William H. Brown
Christopher S. Foote
Brent L. Iverson
Eric Anslyn
http://academic.cengage.com/chemistry/brown
Chapter 5
Alkenes
William H. Brown • Beloit College
5-1
Unsaturated Hydrocarbons
Unsaturated hydrocarbon: Contains one or more
multiple bonds.
‹ Alkene: Contains a carbon-carbon double bond
and has the general formula CnH2n.
‹
‹
Alkyne: Contains a carbon-carbon triple bond and
has the general formula CnH2n-2.
5-2
Unsaturated Hydrocarbons
‹ Arenes:
Benzene and its derivatives (Ch 21-22)
• We do not study arenes until Chapters 21 & 22.
• However, 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 6-20.
5-3
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°.
5-4
Structure of Alkenes
• it takes approximately 264 kJ (63 kcal)/mol to break
the pi bond in ethylene; that is, to rotate one carbon
by 90° with respect to the other so that there is no
overlap between 2p orbitals on adjacent carbons.
5-5
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 (Chapter
2) or a carbon-carbon double bond.
5-6
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.
5-7
Index of Hydrogen Deficiency
• 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.
5-8
Index of Hydrogen Deficiency
Problem: isopentyl acetate has a molecular formula of
C7H14O2. Calculate its IHD.
• reference hydrocarbon C7H16
• IHD = (16-14)/2 = 1
Problem: calculate the IHD for niacin, molecular formula
C6H6N2O.
• reference hydrocarbon C6H14
• IHD = 5
5-9
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 carbons, locate and name substituents,
locate the double bond, and name the main chain.
5-10
Common Names
‹ Despite
the precision and universal acceptance
of IUPAC nomenclature, some alkenes,
particularly low-molecular-weight alkenes, are
known almost exclusively by their common
names.
5-11
Common Names
• the common names methylene, vinyl, and allyl are
often used to show the presence of the following
alkenyl groups:
5-12
The Cis,Trans System
‹ Configuration
is determined by the orientation of
atoms of the main chain.
5-13
The E,Z System
• Uses priority rules (Chapter 3).
• 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).
5-14
The E,Z System
‹
Example: Name each alkene and specify its configuration
by the E,Z system.
a)
b)
c)
d)
Z
E
E
Z
a) Z-(3,4-dimethyl)2-pentene
b) E-2-chloro-2-pentene
c) E-1-chloro-2,3-dimethyl-2-pentene
d) Z-1,1-Bromo-chloro-1-propene
5-15
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.
5-16
Cis,Trans Isomerism
• Trans-cyclooctene is the smallest trans cycloalkene
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.
5-17
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 to more generally as polyenes.
• Following are three dienes.
5-18
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.
5-19
Dienes, Trienes, and Polyenes
• Vitamin A is 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.
5-20
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.
5-21
Terpenes
‹ Terpene:
A compound whose carbon skeleton
can be divided into two or more units identical
with the carbon skeleton of isoprene.
5-22
Terpenes
‹ Myrcene,
C10H16, a component of bayberry wax
and oils of bay and verbena.
‹ Menthol,
from peppermint
5-23
Terpenes
• α-Pinene, from turpentine
• camphor, from the camphor tree
5-24
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.
• 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.
5-25
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.
5-26
Fatty Acids
• the four most abundant fatty acids
5-27
Fatty Acids
• The 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.
5-28
Fatty Acids
• Cis double bonds create 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.
5-29
Alkenes:
Structure and
Nomenclature
End Chapter 5
5-30