Chapter 3
Alkenes and Alkynes
Alkenes and Alkynes
Alkene:A hydrocarbon that contains one or more
carbon-carbon double bonds.
• Ethylene is the simplest alkene.
Alkyne:A hydrocarbon that contains one or more
carbon-carbon triple bonds.
• Acetylene is the simplest alkyne.
Alkenes
Structure:
◦ The VSEPR model predicts bond angles of 120° about
each carbon of a double bond.
◦ In ethylene, the actual angles are close to 120°.
◦ In substituted alkenes, angles about each carbon of the
double bond may be greater than 120° because of
repulsion between groups bonded to the double bond.
Alkenes
Cis-trans isomerism
• Because of restricted rotation about a carbon-carbon
double bond, an alkene with two different groups on
each carbon of the double bond shows cis-trans
isomerism.
• Aka as geometric isomers
Alkenes
Examples

Which of the following compounds can exist as cistrans isomers?
Alkenes—IUPAC Names
To name an alkene;
• The parent name is that of the longest chain that
contains the C=C.
• Number the chain from the end that gives the lower
numbers to the carbons of the C=C.
• Locate the C=C by the number of its first carbon.
• Use the ending -ene to show the presence of the
C=C
• Branched-chain alkenes are named in a manner
similar to alkanes in which substituted groups are
located and named.
Alkenes—IUPAC Names
Examples
Cycloalkenes
To name a cycloalkene:
• Number the carbon atoms of the ring double bond 1 and 2 in
the direction that gives the lower number to the substituent
encountered first.
• Note that it is not necessary to explicitly number the position of
the double bond in a cycloalkene as in linear alkenes.
• Number and list substituents in alphabetical order.
Cycloalkenes
• Alkenes that contain more than one double bond are
named as alkadienes, alkatrienes, and so forth.
• Those that contain several double bonds are referred
to more generally as polyenes (Greek: poly, many).
Examples

Draw the condensed structure of the following
compounds
a. 3,3-dimethylcyclohexene
b. trans-1-bromo-2,3-dimethyl-2-hexene
Examples

Give the IUPAC name for each of the following
compounds
Alkynes—IUPAC Names
Follow the same rules as for alkenes, but use the ending
-yne to show the presence of the triple bond.
Examples

Name the following compounds
Common Names
Common names are still used for some alkenes and
alkynes, particularly those with low molecular weight.
Physical Properties
• Alkenes and alkynes are nonpolar compounds.
• The only attractive forces between their molecules
are London dispersion forces.
• Their physical properties are similar to those of
alkanes with the same carbon skeletons.
• Alkenes and alkynes are insoluble in water but soluble
in one another and in nonpolar organic liquids.
• Alkenes and alkynes that are liquid or solid at room
temperature have densities less than 1.0 g/mL; they
float on water.
Reactions of Alkenes
Reactions of Alkenes
Most alkene addition reactions are exothermic.
◦ the products are more stable (lower in energy) than the
reactants.
◦ Just because they are exothermic doesn’t mean that alkene
addition reactions occur rapidly.
◦ Reaction rate depends on the activation energy.
◦ Many alkene addition reactions require a catalyst.
Addition of HX
Addition of HX (HCl, HBr, or HI) to an alkene gives a
haloalkane.
Acid-Catalyzed hydration reaction
◦ H adds to one carbon of the C=C and X to the other.
◦ Reaction is regioselective. One direction of bond forming (or
bond breaking) occurs in preference to all other directions.
Reactions of Alkenes
◦ Markovnikov’s rule: H adds to the less substituted carbon
and X to the more substituted carbon.
Examples

Give the major product obtained from the acidcatalyzed hydration of each of the following alkenes
reactions
Addition of H2O
Addition of water is
called hydration.
◦ Hydration is acid
catalyzed, most
commonly by H2SO4.
◦ Hydration follows
Markovnikov’s rule; H
adds to the less
substituted carbon and
OH adds to the more
substituted carbon.
Addition of Cl2 and Br2
Addition takes place readily at room temperature.
• Reaction is generally carried out using pure reagents, or
mixing them in a nonreactive organic solvent.
• Addition of Br2 is a useful qualitative test for the
presence of a carbon-carbon double bond.
• Br2 has a deep red color; dibromoalkanes are colorless.
Addition of Cl2 and Br2
• Addition of Br2 is a useful qualitative test for the
presence of a carbon-carbon double bond.
• Br2 has a deep red color; dibromoalkanes are
colorless
Addition of H2—Reduction
Virtually all alkenes add H2 in the presence of a transition
metal catalyst, commonly Pd, Pt, or Ni.
Addition of H2—Reduction
Figure 3.1 The addition of hydrogen to an alkene
involving a transition metal catalyst.
Examples

Predict the products of the following reaction and
determine the major product
Examples

Draw a line-angle for an alkene with the molecular
formula C5H10 that reacts with HCl to give the
indicated chloroalkane as the major product. More than
one alkene may give the same compound as the major
product
Polymerization
From the perspective of the organic chemical industry,
the single most important reaction of alkenes is
polymerization:
• polymer: Greek: poly, many, and meros, part; any longchain molecule synthesized by bonding together many
single parts, called monomers.
• monomer: Greek: mono, single and meros, part.
Polymerization
• Show the structure of a polymer by placing parentheses
around the repeating monomer unit.
• Place a subscript, n, outside the parentheses to indicate
that this unit repeats n times.
• The structure of a polymer chain can be reproduced by
repeating the enclosed structure in both directions.
• The following is a section of polypropene
(polypropylene).
Polymerization
Ethylene Polymers
Polyethylene
Low-density polyethylene (LDPE):
◦ A highly branched polymer; polymer chains do not
pack well and London dispersion forces between
them are weak.
◦ Softens and melts above 115°C.
◦ Approximately 65% of all LDPE is used for the
production of films for packaging and for trash bags.
High-density polyethylene (HDPE):
◦ Only minimal chain branching; chains pack well and
London dispersion forces between them are strong.
◦ Has higher melting point than LDPE and is stronger
◦ Can be blow molded to squeezable jugs and bottles.
Polyethylene
Figure 12.2
Fabrication of
LDPE film.