CHAPTER 12
Unsaturated
Hydrocarbons
UNSATURATED HYDROCARBONS
• Unsaturated hydrocarbons contain one or more carboncarbon multiple bonds.
• Alkenes contain one or more carbon-carbon double
bonds (C=C ).
• Alkynes contain one or more carbon-carbon triple bonds
(C≡C).
• Aromatics contain benzene rings.
NAMING ALKENES & Alkynes
• Step 1: Name the longest chain that contains the C=C or C≡C bond. Use
the IUPAC root and the –ene ending or –yne ending.
• Step 2: Number the longest chain so the carbon atoms joined in the C=C
or C≡C bond get the lowest number possible.
• Step 3: Locate the C=C or C≡C bond with the lower-numbered carbon.
Examples:
1
2
3 4
CH3-CH=CH-CH3
2-butene
6 5
4
3 2 1
CH3-CH2-CH2-CH=CH-CH3
2-hexene
NAMING ALKENES (continued)
• Step 4: Locate and name attached groups.
• Step 5: Combine all the names.
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.
• EXAMPLES:
THE GEOMETRY OF ALKENES (continued)
• Because there is no free rotation about the C=C or 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.
THE GEOMETRY OF ALKENES (continued)
• Geometric isomers have different physical properties.
PHYSICAL PROPERTIES OF ALKENES
ALKENES
similar to alkanes
Nonpolar
Insoluble in water
Soluble in nonpolar solvents
Less dense than water
Unpleasant, gasoline-like
odors
Increasing # carbonsincreasing boiling point,
melting point & density
Very reactive
React by addition reaction
with Br2, H2, HCl, H2O
ALKYNES
Similar to alkanes & alkenes
Nonpolar
Insoluble in water
Soluble in nonpolar solvents
Less dense than water
Low melting & boiling points
Increasing # carbonsincreasing boiling point,
melting point & density
Very reactive
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
ALKENE REACTIONS
• Alkenes are quite reactive chemically.
• Alkene reactions follow the pattern:
• These reactions are called addition reactions.
ALKENE REACTIONS (continued)
ALKENE REACTIONS (continued)
• HALOGENATION
• Halogenation (addition) reactions produce haloalkanes or
alkyl halides.
ALKENE REACTIONS (continued)
• HYDROGENATION
• Hydrogenation (addition) reactions occur in the presence of a
catalyst (Pt, Pd, or Ni).
• The hydrogenation of vegetable oils is an important commercial
process.
• Polyunsaturated molecules contain several double bonds.
• Hydrogenation of polyunsaturated molecules raises their
melting points.
ALKENE REACTIONS (continued)
• MARKOVNIKOV’S RULE
• Unsymmetrical alkene addition reactions follow
Markovnikov’s rule which states that when a molecule of
H-X adds to an alkene, the H predominantly attaches to
the carbon already bonded to the most hydrogen atoms.
“The rich get richer.”
ALKENE REACTIONS (continued)
• ADDITION OF SIMPLE ACIDS
• Addition of simple acids when Markovnikov’s rule is not
required:
• Addition of simple acids following Markovnikov’s rule:
ALKENE REACTIONS (continued)
• HYDRATION
• Hydration (addition of water) reactions follow
Markovnikov’s rule:
• This reaction requires an acid catalyst.
ALKENE REACTIONS (continued)
• ADDITION POLYMERIZATION
• An addition polymer is a polymer formed by the linking
together of many alkene molecules through addition
reactions.
POLYMERIZATION
• Polymers are very large molecules made up of
repeating units.
• A monomers is the starting material that becomes
the repeating units of a polymer.
COPOLYMER
• An addition polymer formed by the reaction of two different
monomers is a copolymer.
COMMON ADDITION POLYMERS
BENZENE
• Aromatic compounds contain the benzene ring or one of its
structural relatives.
• Aliphatic compounds do not contain this structure.
NAMING BENZENE DERIVATIVES
• Guideline 1: When a single hydrogen attached to the
benzene ring is replaced, the compound can be named as a
derivative of benzene.
NAMING BENZENE DERIVATIVES (continued)
• Guideline 2: Some common names are IUPAC-accepted
and used preferentially.
NAMING BENZENE DERIVATIVES (continued)
• Guideline 3: When the benzene ring is part of a more
complex hydrocarbon, the benzene ring is referred to as a
phenyl group.
NAMING BENZENE DERIVATIVES (continued)
• Guideline 4: When two groups are attached to the benzene
ring, their positions can be designated by the prefixes ortho
(o), meta (m), and para (p).
NAMING BENZENE DERIVATIVES (continued)
• Guideline 5: 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.
PROPERTIES OF AROMATIC COMPOUNDS
• 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.
IMPORTANT AROMATIC COMPOUNDS
POLYCYCLIC AROMATIC COMPOUNDS
• Polycyclic aromatic compounds contain two or more fused
benzene rings and are often known to be carcinogenic.