Alkenes and Alkynes
Alkenes - Synthesis and Reactions
 Structure and Properties
 Nomenclature
 Synthesis of Alkenes
 Reactions of Alkenes
Structure and Properties
 Alkene:
 hydrocarbon with one or more C-C double
bond
 also called olefin
H
H
 C=C consists of 1 s bond
and 1 p bond
H
C
C
H
Ethylene
ethene
Structure and Properties
 C=C is a functional group
 BDE (s bond) = ~83 kcal/mol
 BDE (p bond) = ~ 63 kcal/mol
 p bond is weaker than s bond
 reactions take place at the p bond
 sp2 hybridization
 trigonal planar
Structure and Properties
 Trigonal planar geometry
 approximately 120o bond angle for alkenes
 vs. ~109.5o bond angle for alkanes
 Double bonds are shorter than single bonds.
Structure and Properties
 Alkanes
 saturated hydrocarbons
 each C has the maximum # of H’s possible
CH3CH3
 Alkenes
CH2 CH2
 unsaturated hydrocarbons
 fewer H atoms per C than an alkane
 capable of adding hydrogenCH3CH3
CH2 CH2
Structure and Properties
 Element of unsaturation
 a structural feature that reduces the number
of hydrogen atoms by 2 relative to the
corresponding alkane
 ring
 p bond
C6H14
CH3CH
3
CH
6
12
C6H12
 used to help determine possible structures
CH2 CH2
Structure and Properties
 Elements of unsaturation = 1/2 (2C + 2 - H)
 C6H12
 EU = ½ (2x6 +2 – 12) = 1
Structure and Properties
Example: Calculate the elements of unsaturation
for C4H8. Draw 5 structural isomers with this
formula.
Structure and Properties
5 structural isomers of C4H8
Structure and Properties
 To determine the elements of unsaturation for
compounds with heteroatoms (atoms other than
C and H):
 use same formula as given previously
BUT
 Each halogen counts as a hydrogen atom
 Ignore any oxygen atoms
 Each nitrogen counts as 1/2 C
Structure and Properties
Example: Calculate the elements of unsaturation
for C6H9ClO. Draw at least 4 structural isomers.
Structure and Properties
 4 possible structural isomers
The structures you draw should contain
reasonable functional groups….i.e. don’t
make up strange functional groups!
IR
 Alkenes have two characteristic peaks in the
IR:
 sp2 C-H at >3000 cm-1
 C=C at ~1620 – 1680 cm-1
 Conjugated alkene C=C is at lower
frequency
 Isolated alkene C=C is at higher frequency
 C=C peak has variable intensity but is
typically weak to moderate.
sp2
C-H
Alkene
C=C
sp3
C-H
CH3

Nomenclature
CH
=C
2
CH3
Alkenes can be named
CH
=CHCH
or
common
names.
2
3
using eitherCH
IUPAC
names
3
CH2=C
CH3
CH2=CHCH3
CH2=CH2
ethene
ethylene
propene
propylene
CH
=CH
2
CH2=C
2
CH3
CH3
2-methylpropene
CH2=CHCH3
isobutylene
Blue = IUPAC
Red = common
Nomenclature
 To name alkenes:
 Find the longest continuous chain (or ring)
that contains the double bond.
 Base name = name of corresponding alkane
or cycloalkane with ending changed to “ene”
Cl
Hexane
cyclopentane
Br
CH3
hexene
cyclopentene
Nomenclature
 To name alkenes:
 Number from the end of the chain closest to
Br
the double bond
 the double bond is given the lower number
of the two double-bonded carbons
 Cycloalkenes: double bond is always
CH
3
between
carbons 1 and 2
2
1
6
4
3
5
5
1
Cl
2
4
3
Br
CH3
Nomenclature
CH3 the number of the double bond in front of
 Place
Br the base name of the alkene (omit the number
for cycloalkenes unless > 2 double bonds)
Cl
a substituted
2-hexene
a substituted
Br
hex-2-ene
CH3
Br
a substituted
cyclopentene
Cl
Newer IUPAC system places the
position number just before the
“ene” ending
Br
Br
Nomenclature
CH
3
 Name
substitutent groups
in alkanes.
Cl
Br
Br
CH3
in the same manner as
trans-6-chloro-5-methyl-2-hexene
or
trans-6-chloro-5-methylhex-2-ene
3-bromo-4-methylcyclopentene
Nomenclature
 Alkenes as substitutents (often named using
common names)
CH2
CH
CH2
CH2
CHCH2Cl
Methylene group
3-methylenecyclohexene
vinyl group
3-vinyl-1,5-hexadiene
3-vinylhexa-1,5-diene
Allyl group
Allyl chloride
Nomenclature
 For compounds that show geometric isomerism,
add the appropriate prefix:
 cis
 trans
OR
E
Z
 NOTE:
Cycloalkenes are assumed to be cis
unless otherwise indicated.
Nomenclature
 Cis/trans isomers
B
C
A
C C
C C
A
A
cis
C
B
A
trans
Cis: 2 identical groups located on the same
side of the double bond
Trans: 2 identical groups located on opposite
sides of the double bond
Nomenclature
Example: Name the following compounds.
Br
CH3
Nomenclature
Br
CH3
 Some compounds form geometric
C Cisomers that
cannot be named using the Cl
cis/transH
nomenclature
Br
CH3
Cl
C C
C C
Cl
CH3
H
Br
H
 Cis/trans
can’t be used:
Cl nomenclature
CH3
 two identical
groups are not attached to
C
C
adjacent
carbons
in the C=C
Br
H
Nomenclature
 The E-Z system of nomenclature for geometric
C
CC
C
CC
isomers:
 Break the double bond into two halves
Br
CH3
C
Cl
C
H
1BrBr
CC
2 ClCl
CH
1
CH
3 3
CC
HH 2
 Separately, assign priorities to the groups on
Cl
Cl
CH
CH3 in the Cl
CH
3
each
carbon
double
bond
the
3 using
C C
C CC(RC & S configuration
Cahn-Ingold-Prelog
rules
Br
BrBr
H
HH
rules)
Nomenclature
Br
CH3
C
Cl
C
H
 Z (Zusammen) isomer
C
 both high priority groups are on the same
side of the double bond
 similar to cis
1
1
BrC C CH3
2 C C2
 E (Entgegen) isomer
Cl
H
 high priority groups are on the opposite side
of the double bond
1
2
 similar to trans
C C
2
1
Br
Cl
C
C
CH3
H
(Z)-1-bromo-1-chloropropene
C
Nomenclature
 Naming alkenes with more than one double bond:
 Make sure that the longest chain includes as
many C=C as possible.
 2 C=C
diene
 3 C=C
triene
 4 C=C
tetraene
Br
a substituted
octatriene
Nomenclature
 Show the location of each double bond
Br
3-bromo-2, 4, 6-octatriene
3-bromoocta-2,4,6-triene
 Designate the isomer present for each double
bond (use location and E or Z)
Br
(2Z,4E,6E)-3-bromo-2,4,6-octatriene
(2Z,4E,6E)-3-bromoocta-2,4,6-triene
Nomenclature
Example: Name the following compounds.
Br
Br
Nomenclature
Example: Draw the following compounds.
cis-3-methyl-2-pentene
1-ethylcyclohexene
(2E, 4Z)-2,4-hexadiene
Remember: You must show the trigonal planar
geometry around the C=C.
Uses and Physical Properties
 Alkenes are important intermediates in the
synthesis of polymers, drugs, pesticides, and
other chemicals.
 Ethylene is used as a feedstock for:
 ethanol
 ethylene glycol (antifreeze)
 acetic acid
 Propylene is used as a feedstock for:
 isopropyl alcohol
 acetone
Uses and Physical Properties
 Alkenes are important “monomers” for the
production of polymers like poly(vinyl chloride),
and Teflon.
Uses and Physical Properties
 Physical Properties
 Similar to alkanes
 Density
 ~0.6 g/mL to ~ 0.7 g/mL
 Boiling Point
 increases with increasing MW
 decreases with branching
 Polarity
 relatively non-polar
 insoluble in water
Stability of Alkenes
Hheat of hydrogenation
H
 The
is used to compare
the relative
C Cstabilities of alkenes.
 Heat of hydrogenation:
H
C
CH3
3
 The heat released (DH) during a catalytic
HC
hydrogenation
3
 Catalytic hydrogenation:
the addition of H2
to a double (or triple) bond in the presence
of a catalyst
CH3CH=CHCH3 + H2
Pt
CH3CH2CH2CH3
Stability of Alkenes
 As the heat of hydrogenation becomes more
negative, the stability of the alkene decreases.
Stability of Alkenes
 More highly substituted double bonds are more
stable
 larger angular separation between the bulky
alkyl groups
Stability of Alkenes
 For acyclic alkenes, trans isomers are more
stable than cis isomers.
 Trans isomers of cycloalkenes with fewer than 8
carbons are unstable.
 Large amount of ring strain
 Because of ring strain, cycloalkenes with less
than 5 carbons in the ring are less stable than
those with 5 or more carbons.
Stability of Alkenes
Example: Which of the following alkenes is more
stable.
vs.
vs.