5
Organic
Chemistry
William H. Brown &
Christopher S. Foote
5-1
5
Alkenes I
Chapter 5
5-2
5 Unsaturated Hydrocarbons
 Unsaturated
hydrocarbon: contains one or more
carbon-carbon double or triple bonds
 Alkene: contains a carbon-carbon double bond
and has the general formula CnH2n
H
H
C
C
H
H
Ethylene
(an alkene)
 Alkyne:
contains a carbon-carbon triple bond
and has the general formula CnH2n-2
H-C C-H
Acetylene
(an alkyne)
.
5-3
5 Unsaturated Hydrocarbons
 Arenes:
benzene and its derivatives (Ch 20-21)
H
H
H
C
C
C
C
C
C
H
H
H
5-4
5 Benzene & Phenyl Group
 We
do not study benzene and its derivatives until
Chapters 20 & 21
• but, we show structural formulas of compounds
containing the phenyl group before that time
• the phenyl group is not reactive under any of the
conditions we describe in Ch 6-19
C6 H5 Benzene
Ph-
Alternative representations
for the phenyl group
5-5
5 Structure of Alkenes
 The
two carbon atoms of a double bond and the
four atoms attached to them lie in a plane, with
bond angles of approximately 120°
 A double bond consists of
• one sigma bond formed by overlap of sp2 hybrid
orbitals
• one pi bond formed by overlap of parallel 2p orbitals
• it takes approx. 264 kJ (63 kcal)/mol to break the pi
bond in ethylene
5-6
5 Restricted rotation about C=C
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5-7
5 Cis,Trans Isomerism
 Cis,trans
isomers: isomers that have the same
connectivity but a different arrangement of their
atoms in space due to the presence of a ring or
a carbon-carbon double bond
H
H
C
H3 C
H
C
CH 3
C
CH3
cis-2-Butene
mp -139°C, bp 4°C
H3 C
C
H
trans-2-Butene
mp -106°C, bp 1°C
5-8
5 IUPAC Nomenclature
1. Number the longest chain of carbon atoms that
contains the double bond in the direction that gives
the carbons of the double bond the lowest numbers
2. Locate the double bond by the number of its first
carbon
3. Name substituents
4. Number the carbon, locate and name substituents,
locate the double bond, and name the main chain.
6
5
4
3
2
1-Hexene
1
6
54
3
2
1
4-Methyl-1-hexene
5 4
32
1
2-Ethyl-4-methyl-1-pentene
5-9
5 Alkenyl Groups
Alkenyl
Group
Common
Name
Example
CH2 =
methylene
CH2
methylenecyclohexane
CH2 = CH-
vinyl
CH2 = CHCl
vinyl chloride
CH2 = CHCH 2 -
allyl
CH2 = CHCH2 Cl
allyl chloride
5-10
5 Common Names
 Despite
the precision and universal acceptance
of IUPAC nomenclature, some alkenes,
particularly low-molecular-weight ones, are
known almost exclusively by their common
names
CH3
CH2 = CH2
IUPAC: Ethene
Common: Ethylene
CH3 CH= CH 2
CH3 C= CH2
Propene 2-Methylpropene
Propylene
Isobutylene
5-11
5 Configuration
 The
cis,trans system: configuration is
determined by the orientation of atoms of the
main chain
H
CH2 CH3
C
CH3 CH 2
C
H
1
H
trans-3-Hexene
H3 C
2
C
3
CH3
C
4
CH( CH 3 ) 2
cis-3,4-Dimethyl-2-pentene
5-12
5 Configuration - E,Z
• uses priority rules (Chapter 3)
• if groups of higher priority are on the same side,
configuration is Z (German, zusammen)
• if groups of higher priority are on opposite sides,
configuration is E (German, entgegen)
higher
higher
C
lower
higher
C
lower
C
lower
Z (zusammen)
lower
C
higher
E (entgegen)
5-13
5 Configuration - E,Z
Example: name each alkene and specify its
configuration by the E,Z system
Cl
(a)
(b)
Cl
(c)
(d)
Cl
Br
5-14
5 Cis,Trans Isomerism
 Cycloalkenes
• configuration of the double bond in cyclopropene
through cycloheptene must be cis
• cyclopentene is only slightly puckered
5-15
5 Cis,Trans Isomerism
• the configuration of the double bond in cyclohexene
must be cis
• cyclohexene is puckered
5-16
5 Cis,Trans Isomerism
 Cycloalkenes
• trans-cyclooctene is the smallest trans-cycloalkene
stable at 25°C
• the cis isomer is 38 kJ (9.1 kcal)/mol more stable than
the trans isomer.
trans-Cyclooctene
(a pair of enantiomers)
5-17
5 Physical Properties
 Alkenes
are nonpolar compounds
 The only attractive forces between their
molecules are dispersion forces
 The physical properties of alkenes are similar to
those of alkanes
5-18
5 Terpenes
 Terpene:
a compound whose carbon skeleton
can be divided into two or more units identical
with the carbon skeleton of isoprene
head
1
2
3
4
tail
2-Methyl-1,3-butadiene
(Isoprene)
5-19
5 Terpenes
 Myrcene,
C10H16, a
component of bayberry wax
and oils of bay and verbena
 Menthol,
from peppermint
OH
5-20
5 Terpenes
• -Pinene, from turpentine
• camphor, from the camphor tree
O
5-21
5 Terpenes
 Vitamin
A (retinol)
• how many stereoisomers are possible for this terpene
alcohol?
OH
5-22
5 Prob 5.7
Using the VSEPR model, predict all bond angles about
each circled atom.
(a)
(c)
(b)
O
C-OH
CH 2 OH
(d)
(e) HC C-CH= CH 2
5-23
5 Prob 5.12
Name these alkenes and cycloalkenes.
Cl
(a)
(b)
(c)
Cl
(e)
(d)
(g)
(h)
F
F
F
(f)
Cl
Cl
F
(i)
5-24
5 Prob 5.13
Arrange the groups in each set in order of increasing
priority.
(a) - CH 3
(b) - OCH 3
(c) - CH 3
-H
-Br
-CH 2 CH 3
-CH( CH3 ) 2
-CH 2 OH
-B( CH 2 CH 3 ) 2
-CH 2 NH 2
-H
-CH 2 Br
5-25
5 Prob 5.14
Assign an E or Z configuration to each compound. Also
assign a cis or trans configuration to each.
H
COOH
C
(a)
H OOC
C
H OOC
(b)
H
Fumaric acid
COOH
C
H
C
CH2 COOH
Aconitic acid
5-26
5 Prob 5.16
For each molecule that shows cis,trans isomerism, draw
the cis isomer.
CH3
(a)
CH3
CH3
(b)
CH3
CH3
CH3
(d)
(c)
CH3
CH3
5-27
5 Prob 5.17
Draw structural formulas for all compounds of molecular
formula C5H10 that are
(a) alkenes that do not show cis,trans isomerism.
(b) alkenes that do show cis,trans isomerism.
(c) cycloalkanes that do not show cis,trans isomerism.
(d) cycloalkanes that do show cis,trans isomerism.
5-28
5 Prob 5.19
Draw a structural formula of a bromoalkene of molecular
formula C5H9Br that shows
(a) neither E,Z isomerism nor chirality.
(b) E,Z isomerism but not chirality.
(c) chirality but not E,Z isomerism.
(d) both chirality and E,Z isomerism.
5-29
5 Prob 5.23
Which alkenes exist as pairs of cis,trans isomers? For
each that does, draw the trans isomer.
(a) CH2 = CHBr
(b) CH3 CH= CHBr
(c) BrCH= CHBr
(d) ( CH3 ) 2 C= CHCH3
(e) ( CH3 ) 2 CH CH = CH CH3
5-30
5 Prob 5.24
Show that four stereoisomers are possible for this
compound. Draw the stereoisomer with an E
configuration at the double bond and an R configuration
at the stereocenter.
OH
CH3 -CH= CH-CH- CH3
3-Penten-2-ol
5-31
5 Prob 5.28
Show that warburganal is a terpene. Label all
stereocenters and state how many stereoisomers are
possible for a molecule of this structure.
Warburganal
[]25
D -260
CHO
H3 C
OH
CHO
H
H3 C CH3
5-32
5 Prob 5.29
Show that santonin is a terpene. Label all stereocenters
and tell how many stereoisomers are possible for the
molecule of this structure.
CH3
CH3
O
CH3
O
O
5-33
5 Prob 5.30
(a) Show that zoapatanol is a terpene.
(b)Specify the configuration at the double bond to the
seven-membered ring.
(c) How many stereoisomers are possible for this
molecule?
OH
O
HO
O
5-34
5 Prob 5.31 Pyrethrin II
Label all stereocenters in pyrethrin II. How many
stereoisomers are possible for this molecule?
O
H 3 COC
H
CH3
O
H
H3 C
H
CH3
O
C
O
H
CH3
5-35
5 Prob 5.31 Pyrethrosin
Show that pyrethrosin is a terpene. Label all
stereocenters in pyrethrosin and state how many
stereoisomers are possible for it.
H
O
H
O
O
CH3
CH 3
O
H
CH 2
CCH 3
O
5-36
5
Alkenes I
End Chapter 5
5-37