Chapter 12: 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.
– 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
– because of restricted rotation about a carboncarbon double bond, an alkene with two different
groups on each carbon of the double bond shows
– 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
– Branched-chain alkenes are named in a manner
similar to alkanes; substituted groups are located
Physical Properties of Alkenes and Alkynes
– 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
– Alkenes and alkynes that are liquid or solid at
room temperature have densities less than 1.0
g/mL; they float on water.
• Most alkene addition reactions are
– 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.
• From the perspective of the organic chemical
industry, the single most important reaction of
alkenes is polymerization:
– polymer: Greek: poly, many and meros, part
– monomer: Greek: mono, single and meros, part
– 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
following a section of polypropene (polypropylene)
• Low-density polyethylene (LDPE):
– a highly branched polymer; polymer chains do not pack
well and London dispersion forces between them are
– softens and melts above 115°C.
– approximately 65% 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.
Problems with Polymers
1) Disposal- their stability and resistance to
oxidation and attack by chemicals and bacteria
*Only 5% of plastics are recycled
*Account for 20% of solid waste volume
*Incineration releases toxic gases
*Plastisizers cause pollution and health
2) Depleated petroleum reserves
1) Degradable plastics
• Incorporation of light sensitive molecules
that can be degraded by UV light
• Incorporation of biodegradable polymers
that bacteria can decompose
1) Reducing plastic usage
• Different types of plastics need to be sorted
• Lesser quality plastic