Chapter 7: Alkenes: Reactions and Synthesis
[Pages:9]Chapter 7: Alkenes: Reactions and Synthesis
H OH CC
alcohol
H
H
CC
alkane
X
OH
CC
halohydrin
X
X
CC
1,2-dihalide
CC alkene
H
X
CC
halide
HO OH CC 1,2-diol
CO carbonyl
C CC halide
CC alkene
+ XY
Addition
X
Y
CC
Elimination
Electrophilic Addition
H + HBr
H
ether
H Br H
H
Dehydrohalogenation: loss of HX from an alkyl halide to form an alkene
H Br +
H H
KOH
EtOH (ethanol)
H + KBr + H2O
H
1
Hydration: addition of water (H-OH) across a double bond to give an alcohol
H
H+
+ H2O
H
H OH H
H
Dehydration: Loss of water (H-OH) from an alcohol to give an alkene
H
OH
H+
H H
H + H2O
H
Addition of Halogens (X2) to Alkenes: 1,2-dihalides
X2 CC
X
X
CC
alkene
1,2-dihalide
+ Br2
Br
Br
+
Br
Br
not observed
1,2-dibromide has the anti stereochemistry Bromonium ion intermediate controls the stereochemistry
2
Halohydrin Formation
CC alkene
"X-OH"
X
OH
CC
halohydrin
Br2, H2O
X
OH
anti stereochemistry
+ HBr
Organic molecules are sparingly soluble in water as solvent. The reaction is often done in a mix of organic solvent and water using N-bromosuccinimide (NBS) as the electrophilic bromine source.
O
+
N Br
DMSO, H2O
O
OH
Br +
O NH O
Note that the aryl ring does not react!!!
For unsymmterical alkenes, halohydrin formation is Markovnikov-like in that the orientation of the addition of X-OH can be predicted by considering carbocation stability
d+ CH3
Br d+
d+
more d+ charge on the more substituted carbon
CH3
Br2, H2O
H2O adds in the second step and adds to the carbon that has the most d+ charge and ends up on the more substituted end of the double bond
HO CH3
H Br Br adds to the double bond first (formation of bromonium ion) and is on the least substituted end of the double bond
3
Hydration of Alkenes: addition of water (H-OH) across the p-bond of an alkene to give an alcohol.
1. Acid catalyzed hydration- Markovnikov addition of H-OH Not a good method for hydration of an alkene
2. Oxymercuration- Markovnikov addition H-OH
CH3
1) Hg(OAc)2, H2O 2) NaBH4
HO CH3
HH
3. Hydroboration- Anti-Markovnikov addition of H-OH, Syn addition of H-OH
CH3
1) B2H6, THF 2) H2O2, NaOH, H2O
H CH3
HO H
H
H
H
B
B
H
H
H
B2H6 (diborane)
O
tetrahydrofuran (THF)
_
2
H3B
O +
borane-THF complex
4
Reaction of Alkenes with Carbenes to give Cyclopropanes Carbene: highly reactive, 6-electron species.
(sp2-hybridized)
Generation and Reaction of Carbenes:
CHCl3 + KOH
Cl2C: + H2O + KCl
dichlorocarbene
CHCl3, KOH
H Cl
Cl H
Simmons-Smith Reaction (cyclopropanation)
ether
CH2I2 + Zn(Cu)
I-CH2-Zn-I = "H2C:"
methylene
carbene
H CH2I2, Zn(Cu)
ether H
5
The cyclopropanation reaction of an alkene with a carbene takes place in a single step. There is NO intermediate.
As such, the geometry of the alkene is preserved in the product. Groups that are trans on the alkene will end up trans on the cyclopropane product. Groups that are cis on the alkene will end up cis on the cyclopropane product.
H
H
R
R
cis-alkene
CH2I2, Zn(Cu) ether
H
H
R
R
cis-cyclopropane
H
R
R
H
trans-alkene
CH2I2, Zn(Cu) ether
H
R
R
H
trans-cyclopropane
Hydrogenation: Addition of H2 across the p-bond of an alkene to give an alkane. This is a reduction.
H2, PtO2
ethanol
? The reaction uses H2 and a precious metal catalyst. ? The catalysts is not soluble in the reaction media, thus this process
is referred to as a heterogenous catalysis. ? The catalyst assists in breaking the p-bond of the alkene and
the H-H s-bond. ? The reaction takes places on the surface of the catalyst. Thus, the rate
of the reaction is proportional to the surface area of the catalyst. ? To increase the surface area of the catalyst it is finely dispersed on
an inert support such as charcoal (carbon, C) ? Carbon-carbon p-bond of alkenes and alkynes can be reduced to the
corresponding saturated C-C bond. Other p-bond bond such as
C=O (carbonyl) and CN are not easily reduced by catalytic
hydrogenation. The C=C bonds of aryl rings are not easily reduced.
6
Catalysts: Pt2O (Adam's catalyst) or Pd/C mechanism:
The addition of H2 across the p-bond is syn
O
O
C5H11
OH
Linoleic Acid (unsaturated fatty acid)
O
OCH3
H2, PtO2 ethanol H2, Pd/C
H2, Pd/C ethanol
O
CH3(CH2)16CO2H Steric Acid (saturated fatty acid)
O OCH3
C N
H2, Pd/C ethanol
CH3
H2, PtO2
ethanol CH3
C N
H CH3
H CH3 syn addition
of H2
CH3 H
H CH3 Not observed
7
Oxidation of Alkenes to 1,2-Diols and Carbonyl
Hydroxylation: formal addition of HO-OH across the p-bond of an alkene to give a 1,2-diol. This is an overall oxidation.
1) OsO4 2) NaHSO3
H OH OH
H
syn addition
H OO
Os
OO H
osmate ester intermediate
- not usually isolate
- NaHSO3 breaks down the osmate ester to the product
Ozonolysis: oxidative cleavage of an alkene to carbonyl compounds. The p- and s-bonds of the alkene are broken and replaced with C=O doubled bonds.
C=C of aryl rings, CN and C=O do not react with ozone, CC react very slowly with ozone
Ozone (O3):
3 O2
electrical discharge
2 O3
R1
R3
O3, CH2Cl2 -78 ?C
O OO
R2
R4
R1
R3
R2 R4
molozonide
R1
OO R3
R2 O R4
ozonide
+ O_ OO
Zn
R1
R3
O+ O
R2
R4
+ ZnO
8
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