Spencer L. Seager
Michael R. Slabaugh
www.cengage.com/chemistry/seager
Chapter 12
Unsaturated Hydrocarbons
Jennifer P. Harris
UNSATURATED HYDROCARBONS
• Unsaturated hydrocarbons contain carbon-carbon multiple
bonds.
• Alkenes contain carbon-carbon double bonds (C=C ).
• Alkynes contain carbon-carbon triple bonds (C≡C).
• Aromatics contain benzene rings.
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 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
• 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:
THE GEOMETRY OF ALKENES (continued)
• 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.
THE GEOMETRY OF ALKENES (continued)
• 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.
THE GEOMETRY OF ALKENES (continued)
• 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.
THE GEOMETRY OF ALKENES (continued)
• 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.
THE GEOMETRY OF ALKENES (continued)
• Geometric isomers have different physical properties.
PHYSICAL PROPERTIES OF ALKENES
•
•
•
•
•
•
Similar to alkanes
Nonpolar
Insoluble in water
Soluble in nonpolar solvents
Less dense than water
Unpleasant, gasoline-like odors
PHYSICAL PROPERTIES OF ALKENES
ALKENE REACTIONS
• Alkenes are quite chemically reactive.
• 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 can 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 hydrogens. “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)
• Hydration reactions are believed to occur in three steps.
• Step 1:
• H+ from acid catalyst is attracted to the electrons in the
carbon-carbon double bond.
• It becomes bonded to one of the carbon atoms by a
sharing of electrons.
• The other carbon atom from the double bond becomes an
extremely reactive carbocation (positively charged
carbon atom with only three bonds).
• The carbocation attracts the oxygen atom (with two
unshared pairs of electrons) in a water molecule.
ALKENE REACTIONS (continued)
• Step 2:
• One pair of oxygen electrons form a covalent bond with
the carbocation.
ALKENE REACTIONS (continued)
• Step 3:
• H+ is lost to produce the alcohol.
• Note: Catalyst is regenerated in this step.
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
COMMON ADDITION POLYMERS
ALKYNES
• Ethyne (commonly called acetylene) is the simplest alkyne
and is used as a fuel for torches and in making plastics.
ALKYNE NOMENCLATURE
• Alkynes are named in exactly the same ways as alkenes,
except the ending –yne is used.
• Examples:
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:
THE GEOMETRY OF ALKYNES (continued)
• During hybridization, one 2p orbital mixes with the single 2s
orbital to produce two sp hybrid orbitals. Two 2p orbitals are
not hybridized and remain unchanged.
• This gives a linear shape for the sp bonding orbitals with the
unhybridized p orbitals perpendicular to the line of the two sp
hybridized orbitals.
THE GEOMETRY OF ALKYNES (continued)
• A carbon-carbon sigma bond forms by the overlap of one sp
hybrid orbital of each carbon atom.
• The other sp hybrid orbital of each carbon atom overlaps with
a 1s orbital of a hydrogen atom to form a carbon-hydrogen
sigma bond.
• The remaining pair of unhybridized p orbitals of each carbon
atom overlap sideways to form a pair of pi bonds between
the carbon atoms.
ALKYNE PROPERTIES
• 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
BENZENE
• Aromatic compounds contain the benzene ring or one of its
structural relatives.
• Aliphatic compounds don’t contain this structure.
BENZENE (continued)
• 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:
BENZENE (continued)
• This gives rise to two possible benzene structures called
Kekulé structures in honor of the German chemist who
suggested that benzene might be represented by a ring
arrangement of carbon atoms with alternating single and
double bonds between the carbon atoms:
• Which are better represented by:
• Note: that there is only 1 available bonding site on each
carbon atom!
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
IMPORTANT AROMATIC COMPOUNDS
(continued)
• A number of aromatic compounds must be present in our diet
for proper nutrition.
• A vitamin is an organic nutrient that the body cannot
produce in the small amounts needed for good health.
POLYCYCLIC AROMATIC COMPOUNDS
• Polycyclic aromatic compounds contain two or more fused
benzene rings and are often known to be carcinogenic.