Alkenes:
Structure and
Nomenclature
Unit 6
6-1
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)
.
6-2
Unsaturated Hydrocarbons
 Arenes:
benzene and its derivatives
H
H
C C
H C
C H
C6 H5 -
Ph-
C C
H
H
Benzene
Alternative rep res entations
for th e ph enyl grou p
• Basically unreactive
6-3
Structure of Alkenes
A
double bond consists of
• one sigma bond formed by the overlap of sp2 hybrid
orbitals and one pi bond formed by the overlap of
parallel 2p orbitals
• the two carbon atoms of a double bond and the four
atoms bonded to them lie in a plane, with bond angles
of approximately 120°
6-4
Cis,Trans Isomerism in Alkenes
 Cis,trans
isomers: isomers that have the same
connectivity but a different arrangement of their
atoms in space due to the presence of either a
ring or a carbon-carbon double bond
H
H
C
C
H3 C
CH3
cis -2-Buten e
mp -139°C, bp 4°C
H
CH3
C
C
H3 C
H
t rans-2-Butene
mp -106°C, b p 1°C
6-5
Index of Hydrogen Deficiency
 Index
of hydrogen deficiency (IHD): the sum of
the number of rings and pi bonds in a molecule
 To determine IHD, compare the number of
hydrogens in an unknown compound with the
number in a reference hydrocarbon of the same
number of carbons and with no rings or pi bonds
• the molecular formula of the reference hydrocarbon is
CnH2n+2
6-6
Index of Hydrogen Deficiency
IDH =
(H reference - Hmolecule)
2
• for each atom of a Group 7 element (F, Cl, Br, I), add
one H
• no correction is necessary for the addition of atoms of
Group 6 elements (O,S) to the reference hydrocarbon
• for each atom of a Group 5 element (N, P), subtract one
hydrogen
6-7
Index of Hydrogen Deficiency
Problem: isopentyl acetate has a molecular formula of
C7H14O2. Calculate its IHD
O
• reference hydrocarbon C7H16
O
• IHD = (16-14)/2 = 1
Isopentyl acetate
Problem: calculate the IHD for niacin, molecular formula
O
C6H6N2O
N H2
• reference hydrocarbon C6H16
• IHD = (16 - 6)/2 = 5
N
Niacin
6-8
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
4
5
3
2
1-Hexene
6
1
5
4
2
3
1
4-Methyl-1-hexen e
5
4
3
2
1
2-Eth yl-4-methyl-1-pen tene
6-9
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
CH
3
CH2 = CH2
IUPAC: Ethene
Common: Ethylene
CH3 CH= CH 2
CH3 C= CH2
Propene
Propylene
2-Methylpropene
Isobutylene
6-10
Common Names
• the common names methylene, vinyl, and allyl are
often used to show the presence of the following
alkenyl groups
Alk enyl
Grou p
Common
N ame
Example
CH2=
Methylidene
Methylene
H2 C
CH2=CHEth enyl
Vin yl
CH2 =CH
Eth enylcyclop entane
(Vin ylcyclopen tane)
CH2=CHCH2
Allyl
CH2 =CHCH2
3-Propenylcyclopentane
(Allylcyclopentane)
3-Propenyl
IUPA C N ame
(Common name)
Methylidenecyclopen tane
(Methylenecyclopentan e)
6-11
The Cis,Trans System
 Configuration
is determined by the orientation of
atoms of the main chain
H
CH2 CH3
C
CH3 CH
C
H
1
H
trans
2 -3-Hexene
H3 C
2
C
3
C
CH3
4
CH( CH3 ) 2
cis -3,4-D imethyl-2-penten e
6-12
The E,Z System
• uses priority rules
• if groups of higher priority are on the same side, the
configuration is Z (German, zusammen)
• if groups of higher priority are on opposite sides, the
configuration is E (German, entgegen)
h igh er
h igh er
C
low er
h igh er
C
low er
C
low er
Z (zu sammen )
low er
C
higher
E (en tgegen)
6-13
The E,Z System

Example: name each alkene and specify its configuration
by the E,Z system
Cl
(a)
(b)
Cl
Cl
(c)
(d)
Br
6-14
Cis,Trans Isomerism
 Cycloalkenes
• in small-ring cycloalkenes, the configuration of the
double bond is cis
• these rings are not large enough to accommodate a
trans double bond
H
H
Cyclobuten e
H
H
Cyclop entene
H
H
Cyclohexen e
H
H
Cycloh eptene
6-15
Cis,Trans Isomerism
• trans-cyclooctene is the smallest trans cyclooctene
that has been prepared in pure form and is stable at
room temperature
• the cis isomer is 38 kJ (9.1 kcal)/mol more stable than
the trans isomer
• the trans isomer is chiral even though it has no chiral
center
trans- Cyclooctene
(a pair of enantiomers)
6-16
Dienes, Trienes, and Polyenes
 For
alkenes containing two or more double
bonds, change the infix -en- to -adien-, -atrien-,
etc.
• those containing several double bonds are often
referred more generally as polyenes
• following are three dienes
1,4-Pen tadien e
2-Methyl-1,3-bu tadiene
(Isoprene)
1,3-Cyclopentadien e
6-17
Dienes, Trienes, and Polyenes
• for alkenes with n double bonds, each of which can
show cis,trans isomerism, 2n stereoisomers are
possible
• example: 22 = 4 cis,trans isomers are possible for 2,4heptadiene
1
D oub le bond
C2 -C3
C4 -C5
tran s
tran s
cis
cis
2
2 3
4
4
5
6 7
tran s
(2E,4E)-2,4-Heptadien e
cis
tran s
2
cis
4
(2Z,4E)-2,4-Heptad iene
(2E,4Z)-2,4-Hep tadiene
2
4
(2Z,4Z)-2,4-Hep tadiene
6-18
Dienes, Trienes, and Polyenes
• vitamin A, a biologically important compound for
which a number of cis,trans isomers is possible
• there are four double bonds about which cis,trans
isomerism is possible, for 24 = 16 stereoisomers
CH2 OH
V itamin A (retinol)
6-19
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
6-20
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
4
tail
3
2-Methyl-1,3-butad iene
(Isoprene)
6-21
Terpenes
 Myrcene,
C10H16, a component of bayberry wax
and oils of bay and verbena
 Menthol,
from peppermint
OH
6-22
Terpenes
• -Pinene, from turpentine
• camphor, from the camphor tree
O
6-23
Fatty Acids
 Animal
fats and vegetable oils are both triesters
of glycerol, hence the name triglyceride
• hydrolysis of a triglyceride in aqueous base followed
by acidification gives glycerol and three fatty acids
O
O CH2 OCR
R'COCH
1 . NaOH, H2 O
O
2 . HCl, H2 O
CH2 OCR''
A triglyceride
(a triester of glycerol)
RCOOH
CH2 OH
HOCH
CH2 OH
1,2,3-Prop anetriol
(Glycerol)
+
R'COOH
R''COOH
Fatty acids
• fatty acids with no C=C double bonds are called
saturated fatty acid
• those with one or more C=C double bonds are called
unsaturated fatty acids
6-24
Fatty Acids
• the most common fatty acids have an even number of
carbons, and between 12 and 20 carbons in an
unbranched chain
• the C=C double bonds in almost all naturally occurring
fatty acids have a cis configuration
• the greater degree of unsaturation, the lower the
melting point
• triglycerides rich in unsaturated fatty acids are
generally liquid at room temperature and are called
oils
• triglycerides rich in saturated fatty acids are generally
semisolids or solids at room temperature and are
called fats
6-25
Fatty Acids
• the four most abundant fatty acids
COOH Stearic acid (18:0)
(mp 70°C)
COOH Oleic acid (18;1)
(mp 16°C)
COOH Linoleic acid (18:2)
(mp-5°C)
COOH Lin olen ic acid (18:3)
(mp -11°C)
6-26
Fatty Acids
• carbon chains of saturated fatty acids exist largely in
the staggered, anti-conformation
• because of their high degree of order, they pack
together well and are held together by dispersion
forces
• as a result both saturated fatty acids and triglycerides
derived from them are solids at room temperature
• following is a saturated triglyceride
6-27
Fatty Acids
• cis double bonds place kinks in the chains of
unsaturated fatty acids
• unsaturated fatty acids and the triglycerides derived
from them do not pack as well in a crystal lattice as
their saturated counterparts, and have weaker
dispersion forces between their molecules
• butter fat, for example, has a high content of saturated
fatty acids and is a solid at room temperature
• salad oils (from plant oils) have a high content of
polyunsaturated fatty acids and are liquid at room
temperature
6-28