Chapter 12: Alkenes and Alkynes

K. DunlapChem 104

Alkene:Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds.– ethylene is the simplest alkene.

Alkyne:Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds.– acetylene is the simplest alkyne.

Alkenes and Alkynes

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.

Trigonal Planar

– 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.

Cis-Trans Isomerism Cis-Trans Isomerism

cis = same side trans = opposite side

– 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 -eneene 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.

Naming Alkenes

Examples:

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

Naming Alkynes

Linear compound

Names this compounds:

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

Examples of ALKENES

– 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.

Physical Properties of Alkenes and Alkynes

The most common reaction is addition:

• 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.

• From the perspective of the organic chemical industry, the single most important reaction of alkenes is polymerization:– polymer:polymer: Greek: poly, many and meros, part– monomer:monomer: Greek: mono, single and meros, part

Polymers

– 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.

Polymers Cont…

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 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.

Problems with PolymersProblems 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

SolutionsSolutions1) Degradable plastics

• Incorporation of light sensitive molecules that can be degraded by UV light

• Incorporation of biodegradable polymers that bacteria can decompose

2) Reducing plastic usage

3) Recycling• Different types of plastics need to be sorted• Lesser quality plastic• Economical

Polymer CodesPolymer Codes