Chapter 12:
Unsaturated Hydrocarbons
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UNSATURATED HYDROCARBONS
• contain carbon-carbon multiple bonds.
Alkenes C=C double bonds
Alkynes triple bonds
Aromatics
benzene rings
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NAMING ALKENES
Step 1: Name the longest chain that contains the C=C
bond. Use the IUPAC root and the –ene ending.
Step 2: Number the longest chain so the C=C bond gets
the lowest number possible.
Step 3: Locate the C=C bond with the lowest-numbered
carbon.
Examples:
1
2 3 4
CH3-CH=CH-CH3
2-butene
1 2
3 4 5 6
CH3-CH2-CH2-CH=CH-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 ﾐ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 C=C 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 C=C bond firmly in place.
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• Because there is no free rotation about the C=C 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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MARKOVNIKOV’S RULE
• Unsymmetrical alkene addition reactions follow
Markovnikov’s 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 Markovnikov’s rule:
CH2 = CH – CH3 + HBr  CH2 – CH – CH3
|
|
H
Br
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HYDRATION
• Hydration (addition of water) reactions follow
Markovnikov’s rule:
H2SO4
CH2 = CH – CH3 + H2O  CH2 – CH – CH3
|
H
|
OH
• This reaction requires an acid catalyst.
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ADDITION POLYMERIZATION: Don’t memorize
• An addition polymer is a polymer formed by the linking
together of many alkene molecules through addition
reactions.
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POLYMERIZATION: Don’t 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: Don’t 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 C≡C 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 don’t 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
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