Frederick A. Bettelheim
William H. Brown
Mary K. Campbell
Shawn O. Farrell
Omar J. Torres
www.cengage.com/chemistry/bettelheim
Chapter 12
Alkenes and Alkynes
William H. Brown • Beloit College
Alkenes and Alkynes
Alkene: A hydrocarbon that contains one or more carboncarbon double bonds.
• Ethylene is the simplest alkene.
Alkyne: A hydrocarbon that contains one or more carboncarbon 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.
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
Alkynes—IUPAC Names
Follow the same rules as for alkenes, but use the ending
-yne to show the presence of the triple bond.
Common Names
Common names are still used for some alkenes and
alkynes, particularly those with low molecular weight.
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).
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.
• 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.
• Markovnikov’s rule: H adds to the less substituted
carbon and X to the more substituted carbon.
Addition of HX
Chemists account for the addition of HX to an alkene by a
two-step reaction mechanism.
• We use curved arrows to show the repositioning of
electron pairs during a chemical reaction.
• The tail of an arrow shows the origin of the electron
pair (either on an atom or in the double bond).
• The head of the arrow shows its new position.
• Curved arrows show us which bonds break and which
new ones form.
Common Mechanism Steps
Pattern 1: Add a proton
An acid is a proton donor and a base is a proton acceptor.
We can use curved arrows to show how a proton transfer
occurs.
Patterns 2: Take a proton away.
If we run the above reaction, in reverse it corresponds to
taking a proton away.
Common Mechanism Steps
• Pattern 3: Reaction of an electrophile and a nucleophile
to form a new covalent bond.
• An electrophile is an electron-poor species that can
accept a pair of electrons to form a new covalent bond.
• A nucleophile is an electron-rich species that can donate
a pair of electrons to form a new covalent bond.
Common Mechanism Steps
• Pattern 4: Reaction of a proton donor with a carbon-
carbon double bond to form a new covalent bond. The
double bond provides both of the electrons that forms the
new covalent bond. In the following reaction we show the
hydronium ion as the proton donor.
• While the above equation is the most accurate way to
show the proton transfer, we will simplify the equation to
just show the proton and formation of the new covalent
bond.
Addition of HCl to 2-Butene
Step 1: Add a proton
• Reaction of the carbon-carbon double bond with H+
gives a secondary (2°) carbocation intermediate, a
species containing a carbon atom with only three
bonds to it and bearing a positive charge.
Step 2: Reaction of an electrophile and a nucleophile to
form a new carbon-carbon bond.
• Reaction of the carbocation intermediate with chloride
ion completes the addition.
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 H2O
• Step 1: Add a proton
• Step 2: Reaction of an electrophile and a nucleophile to
form a new covalent bond.
Step 3; Take a proton away.
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 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 12.1 The addition of hydrogen to an alkene
involving a transition metal catalyst.
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).
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.
Polyethylene
Figure 12.3 Blow molding an HDPE container.
Chapter 12 Alkenes and Alkynes
End
Chapter 12