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.
Ethylene
(an alkene)
Acetylene
(an 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 carboncarbon 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 cis-trans 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. Cis-2,3-dimethylcyclohexene
b. 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.
Naming Alkynes

Name the following compounds
Common Names
Common names are still used for some alkenes
and alkynes, particularly those with low
molecular weight.
Ethene
Ethylene
Ethyne
Acetylene
Propene
Propylene
Propyne
Methylacetylene
2-
2-Methylpropene
Isobutylene
2-butyne
Dimethylacetylene
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
Table 3.1
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
Reactions of Alkenes
◦ Reaction is regioselective. One direction of bond forming
(or bond breaking) occurs in preference to all other
directions.
◦ 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
Also called Halogenation
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 H2—Reduction
Also called Hydrogenation
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 long-chain
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).
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.