reactions of alkenes. some reaction types : addition elimination substitution
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State University of New York at Albany State University of New York at Albany
Reactions of AlkenesReactions of Alkenes
Some Reaction TypesSome Reaction Types::
C X + Y C Y + X
C CC C
X Y
+ X Y
C C X Y+ C C
X Y
Addition Addition
Elimination Elimination
SubstitutionSubstitution
Addition ReactionsAddition Reactions
The vast majority of the reactions of The vast majority of the reactions of alkenes are additions.alkenes are additions.
When the addition reaction is initiated by When the addition reaction is initiated by attack of the pi bond electrons of the double attack of the pi bond electrons of the double bond on an electrophile to form a bond on an electrophile to form a carbocation, the reaction is known as an carbocation, the reaction is known as an electrophilic additionelectrophilic addition..
Addition of HX to alkenesAddition of HX to alkenes In the addition of H-X to alkenes, H bonds to the In the addition of H-X to alkenes, H bonds to the
carbon of the double bond with the greater number carbon of the double bond with the greater number of hydrogens. This is called of hydrogens. This is called Markovnikov’sMarkovnikov’s rule. rule.
C C + H X C
H
C
X
CH2CH3 CH + H Br CH3 CH
Br
CH2
H
Examples:
General Reaction:
HCl
Cl
CH2CH3 CH + H CH3 CH CH2
H
CH3 CH CH2
H
BrCH3 CH
Br
CH2
H
Mechanism:
1.
2.
The reaction is The reaction is regiospecificregiospecific, and the product obtained, and the product obtainedis called the Markovnikov product. It is formed becauseis called the Markovnikov product. It is formed becausethe reaction proceeds to form the most stable carbo-the reaction proceeds to form the most stable carbo-cation intermediate.cation intermediate.
CH2CH3 CH + H Br CH3 CH
H
CH2
Br
Example:ROOR
When HBr is added in the presence of peroxides,When HBr is added in the presence of peroxides,the the anti-Markovnikovanti-Markovnikov product is formed. product is formed.
The reason for the change in The reason for the change in regiochemistryregiochemistry is isthat the reaction proceeds via a free radical that the reaction proceeds via a free radical mechanism rather than a carbocation mechanism rather than a carbocation mechanism. The production of free radicals ismechanism. The production of free radicals isinitiated by the presence of the peroxide.initiated by the presence of the peroxide.
RO ORheat
R O + O R
R O + H Br R OH + Br
CH2CH3 CH Br+ CH2CH3 CH
Br
CH2CH3 CH
Br
H Br CH2CH3 CH
BrH
+ Br
MechanismMechanism::
Note that reaction proceeds to form the most stableNote that reaction proceeds to form the most stableradical.radical.
3. Addition of Water—Hydration of Alkenes3. Addition of Water—Hydration of Alkenes
C C + C
H
C
OH
H2OH
General Reaction:General Reaction:
CH2CH3 CH + CH3 CH
OH
CH2
H
Example:
H2OH
This reaction follows Markovnikov’s rule.This reaction follows Markovnikov’s rule.
H OCH2CH3 CH + CH3 CH CH2
H
CH3 CH CH2
H
CH3 CH
O
CH2
H
Mechanism:
1.
2.
H
H
OH2
+ H2O
HH
CH3 CH
O
CH2
HHH
3. CH3 CH
OH
CH2
H
+ H3O
4. Addition of Sulfuric Acid4. Addition of Sulfuric Acid
C C + C
H
C
OSO3H
General Reaction:
HH2SO4
alkyl hydrogen sulfate
CH2CH3 CH + CH3 CH
OSO3H
CH2
Example:
H2SO4
H
The alkyl hydrogen sulfate can be converted The alkyl hydrogen sulfate can be converted to an alcohol by boiling in water.to an alcohol by boiling in water.
C
H
C
OSO3H
H2OC
H
C
OH
+ HSO4
The reaction proceeds by an SThe reaction proceeds by an SNN1 mechanism 1 mechanism
in which the OSOin which the OSO33H acts as a leaving group.H acts as a leaving group.
CH3CH3 CH CH3 CH CH2
H
CH3 CH CH2
H
CH3 CH
O
CH2
H
Mechanism:
1.
2.OH2
+
HH
CH3 CH
O
CH2
HHH
3. CH3 CH
OH
CH2
H
+ H3O
OSO3H
O S OH
O
O
good leaving group
5. Oxymercuration-Demercuration5. Oxymercuration-Demercuration
C C + C
OH
C
HgOAc
General Reaction:
H2OHg(OAc)2
mercuric acetate
NaBH4C
OH
C
H
Example:
1. Hg(OAc)2/H2O
2. NaBH4
CH3
OH
The addition is anti, and Markovnikov The addition is anti, and Markovnikov orientation is observed.orientation is observed.
O
O
Hg O
O
O
O
Hg O
O
+
O
O
Hg
Hg
OAc
Hg
OAc
OH2
CH3
O
HgOAc
H
H
mercuronium ion
CH3
O
HgOAc
H
HOH2
NaBH4CH3
OH
Mechanism:Mechanism:
6. Alkoxymercuration-Demercuration6. Alkoxymercuration-Demercuration
Example:
1. Hg(OAc)2/CH3OH
2. NaBH4
CH3
OCH3
When mercuration takes place in an alcohol When mercuration takes place in an alcohol solvent, ethers are formed. The mechanism solvent, ethers are formed. The mechanism is the same as that for oxymercuration-is the same as that for oxymercuration-demercuration, except that the mercuroniumdemercuration, except that the mercuroniumion is attacked by the alcohol instead of water.ion is attacked by the alcohol instead of water.
7. Hydroboration of Alkenes—anti-Markovnikov7. Hydroboration of Alkenes—anti-Markovnikov addition. addition.
C C + C
H
C
BH2
General Reaction:
C
H
C
OH
B2H6
H2O2
alkyl borane
Example:
1. BH3/THF
2. H2O2/OH
H
CH3
OHOH
H
=
BH
H
H
BH3
H
CH3
BH2
The reaction is anti-Markovnikov. The The reaction is anti-Markovnikov. The addition is syn and the reaction is addition is syn and the reaction is stereospecific.stereospecific.
Mechanism for hydroboration step:Mechanism for hydroboration step:
8. Catalytic Hydrogenation:8. Catalytic Hydrogenation: This reaction is formally a reduction, with HThis reaction is formally a reduction, with H22 adding adding
across the double bond to give an alkane. The across the double bond to give an alkane. The process usually requires a catalyst containing Pt, process usually requires a catalyst containing Pt, Pd, or Ni. The addition is syn.Pd, or Ni. The addition is syn.
C C + C
H
C
H
General Reaction:
H2
Pt
Example:H
CH3
H
H2/Pt
9. Addition of Halogens to Alkenes9. Addition of Halogens to Alkenes
C C + C
X
C
X
General Reaction:
X2
Example:CH3
Br
Br
Br2
Mechanism:Mechanism:
Br
Br
bromonium ion
Br Br+
BrCH3
Br
Br
+ Br
+
The reaction is stereospecific and anti.The reaction is stereospecific and anti.
10. Halohydrin formation.10. Halohydrin formation.
Br
Br
bromonium ion
CH3
O
Br
H
HOH2
Br Br+
CH3
O
Br
OH2H
H
CH3
OH
Br
+
Br+
H3O+
11. Epoxidation of Alkenes.11. Epoxidation of Alkenes.
An epoxide (oxirane) is a three membered An epoxide (oxirane) is a three membered ring cyclic ether.ring cyclic ether.
C C + C C
General Reaction:
RCOOOH
Operacid
+ RCOOH
epoxide
Example:O
RCOOOHRCOOH+
Mechanism.Mechanism.
O
O
H
O
R
O
HO
OR
O
+ RCOOH
This is a concerted electrophilicThis is a concerted electrophilicaddition. The addition is syn.addition. The addition is syn.One of the peracids that is oftenOne of the peracids that is oftenused is “meta-chloroperoxy-used is “meta-chloroperoxy-benzoic acid (MCPBA). If youbenzoic acid (MCPBA). If yousee MCPBA as a reactant, thissee MCPBA as a reactant, thismeans that you are using a permeans that you are using a peracid.acid.
12. Acid Catalyzed Epoxide Ring Opening.12. Acid Catalyzed Epoxide Ring Opening.
Epoxides react with water in the presence of aEpoxides react with water in the presence of acatalytic amount of acid to form diols. The orientationcatalytic amount of acid to form diols. The orientationis of the -OH groups is anti.is of the -OH groups is anti.
C C
O
H3OC C
OH
OH
General Reaction:
a glycol
Mechanism.Mechanism.
CH3
O
OH
H
HOH2
CH3
OH
OH
OH
O
H
O
H
OH2
CH3
O
OH
H
H
trans 1,2-cyclopentane diol
13. Syn Hydroxylation of Alkenes.13. Syn Hydroxylation of Alkenes.
Hydroxylation is the addition of an OH group to eachHydroxylation is the addition of an OH group to eachend of a double bond to form a glycol. The reactionend of a double bond to form a glycol. The reactioncan be accomplished with OsOcan be accomplished with OsO44/H/H22OO22, or KMnO, or KMnO44/OH/OH--..
C C
General Reaction:
OsO4/H2O2C C
OH OH
or KMnO4/OH
Example:OsO4/H2O2 OH
CH3
OH
Mechanism for OsOMechanism for OsO44 Hydroxylation. Hydroxylation.
O
Os
O
O O
osmate ester
O
O O
O
H2O2
O
O O
O
OH
OH
OsO4+
Mechanism for MnOMechanism for MnO44 Hydroxylation. Hydroxylation.
O
Mn
O
O O
OH/H2O
O
O O
O
OH
OH
MnO2+O
O O
O
14. Oxidative Cleavage of Alkenes w/ Permanganate.14. Oxidative Cleavage of Alkenes w/ Permanganate.
C C
H
R'
R
R
General Reaction:
KMnO4 (warm, conc)R C C H
OH
R R'
OH
O
R
R
+ O
H
R'
HO R'
O
acid
KMnO4
warm/conc.
O
HO
O
+
Example of oxidative cleavage withExample of oxidative cleavage withpermanganate.permanganate.
15. Ozonolysis.15. Ozonolysis.
Ozone cleaves double bonds to give ketonesOzone cleaves double bonds to give ketonesand/or aldehydes.and/or aldehydes.
C C
H
R'
R
R
General Reaction:
O
R
R
+ O
H
R'O3
O O
O R'R
R H
CH3S
ozonide
1. O3
2. (CH3)2S
O
O
=
CHO
CHO
Example
16. Simmons-Smith Reaction.16. Simmons-Smith Reaction.
C C
General Reaction:
+ ICH2ZnI
CH2
+ ZnI2(or CH2I2/ZnCuCl)
Example:
CH2I2
Zn, CuCl
17. Addition of Carbenes to Double Bonds.17. Addition of Carbenes to Double Bonds.
When CHBrWhen CHBr33 or CHCl or CHCl33 is treated with aqueous KOH, is treated with aqueous KOH,
a carbene is formed.a carbene is formed.
CHBr3 + KOH CBr2 + Br + H2O
dibromocarbene
Carbenes react at double bonds to form cyclopropanes.Carbenes react at double bonds to form cyclopropanes.
Example:
CHBr3
NaOH, H2O
BrBr