Alkenes-Reactions and Synthesis

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Chapter 7: Alkenes: Reactions and Synthesis
H
OH
H
H
C C
C C
alcohol
alkane
X
OH
HO
OH
C C
C C
halohydrin
C C
1,2-diol
alkene
X
X
C O
C C
carbonyl
1,2-dihalide
H
X
C
C C
C C
halide
halide
Addition
X
Y
C C
+
X Y
C C
alkene
Elimination
Electrophilic Addition
H
H
Br
ether
+ HBr
H
H
H
Dehydrohalogenation: loss of HX from an alkyl halide to form
an alkene
H
Br
EtOH
(ethanol)
H
+ KOH
+ KBr + H
2
O
H
H
H
1
Hydration: addition of water (H-OH) across a double bond to
give an alcohol
H
H
OH
H
+
+ H
2
O
H
H
H
Dehydration: Loss of water (H-OH) from an alcohol to give an
alkene
H
OH
H
+
H
+ H
2
O
H
H
H
Addition of Halogens (X
2
) to Alkenes: 1,2-dihalides
X
2
X
X
C C
C C
alkene
1,2-dihalide
Br
Br
+ Br
2
+
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
Br
2
, H
2
O
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
OH
O
DMSO, H
2
O
Br
+
N
Br
+
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
+
CH
3
Br
d
+
more
d
+ charge on the
more substituted carbon
d
+
H
2
O 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
CH
3
Br
2
, H
2
O
HO
CH
3
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
CH
3
1) Hg(OAc)
2
, H
2
O
2) NaBH
4
HO
CH
3
H
H
3. Hydroboration- Anti-Markovnikov addition of H-OH,
Syn addition of H-OH
CH
3
1) B
2
H
6
, THF
2) H
2
O
2
, NaOH, H
2
O
H
CH
3
HO
H
H
H
H
_
O
tetrahydrofuran
(THF)
B
B
2
H
3
B
O
+
H
H
H
B
2
H
6
(diborane)
borane-THF
complex
4
Reaction of Alkenes with Carbenes to give Cyclopropanes
Carbene: highly reactive, 6-electron species.
(
sp
2
-hybridized)
Generation and Reaction of Carbenes:
CHCl
3
+ KOH Cl
2
C: + H
2
O + KCl
dichlorocarbene
H
CHCl
3
, KOH
Cl
Cl
H
Simmons-Smith Reaction (cyclopropanation)
CH
2
I
2
+ Zn(Cu)
ether
I-CH
2
-Zn-I = “H
2
C:”
methylene
carbene
CH
2
I
2
, Zn(Cu)
H
ether
H
5
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