Chapter 7: Alkenes: Reactions and Synthesis
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
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