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 that contains one or more
  carbon-carbon triple bonds.
• Acetylene is the simplest alkyne.

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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.

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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.

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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.
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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.

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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
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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.

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Chapter 12 Terpenes
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Chapter 12 Alkenes and Alkynes
The most common reaction is addition to the
double bond.
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Chapter 12 Reactions

1. Combustion (but we already spoke about it in
   chp.11)
2. Addition to the double bond.
3. Polymerization

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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.

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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.

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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.

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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.

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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.

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H20

• Step 1
• Step 2
• Step 3

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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.

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H2

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

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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
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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.

polypropene (polypropylene)
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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.

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Polymerization