
+ X2 X XC CC C
alkenes react with X2 to form vicinal dihalides
+ X2 X XC CC C
+ H2O OH
+ H—X
+ X2 X C CC C
alkenes react with X2 to form vicinal dihalides
alkenes react with X2 in water to give vicinal halohydrins
Cl2
anti addition: only product
H2O
H2C CH2 BrCH2CH2OH+ Br2
H2O
(70%)
Examples
H
H
OH
Cl
H
H
Cl2
anti addition: only product
H2O
H2C CH2 BrCH2CH2OH+ Br2
H2O
(70%)
Examples
H
H
OH
Cl
H
H
Perspective formula
Fischer projection
(77%)
H3C
C CH2
H3C
CH3
OH
C CH2BrCH3
Markovnikov's rule applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens.
Br2
H2O
Regioselectivity
CH2
H3C
CH3C
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
H3C CH2
H3C
Br: :
H
OH ..
Br: :
H
O H..
C
Explanation
Question
A. C.
B. D.
Br2, H2O
Br Br
OH
Br
OH
Br
OH
OH
What is the product (in addition to its enantiomer) of the following reaction?
Conversion of Alkenes to Vicinal Diols
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.
Syn Dihydroxylation