Unsaturated Hydrocarbons Alkenes, Alkynes, and Aromatics
Dr. Sean Bonness
Chemistry Department
El Camino College
Unsaturated Hydrocarbons
• Alkenes contain one or more double bonds
• Alkynes contain one or more triple bonds
• Aromatic hydrocarbons have 3 double bonds
alternating with 3 single bonds in a six
Carbon ring.
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Nomenclature of Alkenes
• General formula for alkenes CnH2n.
– For example, ethylene and propylene
– What are some structural isomers for C4H8
Nomenclature of Alkenes (cont.)
• IUPAC rules for naming alkenes are similar
to those used for alkanes.
 Step 1 - Name the longest chain that has the
double bond and end the name with -ene.
 Step 2 - Number the longest chain of Carbon
atoms so that the C atoms joined by double
bond are the lowest number.
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Nomenclature of Alkenes (cont.)
• IUPAC rules for naming alkenes are similar
to those used for alkanes.
 Step 3 -Locate the double bond by the lower
numbered C atom.
 Step 4 - Locate and name attached groups.
 Step 5 - Combine the names for the attached
groups and the longest chain into the name.
Nomenclature of Alkenes (cont.)
• Name the following alkenes:
 CH3-CH2-CH=CH-CH2-CH3
 CH3-CH-CH2-CH2-CH=CH2
|
CH3
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Nomenclature of Alkenes (cont.)
• Name the following alkenes:

CH3-CH2
|
C=CH2
|
CH2-CH2-CH2
|
CH3-CH2
Nomenclature of Alkenes (cont.)
• Name the following alkenes:

Note - in cyclic alkenes, the rings is numbered
to give the lowest numbers to the double bonded
carbons (they become carbons 1 and 2).
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Nomenclature of Alkenes (cont.)
• Some compounds have more than one double
bond.
– Same nomenclature as the alkenes with 1 double
bond but with endings -diene, -triene are used.
– Also, the locations of all multiple double bonds
must be indicated, including those in rings.
CH2=CH-CH=CH2
Nomenclature of Alkenes (cont.)
• Name the following compounds
CH3
|
CH2=C-C=CH-CH=CH2
|
CH3
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Geometry of Alkenes
• Hybridization involves mixing a 2s orbital
and two 2p orbitals of a C atom to form
three hybrid sp2 orbitals
• sp2 hybrid orbitals lie in the same plane and
have angles of 120°.
• The unhybridized 2p orbital is perpendicular
to the plane of sp2 hybrid orbitals.
Geometry of Alkenes (cont.)
• Example of ethylene
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Geometry of Alkenes (cont.)
• Alkenes exhibit geometric isomerism, the
same type of stereoisomerism we saw for
cycloalkanes.
– Example, 2-butene
Geometry of Alkenes (cont.)
• cis-trans stereoisomerism is found only in
alkenes that have two different groups
attached to each double bonded C atom.
– Example
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Geometry of Alkenes (cont.)
• Does the following molecule exhibit
geometric isomerism? If so, draw structural
formulas and name the compound.
Geometry of Alkenes (cont.)
• Does the following molecule exhibit
geometric isomerism? If so, draw structural
formulas and name the compound.
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Preparation of Alkenes
• Hydrogenation of an alkyne.
• Add one H to each C of the triple bond and change
the triple bond to a double bond.
• By appropriate choice of reagent this method can
be used to make pure cis or trans alkenes
Addition reactions
• Addition of X2
• The structure of the product bonds one X
to each C atom of the double bond to form
a single bond.
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Addition reactions (cont.)
• Mechanism for addition of X2
• Addition to ring and alkene chains are trans.
Addition reactions (cont.)
• Addition of HX(dry)
• Mechanism is
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Addition reactions (cont.)
• Addition of HBr in presence of peroxides
• HBr adds in the opposite manner to the
double bond if no peroxide was present.
Addition reactions (cont.)
• Addition of HF(aq)
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