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Chapter 4 Alkenes and Alkynes: Structure and Preparation
Chapter 4 Alkenes and Alkynes:Structure and Preparation
Examples:
Isobutylene, -Pinene, Farnesene, Cicutoxin
CH3
CH3
CH2H
CH3 CH3
CH3
CH3CH2
CH3CH3
CH3
CH3
OH
OH
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
Nomenclature:Replace the aneending of the parent hydrocarbon with ene. The longest
continuous chain which includes the double bond forms the base name., and
the chain is numbered in such a way, so that the alkene carbons obtain the
lowest possible numbers.
CH2
CH3
CH3
CH3
CH3
CH3
CH3
but-1-ene(2E)-pent-2-ene
(2E)-5-methylhex-2-ene
C-C double bonds outrank alkyl groups and halogens in determining the
main carbon chain. Hydroxyl groups, however, outrank the double bond.
CH3
Br
CH2
1
2
34
5
6
OH
CH2
1
2
34
56
6-bromo-3-ethylhex-1-ene
4-ethylhex-5-en-1-ol
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
Cycloalkenes and their derivatives are named by adapting cycloalkane
terminology.
Cl CH3
CH3 CH3
1,3-dimethylcycloheptene
3-chloro-1-methylcyclohexene
cyclopentene
OH
cyclohex-2-en-1-ol
Structure and Bonding in Alkenes
C C
H
H H
H
1.33 A
1.08 A
o
o o
o
116.6
121.7
C C
H
HHH
HH
1.54 A
1.09 A
o
o
Alkene Alkane
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
Hybridization at C: sp2
CC -bondCC -bond
Isolated double bonds: conjugated double bonds
CH2CH2
CH3
CH2
Delocalized systems are more stable !!!
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
ISOMERISM IN ALKENES.
There are four possible Isomers of butene:
CH3
CH2
CH3 CH2
CH3
CH3
CH3
CH3CH3
but-1-ene
2-methylprop-1-ene
(2E)-but-2-ene(2Z)-but-2-ene
trans cis
Naming by the E-Znotational system
Prioritize substituents:
1. Higher atomic number, higher priority (e.g., NH2shows higher priority than CH3).
2. If the group has the same atomic number, go to thenext atom (e.g, CH2CH3has higher priority than CH3).
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
When the two groups of higher priority, each ondifferent carbon atoms of the double bond, are on thesame side of the molecule, the name of the molecule
is prefixed with (Z) (from the German zusammen,together).
When the groups of higher priority are on oppositesides of the molecule, the designation (E) (from theGerman entgegen, across) is used.
Examples:
Br CH3
Cl H
high
low
high
low
Br
Cl
OH
CH3
CH3
high
high
low
low
CH3
H
CH3
CH3
O
OH
low
lowhigh
high
(1Z)-1-bromo-1-chloroprop-1-ene
(3E)-4-bromo-4-chloro-3-isopropylbut-3-en-1-ol
(3E)-3-(1-hydroxyethyl)pent-3-en-2-one
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
Stabilities:
We distinguish mono- di- tri- tetrasubstituted alkenes. Generally, the higher
the substitution order the more stable the alkene.
CH2
CH3
CH3
CH3
CH2
CH3
CH3
CH3CH3
CH3
CH3CH3
CH3CH3
Preparation of Alkenes
Y
H
H
H
H
X
H
H
H
H
+ XY
Alkene formation requires X and Y-substituents to be on adjacent carbons.
Principal methods would be deydration of alcohols (X=H, Y=OH),
dehydrohalogenation of alkylhalides (X=H, Y= Cl, Br, I)
Dehydration of alcohols
An acid catalyst is needed for the reaction to proceed.
H
H
H
H
H
OH
HH
H H
H+
alcohol alkene
additional product would be water (dehydration)
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
other examples:
OH
CH3
CH3
CH3
OH
CH3
CH3
CH2
CH3
CH3
OH
CH3
CH3
CH3CH3
+
CH3
CH2CH3
90% 10%
From the last example we see, that there is a preference to from the highest
substituted alkene. We call this behaviorregioselective. The first to look into
this regioselectivity was Zaitsev, therefore the rule to determine the mainproduct is called Zaitsev rule.
OH
CH3 CH3 CH3 CH3 +
CH3
CH3
75%
25%
In case we have a chance to produce cis and trans products, than there is a
tendency to form the more stable trans product. This behavior is called
stereoselective.
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
Mechanism:CH3
CH3 CH3
OHCH3
CH3
CH2
CH3
CH3 CH3
O
H
H
O+
HH
H
OH
H
CH3
CH3 CH3
O+
H
H
CH3
CH3 CH3
O+
H
CH3
C+
CH3 CH3H
O
H
CH3
C+
CH3
H
++
+
+H
O
H
CH3
CH3 CH2
+ H
O+
HH
H2
SO4
For primary carbons, there is no carbocation intermediate. Two steps, bond
cleavage and bond formation, occur simultaneously
H
OH
H
H
O+
H
CH2 CH2+ +H
O+
H
HH
O H
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
Dehydrohalogenation of Alkyl Halides
Means elimination of a hydrogen and a halogen, which are bound to two
different carbon atoms.
The regioselectivity follows Zaitsev rules and overall the reaction is
regioselective, meaning the most stable alkene is formed in the elimination
process.
E2- Reaction Mechanism
In the E2 mechanism the two leaving groups have to be in an anti
configuration, which leads to a stereospecific reaction.
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
Mechanism for E2
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
:
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
Alkynes
Hydrocarbons with a triple bond are named alkynes general formula CnH2n-2.
Follow the hydrocarbon nomenclature end substitute anewith yne.
The carbons in a triple bond are sp-hybridized.
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Chapter 4 Alkenes and Alkynes: Structure and Preparation
Preparation:
a) double dehydrohalogenation of geminal alkyl dihalides
CH3
Cl
Cl
CH3
CH
3 NaNH2/ NH
3
b) double dehydrohalogenation of vicinal alkyl dihalides
CH3
Br
Br
CH3
CH
3 NaNH2/ NH
3
Important Diynes : tumor inhibitors:
Dynemicin A:
OH
OHNHO
O OH
O
CH3 O
OH
O
CH3