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Chapter 12: Alkenes and Alkynes

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Chapter 12: Alkenes and Alkynes Alkene: A hydrocarbon that contains one or more carbon-carbon double bonds. Ethylene is the simplest alkene. Alkyne: A hydrocarbon ... – PowerPoint PPT presentation

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Title: Chapter 12: Alkenes and Alkynes


1
Chapter 12 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.

2
Chapter 12 Alkenes and Alkynes
  • Cis-trans isomerism Double Bond No Free
    rotation!!!
  • 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.

3
Chapter 12 Alkenes and Alkynes
  • 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.

4
Chapter 12 Alkenes and Alkynes
  • To name an alkyne
  • Follow the same rules as for alkenes, but use the
    ending -yne to show the presence of the triple
    bond.

Common names are still used for some alkenes and
alkynes, particularly those with low molecular
weight.
5
Chapter 12 Alkenes and Alkynes
  • 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.

6
Chapter 12 Alkenes and Alkynes
  • 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).

Alkenes and alkynes physical properties are very
similar to the alkanes properties
7
Chapter 12 Alkenes and Alkynes
  • Terpene A compound whose carbon skeleton can be
    divided into five-carbon units identical with the
    carbon skeleton of isoprene.
  • Terpenes illustrate an important principle of the
    molecular logic of living systems.
  • In building large molecules, small subunits are
    bonded together by a series of enzyme-catalyzed
    reactions and then chemically modified by
    additional enzyme-catalyzed reactions.

8
Chapter 12 Terpenes
9
Chapter 12 Alkenes and Alkynes
The most common reaction is addition to the
double bond.
10
Chapter 12 Reactions
  1. Combustion (but we already spoke about it in
    chp.11)
  2. Addition to the double bond.
  3. Polymerization

11
Chapter 12 Alkenes and Alkynes
  • 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.

12
HX
  • Addition of HX (HCl, HBr, or HI) to an alkene
    gives a haloalkane.
  • 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.
  • Markovnikovs rule H adds to the less
    substituted carbon and X to the more substituted
    carbon.

13
HX
  • Chemists account for the addition of HX to an
    alkene by a two-step reaction mechanism.
  • We use curved arrows to show the repositioning of
    electron pairs during a chemical reaction.
  • The tail of an arrow shows the origin of the
    electron pair (either on an atom or in the double
    bond).
  • The head of the arrow shows its new position.
  • Curved arrows show us which bonds break and which
    new ones form.

14
HX
  • Step 1
  • Reaction of the carbon-carbon double bond with H
    gives a secondary (2) carbocation intermediate,
    a species containing a carbon atom with only
    three bonds to it and bearing a positive charge.
  • Step 2
  • Reaction of the carbocation intermediate with
    chloride ion completes the addition.

15
H20
  • 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.

16
H20
  • Step 1
  • Step 2
  • Step 3

17
Cl2 / Br2
  • Addition takes place readily at room temp.
  • 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.

18
H2
  • Virtually all alkenes add H2 in the presence of
    a transition metal catalyst, commonly Pd, Pt, or
    Ni.

19
H2
?
lt Wilhelm Normann invented what he called fat
hardening. Yoday known as Hydrogenation
The process of hydrogenation adds hydrogen
atoms to cis-unsaturated fats, eliminating double
bonds and making them into partially or
completely saturated fats. However, partial
hydrogenation converts a part of cis-isomers into
trans-unsaturated fats instead of hydrogenating
them completely .
Coronary heart disease
http//en.wikipedia.org/wiki/Trans_fat
20
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.

polypropene (polypropylene)
21
Polymerization
  • 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.

22
Polymerization
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