Chapter 12: Unsaturated Hydrocarbons

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Chapter 12Unsaturated Hydrocarbons
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• UNSATURATED HYDROCARBONS
• contain carbon-carbon multiple bonds.
• Alkenes CC double bonds
• Alkynes triple bonds
• Aromatics benzene rings

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• NAMING ALKENES
• Step 1 Name the longest chain that contains the
  CC bond. Use the IUPAC root and the ene
  ending.
• Step 2 Number the longest chain so the CC bond
  gets the lowest number possible.
• Step 3 Locate the CC bond with the
  lowest-numbered carbon.

Examples 1 2 3 4 CH3-CHCH-CH3
2-butene 1 2 3 4 5
6 CH3-CH2-CH2-CHCH-CH3 2-hexene
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• Step 4 Locate and name attached groups.
• Step 5 Combine all the names.

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• NAMING ALKENES WITH MULTIPLE DOUBLE BONDS
• Step 1 Follow the same naming instructions for
  alkenes with one double bond, except use the
  endings ?diene, ?-triene, and the like to denote
  the number of double bonds.
• Step 2 Indicate the location of all the multiple
  bonds, including those with rings.
• EXAMPLE

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• THE GEOMETRY OF ALKENES
• In CC bonds, sp2 hybrid orbitals are formed by
  the carbon atoms, with one electron left in a 2p
  orbital. A representation of sp2 hybridization
  of carbon

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• During hybridization, two of the 2p orbitals mix
  with the single 2s orbital to produce three sp2
  hybrid orbitals. One 2p orbital is not
  hybridized and remains unchanged.

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• This gives a planar shape for the sp2 bonding
  orbitals with the unhybridized p orbital
  perpendicular to the plane of the three sp2
  hybridized orbitals.

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• The planar geometry of the sp2 hybrid orbitals
  and the ability of the 2p electron to form a pi
  bond bridge locks the CC bond firmly in place.

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• Because there is no free rotation about the CC
  bond, geometric isomerism is possible.
• cis- isomers have two similar or identical groups
  on the same side of the double bond.
• trans- isomers have two similar or identical
  groups on opposite sides of the double bond.

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• PHYSICAL PROPERTIES OF ALKENES
• Similar to alkanes
• Nonpolar
• Insoluble in water
• Soluble in nonpolar solvents
• Less dense than water
• Unpleasant, gasoline-like odors

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• ALKENE REACTIONS
• Alkenes are quite chemically reactive
• Alkene reactions follow the pattern
• These reactions are called addition reactions.

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• HALOGENATION
• Halogenation (addition) reactions produce
  haloalkanes or alkylhalides.

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• HYDROGENATION
• Hydrogenation (addition) reactions can occur in
  the presence of a catalyst (Pt, Pd, or Ni).
• The hydrogenation of vegetable oils is an
  important commercial process.

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• MARKOVNIKOVS RULE
• Unsymmetrical alkene addition reactions follow
  Markovnikovs rule When a molecule of H-X adds
  to an alkene, the H attaches to the carbon
  already bonded to the most hydrogens. The rich
  get richer.

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• ADDITION OF SIMPLE ACIDS
• Addition of simple acids following Markovnikovs
  rule

CH2 CH CH3 HBr ? CH2 CH CH3


H Br
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• HYDRATION
• Hydration (addition of water) reactions follow
  Markovnikovs rule
• This reaction requires an acid catalyst.

H2SO4
CH2 CH CH3 H2O ? CH2 CH CH3


H OH
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• ADDITION POLYMERIZATION Dont memorize
• An addition polymer is a polymer formed by the
  linking together of many alkene molecules through
  addition reactions.

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• POLYMERIZATION Dont memorize
• Polymers very large molecules made up of
  repeating units
• Monomer the starting material that becomes the
  repeating units of a polymer

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• COPOLYMER Dont memorize
• An addition polymer formed by the reaction of two
  different monomers is a copolymer.

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• ALKYNES
• Ethyne (commonly called acetylene) is the
  simplest alkyne and is used as a fuel for torches
  and in making plastics.

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• Alkynes are named in exactly the same ways as
  alkenes, except the ending yne is used.
• Examples

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• THE GEOMETRY OF ALKYNES
• In CC bonds, sp hybrid orbitals are formed by
  the carbon atoms, with two electrons left in
  unhybridized 2p orbitals. A representation of sp
  hybridization of carbon

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• PHYSICAL PROPERTIES OF ALKYNES
• Similar to alkanes and alkenes
• Nonpolar
• Insoluble in water
• Soluble in nonpolar solvents
• Less dense than water
• Low melting and boiling points
• CHEMICAL PROPERTIES OF ALKYNES
• Similar to alkenes
• React by addition reaction with Br2, H2, HCl, H2O
• Require twice as many moles of addition reagent
  as alkenes in reactions that go on to completion

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• BENZENE
• Aromatic compounds contain the benzene ring or
  one of its structural relatives.
• Aliphatic compounds dont contain this structure.

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• In benzene, the six p orbital bonding electrons
  of the sp2 hybridized carbon atoms can move
  freely around the ring.
• A hybrid orbital view of the benzene structure

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• When two groups are attached to the benzene ring,
  their positions can be designated by the prefixes
  ortho (o), meta (m), and para (p).

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• When two or more groups are attached, their
  positions can be indicated by numbering the ring
  so as to obtain the lowest possible numbers for
  the attachment positions.

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• PHYSICAL PROPERTIES OF AROMATIC COMPOUNDS
• Similar to alkanes and alkenes
• Nonpolar
• Insoluble in water
• Hydrophobic
• CHEMICAL PROPERTIES OF AROMATIC COMPOUNDS
• Aromatic rings are relatively stable chemically
  and often remain intact during reactions
• Benzene does not react like alkenes and alkynes
• Benzene does undergo substitution reactions, in
  which a ring hydrogen is replaced by some other
  group