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Alkenes and Alkynes

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Chapter 3 Alkenes and Alkynes Polyethylene Figure 12.2 Fabrication of LDPE film. Alkenes and Alkynes Alkene: A hydrocarbon that contains one or more carbon-carbon ... – PowerPoint PPT presentation

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Title: Alkenes and Alkynes


1
Chapter 3
  • Alkenes and Alkynes

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
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.

4
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

5
Alkenes
6
Examples
  • Which of the following compounds can exist as
    cis-trans isomers?

7
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.

8
AlkenesIUPAC Names
  • Examples

9
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.

10
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).

11
Examples
  • Draw the condensed structure of the following
    compounds
  • a. 3,3-dimethylcyclohexene
  • b. trans-1-bromo-2,3-dimethyl-2-hexene

12
Examples
  • Give the IUPAC name for each of the following
    compounds

13
AlkynesIUPAC Names
  • Follow the same rules as for alkenes, but use
    the ending -yne to show the presence of the
    triple bond.

14
Examples
  • Name the following compounds

15
Common Names
  • Common names are still used for some alkenes and
    alkynes, particularly those with low molecular
    weight.

16
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.

17
Reactions of Alkenes
18
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.

19
Addition of HX
  • Addition of HX (HCl, HBr, or HI) to an alkene
    gives a haloalkane.
  • Acid-Catalyzed hydration reaction
  • H adds to one carbon of the CC and X to the
    other.
  • Reaction is regioselective. One direction of bond
    forming (or bond breaking) occurs in preference
    to all other directions.

20
Reactions of Alkenes
  • Markovnikovs rule H adds to the less
    substituted carbon and X to the more substituted
    carbon.

21
Examples
  • Give the major product obtained from the
    acid-catalyzed hydration of each of the following
    alkenes reactions

22
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.

23
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.

24
Addition of Cl2 and Br2
  • 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

25
Addition of H2Reduction
  • Virtually all alkenes add H2 in the presence of
    a transition metal catalyst, commonly Pd, Pt, or
    Ni.

26
Addition of H2Reduction
  • Figure 3.1 The addition of hydrogen to an alkene
    involving a transition metal catalyst.

27
Examples
  • Predict the products of the following reaction
    and determine the major product

28
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

29
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.

30
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).

31
Polymerization
32
Ethylene Polymers
33
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.

34
Polyethylene
  • Figure 12.2 Fabrication of LDPE film.
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