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SYNTHESIS OF ALKENES VIA
ELIMINATION REACTIONS
Preparation of alkenes
1- Dehydration of alcohols:
• Mineral acids (H2SO4, H3PO4)
• General equation:
H+ /
C
C
C
OH H
CH 3CH 2OH
OH
+
H2O
CH2
+
H2O
H2SO 4 / heat
or H 3PO4 / Heat
Ethanol
C
H2C
Ethene
+
H / heat
+ H2O
H
cyclohexanol
2
cyclohexene
Dehydration of a Primary Alcohol: An E2 reaction
H+
OH
OH 2
E2
H
A
+ HA
+ H 2O
Dehydration of a Secondary or tertiary Alcohol: An E1 reaction
HSO4
H
H
H
C
C
H
H
+
C
E1
C
H
C
C
- H2O
OH2
OH
C
C
+
H2SO4
3
H
Saytzeff's Rule
In every instance in which more than one alkene can be formed,
the major product is the alkene with the most alkyl substituents
attached on the double bonded carbon.
OH
CH3CH2CHCH3
2- butanol
H+
CH3CH2CH=CH2 + H2O
1-Butene
(minor)
∆
CH3CH=CHCH3 + H2O
2- Butene
(major)
Rearrangement during Dehydration of Secondary Alcohols
OH2
OH
H+
- H2O
rearrangement
Secondary carbocation
less stable
Tertiary carbocation
more stable
+
Minor
Major
5
2- Dehydrohalogenation of Alkyl Halides
General equation:
-C - C - + KOH
H
X
H2C
+ KOH
CH2
Alcohol
∆
Alcohol
H2C=CH2
H2C
heat
H
X
an alkyl halide
CH2
+
+ KX + H2O
KX
+ H2O
Alkene
Br
H3C
CH3
H
+
KOH
alcohol / heat
H3C
CH3
2-Butene major
+ H3C
CH2
1-Butene minor
H
Br
H
CH3
+
KOH
Alcohol / heat
CH3
1-methylcyclohexene
major
+
CH3
3methylcyclohexene
minor
OH
H
C
C
H
E2
C
C
+ H2O
+ X-
X
7
3- Dehydrohalogenation of vicinal dihalides
General equation:
Methanol
-CX – CX- + Zn
∆
-C=C- + Zn X2
i) By Zn in acetic acid or ethanol
H 3C
H
C
H
C
Br
Zn
CH 3
CH3COOH or EtOH
H 3C
C
H
C
H
CH 3
+
ZnBr2
Br
ii) By NaI in acetone
H3C
H
C
H
C
Cl
Cl
2 NaI
CH3
acetone
H 3C
C
H
C
H
CH3
+ I2 + 2 NaCl
Reaction of alkenes
1- Additions to the Carbon-Carbon Double Bond
General equation:
H2C=CH2 +A-B
H2A-C-C-H2B
1-1- Addition of hydrogen: Hydrogenation (reduction)
A
A
+
A
Pt or Ni or Pd
H2
A
H3C
CH2
A
A
A
H
An alkene
H2C
A
H
An alkane
+
Pt
H2
CH2
+
H2
H3C
Pt
H3C
CH3
CH
9 3
1-2- Addition of Halogens: Halogenation
General equation:
A
A
A
+
A
X2
A
A
A
Inert solvent
A
X
(X= Cl or Br)
X
Cl
H3C
CH3
+
Cl 2
CCl 4
H3C
CH3
Cl
Br
+
Br2
CCl 4
Br
Br
+
Br
Br
Br
Br
Br
11
1-3- Addition of Hydrogen Halides: Hydrohalogenation
General equation:
A
A
A
+
A
A
A
HX
A
A
H
(X= Cl or Br or I)
X
Cl
H3C
CH3
+
HCl
CCl 4
H3C
CH3
H
H
+
HI
CCl 4
I
12
H
+
HX
X
+
H
formation of the more stable
carbocation
X
13
MARKOVNIKOV’S RULE
In addition of HX to unsymmetrical alkenes the hydrogen halide adds to
the double bonded carbon that bears the greater number of hydrogen
atoms and the negative halide ion adds to the other double bonded
carbon.
H3C
H3C
H3C
CH3
major
Br
CH2
H3C
H3C
CH3
+
HCl
CH 2Br
minor
H3C
H3C
CH3
Cl
1-5- Addition of H2O: Hydration
The addition of water to a double bond usually follows
Markovnikov’s rule.
A
A
+
A
A
+
H2O
H
A
A
A
A
H
OH
OH
H3C
CH3
+
+
H
H2O
H3C
CH3
H
H
+
+
CH3
H2O
H
OH
CH3
15
2- OZONOLYSIS
Alkene+O3
A
ozonoid + H2O
A
A
+
A
Aldehyde+ Ketone
A
A
O3
A
H2O, Zn
A
O
A
A
O
+
A
O
A
O
O
H3C
CH3
1) O 3
H3C
2) H 2O, Zn
1) O 3
H3C
CH3
O
2
H3C
+
O
CH3
O
2) H 2O, Zn
16
3- Oxidation
KMnO 4 / OH
OH
-
OH
4- Epoxidation
H3 O
RCOOOH
+
OH
O
OH
17
Electrophilic Addition to Conjugated Dienes:
1,4-Addition
• The addition of a reagent to a pair of
adjacent carbons is called 1,2-addition.
Br
+
Br 2
CCl4
1,2-addition
1,4-Pentadiene
Br
4,5-Dibromopentene
Br
Br
+
Br 2
Br
CCl4
1,2-addition
Br
Br
4,5-Dibromopentene
Br
1,2,4,5-Tetrabromopentane
18
• Treatement of a conjugated diene with
bromine under the similar conditions gives, in
addition to the exepected 1,2-addition
product, an unexcepted 1,4-addition product.
1,2-addition
expected product
1,4-addition
unexpected product
Br
+
1,3-Butadiene
Br 2
CCl4
Br
+
Br
3,4-Dibromobutene
Br
1,4-Dibromo-2-butene
19
Resonance
The addition of bromine to 1,3-butadiene results
in a secandary carbocation. This carbocation
is also called an allylic carbocation.
H 2C
C
H
C
H
CH 2
H
C
H 2C
Br
C
H
CH 2
resonance
H 2C
Br
Br
1,2-addition
Br
C
H
CH 2
Br
Allylic carbocation (II)
Allylic carbocation (I)
H 2C
C
H
H
C
Br
Br -
C
H
Br -
1,4-addition
CH 2
H 2C
Br
C
H
C
H
CH 2
Br
20
+
H
Cl
CCl 4
1,3-Butadiene
Allylic carbocation equivalent to
+
(b)
1,2-addition
Cl -
+
+
+
(a)
(a)
Cl
1,4-addition
(b)
Cl
21
The Diels-Alder reaction
The reaction is one between a conjugated diene
and a compound containing a double bond
called a dienophile. The product of a DielsAlder reaction is offten called an adduct.
+
Adduct
Dienophile
Diene
O
O
100°C
+
O
Benzene
O
1,3-Butadiene
O
Maleic anhydride
O
22