This document provides an overview of alkenes and alkynes. It defines alkenes as hydrocarbons containing one or more carbon-carbon double bonds, with ethylene as the simplest example. Alkynes are defined as hydrocarbons with one or more carbon-carbon triple bonds, with acetylene as the simplest example. The document then discusses structural features, naming conventions, common reactions such as addition, and important industrial polymers derived from alkenes such as polyethylene.

1. Chapter 12: Alkenes and Alkynes
Chem 104
K. Dunlap

2. 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.

3. Trigonal Planar
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.

4. 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.

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6. Naming Alkenes
– 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; substituted groups are located
and named.

7. Examples:

8. Naming Alkynes
– follow the same rules as for alkenes, but use the
ending -yne to show the presence of the triple
bond.

9. Linear compound

10. Names this compounds:

11. • Common names are still used for some
alkenes and alkynes, particularly those of low
molecular weight.

12. Examples of ALKENES

13. 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
liquids.
– Alkenes and alkynes that are liquid or solid at
room temperature have densities less than 1.0
g/mL; they float on water.

14. The most common reaction is addition:

15. • 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.

16. Polymers
• 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

17. Polymers Cont…
– 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.

18. following a section of polypropene (polypropylene)

19. • 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% 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.

22. 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
problems
2) Depleated petroleum reserves

23. Solutions
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
2) Recycling
• Different types of plastics need to be sorted
• Lesser quality plastic
• Economical

24. Polymer Codes