Chapter 5 – Structure and Preparation of Alkenes
Double bond - now dealing with sp2 hybrid carbon
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5.1 – Structure and Nomenclature of Alkenes
1-butene
1-hexene
Br
2,3-dimethyl-2-butene
6-bromo-3-propyl-1-hexene
2-methyl-2-hexene
HO
5-methyl-4-hexen-1-ol
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Common Alkene Substituents
H 2C C
H
vinyl
CH 2=CHCH2
allyl
CH 2=C
CH3
isopropenyl
Cycloalkenes
Br
Cl
cyclohexene
3-bromocyclooctene
1-chlorocyclopentene
5.2 Structure and bonding in ethylene
Figure 5.1
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5.3-5.4 cis-trans isomerism in alkenes
1-butene
2-methylpropene
cis-2-butene
trans-2-butene
H
H
H
O
Br
CH2 CH3
Cl
CH 3
Cinnamaldehyde (trans alkene - E)
See Table 5.1 for priority rules
cis alkene (Z)
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Interconversion of cis and trans-2-butene
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5.5-5.6 Heats of combustion of isomeric C4H8 alkenes
Figure 5.3
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5.5-5.6 Heats of combustion of isomeric C4H8 alkenes
Figure 5.2
Generally, the more substituted an alkene, the more stable
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Molecular models of cis-2-butene and trans-2-butene
Figure 5.4
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5.7 Cycloalkenes - trans not necessarily more stable than cis
H
H
H
H
C-12 cis and trans ~ equal in energy
H H
H H
H
CO2 H
H
Sterculic acid (natural product)
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5.8 Preparation of Alkenes - Elimination reactions
X C C Y
+
X Y
-C
-C
5.9 Dehydration of Alcohols
H+
H C C OH
+
H OH
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5.10 Zaitsev Rule
Dehydration usually results in more highly substituted alkene being major
product - Zaitsev rule (regioselectivity)
CH3
OH
C CH 2 CH 3
CH 3
H2 SO4
CH 2CH 3
CH 2 C
80 oC
+
CH 3
10%
H3 C
C CHCH 3
H3 C
90%
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5.10 Zaitsev Rule
HO CH 3
CH 3
CH2
H+
+
OH
H+
+
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5.11 Stereoselectivity in Alcohol Dehydration
One stereoisomer is usually favoured in dehydrations
H2SO4, 
+
OH
75%
25%
When cis and trans isomers are possible in this reaction the
more stable isomer is usually formed in higher yield
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5.12 Acid-catalyzed Alcohol Dehydration – E1
OH
H2 SO4
+
E1
H 2O
heat
protonation
OH 2
deprotonation
dissociation
H
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5.13 Carbocation Rearrangements in E1 Reactions
H3 PO4
OH
heat
3%
33%
64%
H
OH 2
H
H
Cation rearrangement leads to more stable cation
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Orbital representation of methyl migration
Figure 5.6
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5.13 Hydride shifts to more stable carbocations
H2 SO4 , 
OH
1- butene
12%
H
H
H
H
H
C
C
C
C
H
H
H
H
cis-2-butene
32%
tr ans-2-butene
56%
H
H
H
H
C
C
C
C
H
H
H
H
H
1o carbocation?????
5.14 Dehydrohalogenation - Elimination with loss of H-X
H C C X
alkyl
halide
+ NaOCH 2CH3
C C
base
(sodium ethoxide)
Cl
alkene
+ HOCH 2CH3 + NaX
conjugate
acid
salt
NaOCH 2CH 3
100%
H
HOCH2CH3, 55 oC
Zaitsev rule followed for regioisomers when a small base such as NaOCH3,
NaOCH2CH3 is used. Trans usually favoured over cis.
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5.15 The E2 Mechanism - Elimination Bimolecular
• Reaction occurs under basic conditions
B

B
H
H
X
X

+ B-H
+ X
• Reaction is concerted
• Rate depends on [base][alkyl halide] i.e. Bimolecular - E2
• C-H bond breaking, C=C bond forming and C-X bond breaking
events all occur at the same time
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The E2 Mechanism - Elimination Bimolecular
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5.16 Anti Elimination faster than Syn Elimination
Br
Br
H
500 times faster
KOC(CH3)3
HOC(CH3)3
H
4-t er t-Butylcyclohexene
E2 Elimination usually faster when H and leaving group are anti
periplanar as opposed to syn periplanar.
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Conformations of cis- and trans-4-tert-butylcyclohexyl
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Favourable conformations for fast elimination
E2 Elimination usually faster when H and leaving group are anti
periplanar as opposed to syn periplanar.
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Not covering Section 5.17 (Isotope Effects)
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5.18 Different Halide Elimination Mechanism - E1
CH3CH2OH
+
heat
Br
2-methyl-1-butene
2-methyl-2-butene
25%
75%
CH3CH2OH
H
H
CH3CH2OH
R.D.S. is now unimolecular, E1 - usually under neutral/acidic conditions
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