Alkenes%20and%20Alkynes

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Chapter 3

• Alkenes and Alkynes

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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.

Acetylene (an alkyne)
Ethylene (an alkene)
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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.

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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.
• Aka as geometric isomers

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Alkenes
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Examples

• Which of the following compounds can exist as
  cis-trans isomers?

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AlkenesIUPAC Names

• To name an alkene
• The parent name is that of the longest chain that
  contains the CC.
• Number the chain from the end that gives the
  lower numbers to the carbons of the CC.
• Locate the CC by the number of its first carbon.
• Use the ending -ene to show the presence of the
  CC
• Branched-chain alkenes are named in a manner
  similar to alkanes in which substituted groups
  are located and named.

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AlkenesIUPAC Names

• Examples

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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.

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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).

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Examples

• Draw the condensed structure of the following
  compounds
• a. Cis-2,3-dimethylcyclohexene
• b. 1-bromo-2,3-dimethyl-2-hexene

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Examples

• Give the IUPAC name for each of the following
  compounds

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AlkynesIUPAC Names

• Follow the same rules as for alkenes, but use
  the ending -yne to show the presence of the
  triple bond.

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Naming Alkynes

• Name the following compounds

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Common Names

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

Ethene Ethylene
2-
Propene Propylene
2-Methylpropene Isobutylene
Ethyne Propyne 2-butyne Acetylene Methyla
cetylene Dimethylacetylene
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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.

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Reactions of Alkenes
Table 3.1
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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 doesnt mean
  that alkene addition reactions occur rapidly.
• Reaction rate depends on the activation energy.
• Many alkene addition reactions require a catalyst.

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Addition of HX

• Addition of HX (HCl, HBr, or HI) to an alkene
  gives a haloalkane.
• Acid-Catalyzed hydration reaction

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Reactions of Alkenes

• Reaction is regioselective. One direction of bond
  forming (or bond breaking) occurs in preference
  to all other directions.
• Markovnikovs rule H adds to the less
  substituted carbon and X to the more substituted
  carbon.

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Examples

• Give the major product obtained from the
  acid-catalyzed hydration of each of the following
  alkenes reactions

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Addition of H2O

• Addition of water is called hydration.
• Hydration is acid catalyzed, most commonly by
  H2SO4.
• Hydration follows Markovnikovs rule H adds to
  the less substituted carbon and OH adds to the
  more substituted carbon.

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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.

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Addition of Cl2 and Br2
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Addition of H2Reduction

• Also called Hydrogenation
• Virtually all alkenes add H2 in the presence of
  a transition metal catalyst, commonly Pd, Pt, or
  Ni.

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Addition of H2Reduction

• Figure 3.1 The addition of hydrogen to an alkene
  involving a transition metal catalyst.

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Examples

• Predict the products of the following reaction
  and determine the major product

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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

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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.

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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).

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Ethylene Polymers
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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 115C.
• 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.