Chapter 7: Alkenes: Reactions and Synthesis
H
X
OH
H
H
C C
C C
alcohol
alkane
OH
HO
C C
OH
C C
C C
halohydrin
1,2-diol
alkene
X
X
C O
C C
carbonyl
1,2-dihalide
C C
C
C C
halide
halide
H
X
Addition
X
C C
+
X Y
Y
C C
alkene
Elimination
Electrophilic Addition
H
H
ether
+
Br
HBr
H
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
H+
+
OH
H2O
H
H
H
Dehydration: Loss of water (H-OH) from an alcohol to give an
alkene
H
OH
H
H+
+ H2O
H
H
H
Addition of Halogens (X2) to Alkenes: 1,2-dihalides
X2
X
X
C C
C C
alkene
1,2-dihalide
Br
Br
+
+
Br2
Br
Br
not observed
1,2-dibromide has the anti stereochemistry
Bromonium ion intermediate controls the stereochemistry
2
Halohydrin Formation
"X-OH"
X
OH
C C
C C
alkene
halohydrin
Br2, H2O
X
+ HBr
OH
anti
stereochemistry
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
+
O
OH
N Br
Br
DMSO, H2O
+
N H
O
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
more d+ charge on the
more substituted carbon
Br d+
d+
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
CH3
Br2, H2O
HO
H
CH3
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
HO
1) Hg(OAc)2, H2O
2) NaBH4
CH3
H
CH3
H
3. Hydroboration- Anti-Markovnikov addition of H-OH,
Syn addition of H-OH
1) B2H6, THF
2) H2O2, NaOH, H2O
CH3
H
HO
H
H
B
H
H
B
H
B2H6 (diborane)
H
O
tetrahydrofuran
(THF)
2
CH3
H
_
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
CH2I2, Zn(Cu)
H
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
CH2I2, Zn(Cu)
ether
cis-alkene
H
R
R
H
H
H
R
R
cis-cyclopropane
CH2I2, Zn(Cu)
ether
trans-alkene
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 C≡N 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
H2, PtO2
ethanol
O
OH
C5H11
H2, Pd/C
CH3(CH2)16CO2H
Linoleic Acid (unsaturated fatty acid)
Steric Acid (saturated fatty acid)
O
O
OCH3
H2, Pd/C
OCH3
ethanol
C
H2, Pd/C
N
CH3
CH3
C
ethanol
H
H2, PtO2
ethanol
H
N
CH3
CH3
H
CH3
H
syn addition
of H2
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.
H
1) OsO4
2) NaHSO3
H
H
O O
Os
O O
H
OH
syn addition
OH
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, C≡N and C=O do not react with ozone,
C≡C react very slowly with ozone
3 O2
Ozone (O3):
R1
R2
R3
R4
O3, CH2Cl2
-78 °C
O
R1
R2
O
O
R3
R4
molozonide
electrical
discharge
R1
R2
2 O3
O O
O
ozonide
R3
R4
O
Zn
+
O
O
_
R1
R3
O
+
R2
O
R4
+ ZnO
8
1) O3
2) Zn
O
+
1) O3
2) Zn
O
H
O
1) O3
2) Zn
+
O=
C
H2
O
H
O
Oxidative Cleavage of 1,2-Diols to Carbonyl Compounds
HO
R1
R2
OH
R3
R4
NaIO4
R1
THF, H2O
R2
R3
O
+
O
R4
+ ZnO
OH
O
I
O
O
O
R1
R2
OH
OH
periodate intermediate
R3
R4
O
NaIO4
H
H
O
9