Chapter 5 – Structure and Preparation of Alkenes
Double bond - now dealing with sp2 hybrid carbon
5.1 Nomenclature
1-butene
1-hexene
Br
2,3-dimethyl-2butene
6-bromo-3-propyl-1-hexene
2-methyl-2-hexene
HO
5-methyl-4-hexen-1-ol
Common Alkene Substituents
H2C C
H
vinyl
CH2=CHCH2
CH2=C
CH3
allyl
isopropenyl
Cycloalkenes
Br
Cl
cyclohexene
1-chlorocyclopentene
3-bromocyclooctene
5.2 Structure and bonding in ethylene
Figure 5.1
5.3-5.4 cis-trans isomerism in alkenes
1-butene
2-methylpropene
cis-2-butene
H
H
H
trans-2-butene
Br
CH2 CH3
Cl
CH 3
O
Cinnamaldehyde
(trans alkene - E)
cis alkene (Z)
See Table 5.1 for priority rules
Interconversion of cis and trans-2-butene
5.5-5.6 Heats of combustion of isomeric C4H8 alkenes
Figure 5.3
5.5-5.6 Heats of combustion of isomeric C4H8 alkenes
Figure 5.2
Generally, the more substituted an alkene, the more stable
Molecular models of cis-2-butene and trans-2-butene
Figure 5.4
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
CO2H
H
Sterculic acid (natural product)
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
5.10 Zaitsev Rule
Dehydration usually results in more highly substituted alkene being
major product - Zaitsev rule (regioselectivity)
CH3
OH
C CH2CH3
CH3
H2SO4
CH2
80 oC
CH2CH3
C
+
CH3
H3C
C CHCH3
H3C
10%
HO CH 3
90%
CH 3
CH 2
H+
+
OH
H+
+
5.11 Stereoselectivity in Alcohol Dehydration
One stereoisomer is usually favoured in dehydrations
H2 SO4, 
+
OH
75%
25%
When cis and trans isomers are possible in this reaction
the more stable isomer is usually formed in higher yield
5.12 Acid-catalyzed Alcohol Dehydration E1 and E2
E1
OH
H2 SO4
+
H 2O
heat
protonation
OH 2
deprotonation
dissociation
H
E2 on 1o alcohols – concerted with no cation
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
Orbital representation of methyl migration
Figure 5.6
5.13 Hydride shifts to more stable carbocations
H 2SO4, 
OH
1- butene
12%
H
H
H
H
H
C
C
C
C
H
H
H
H
tr ans-2-butene
56%
H
ci s-2-butene
32%
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
+ NaOCH2CH3
C C
base
(sodium ethoxide)
Cl
alkene
+ HOCH2CH3 + NaX
conjugate
acid
salt
NaOCH 2 CH 3
100%
H
HOCH2 CH3, 55 oC
Zaitsev rule followed for regioisomers when a small base such as
NaOCH3, NaOCH2CH3 is used. Trans usually favoured over cis.
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
The E2 Mechanism - Elimination Bimolecular
5.16 Anti Elimination faster than Syn Elimination
Br
Br
H
500 times faster
KOC(CH3) 3
HOC(CH3)3
H
4-ter t -Butylcyclohexene
E2 Elimination usually faster when H and leaving group are
anti periplanar as opposed to syn periplanar.
Conformations of cis- and trans-4-tert-butylcyclohexyl
Favourable conformations for fast elimination
E2 Elimination usually faster when H and leaving group are
anti periplanar as opposed to syn periplanar.
Not covering Section 5.17 (Isotope Effects)
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, therefore E1 - usually under neutral or
acidic conditions (compare with dehydration earlier)