Chapter 11
The Unsaturated Hydrocarbons:
Alkenes, Alkynes, and Aromatics
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1. Structure
Alkenes are hydrocarbons with a double bond.
CnH2n
Alkynes are hydrocarbons with a triple bond.
CnH2n-2
Alkenes and alkynes are unsaturated (don’t have the
maximum number of hydrogens bonded to each carbon).
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1. Comparison
3
1. Geometry
4
1. Geometry [3.4 Lewis structures]
Four groups of electrons
Ethane
tetrahedral
extend toward the corners of a regular tetrahedron
bond angle = 109.5
o
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1. Geometry [3.4 Lewis structures]
Three groups of electrons
Ethene
All in the same plane
Trigonal planar
Bond angle = 120o
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1. Geometry [3.4 Lewis structures]
Two groups of electrons
Ethyne
Linear
Bond angle = 180o
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1. Physical properties
Name
ethene
propene
1-butene
methylpropene
ethyne
propyne
1-butyne
2-butyne
Melting point
-160.1oC
-185.0oC
-185.0oC
-140.0oC
-81.8oC
-101.5oC
-125.9oC
-32.3oC
Boiling point
-103.7oC
-47.6oC
-6.1oC
-6.6oC
-84.0oC
-23.2oC
8.1oC
27.0oC
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1. Physical properties
In each case, the alkyne has a higher boiling point than
the alkene.
Its structure is more linear.
The molecules pack together more efficiently.
Intermolecular forces are stronger.
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2. Nomenclature
The root name is based on the longest chain that
includes both carbons of the multiple bond.
The –ane ending is changed to –ene for double bonds
and –yne for triple bonds.
ethyne
ethene
propyne
propene
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2. Nomenclature
The chain is numbered from the end nearest the multiple
bond.
1-butene
[not 3-butene]
2-pentyne
[not 3-pentyne]
The position of the multiple bond is indicated with the
lower-numbered carbon in the bond.
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2. Nomenclature
Determine the name and number of each substituent
and add in front of the name of the parent compound.
5-chloro-4-methyl-2-hexene
2,6-dimethyl-3-octene
5-bromo-4-ethyl-2-heptene
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2. Nomenclature
Alkenes with more than one double bond are called
alkadienes (2 double bonds)
alkatrienes (3 double bonds)
etc…
Each double bond is designated by its lower-numbered
carbon.
2,4-hexadiene
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2. Nomenclature
Cycloalkenes must be numbered so the double bond is
between carbons one and two.
3-chloro-cyclopentene
4-ethyl-5-methylcyclooctene
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2. Nomenclature
Name the following compounds.
CH3CH=C(CH2CH3)2
H2C=C-CH2-CH=CH2
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2. Nomenclature
Name the following compounds.
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2. Nomenclature
Write a structural formula for each of the following
compounds.
1-hexene
1,3-dicholoro-2-butene
4-methyl-2-hexyne
1,4-cyclohexadiene
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2. Nomenclature
Draw a structural formula for each of the following
compounds:
1-bromo-3-hexyne
2-butyne
dichloroethyne
9-iodo-1-nonyne
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3. Geometric isomers
Rotation around a double bond is restricted, in much the
same was as rotation is restricted for the cycloalkanes.
In the alkenes, geometric isomers occur when there are
two different groups on each of the double-bonded
carbon atoms.
1,2-dichloroethene
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3. Geometric isomers
Time for the first Chapter 11 Journal question!
[Use tag “difference”]
In your own words, explain how constitutional isomers
and geometric isomers are different. Be sure to consider
BOTH their differences and their similarities! You might
want to use examples of actual molecules.
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3. Cis-trans isomers
If both constituents are on the same side of the double
bond, the isomer is cis-.
cis-1,2-dichloroethene
If the constituents are on opposite sides of the double
bond, the isomer is trans-.
trans-1,2-dichloroethene
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3. Cis-trans isomers
Alkenes without substituents also may exhibit cis-trans
isomerism.
trans-4-octene
cis-4-octene
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3. Cis-trans isomers
In order for cis and trans isomers to exist, neither doublebonded carbon may have two identical substituents.
2-methyl-2-butene
no cis/trans isomerism
1-butene
no cis/trans isomerism
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3. Cis-trans isomers
Which of the following compounds can exist as geometric
isomers?
1-bromo-1-chloro-2-methylpropene
1,1-dichloroethene
1,2-dibromoethene
3-ethyl-2-methyl-2-hexene
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4. Alkenes in nature
Ethene (ethylene) and ripening
Ripening agents
Ripening bowl
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5. Reactions of alkenes and alkynes
The most common reactions of alkenes and alkynes are
addition reactions.
Hydrogenation: addition of H2
Halogenation: addition of X2
Hydration: addition of H2O
Hydrohalogenation: addition of HX
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5. General addition reaction
A double bond consists of
a sigma bond: two electrons
concentrated on a line between the
two connected atoms;
a pi bond: two electrons concentrated
in planes above and below the sigma
bond.
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5. General addition reaction
In an addition reaction, the pi bond is lost and its
electrons become part of the single bonds to A and B.
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5. General addition reaction
For hydrogenation, halogenation, hydration, and
hydrohalogenation, identify the A and B portions of what
is being added to the double bond.
hydrogenation, H2
halogenation, X2 (where X = F, Cl, Br, or I)
hydration, H2O
hydrohalogenation, HX (where X = F, Cl, Br, or I)
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5. Hydrogenation
In hydrogenation of an alkene, one molecule of hydrogen
(H2) adds to one mole of double bonds.
Reaction conditions:
platinum, palladium, or nickel catalyst
[sometimes] heat and/or pressure
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5. Hydrogenation
In hydrogenation of an alkyne, two molecules of
hydrogen (H2) add to one mole of triple bonds.
Reaction conditions: same as for alkenes.
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5. Hydrogenation
Compare the products resulting from the hydrogenation
of trans-2-pentene and cis-2-pentene.
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5. Hydrogenation
Compare the products resulting from the hydrogenation
of 1-butene and cis-2-butene.
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5. Vegetable oil and margarine
Why does hydrogenation make oils more solid?
MP = 13-14oC
MP = 69.6oC
MP = 62.9oC
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5. Halogenation
In halogenation of an alkene, one mole of a halogen (Cl2,
Br2, I2) adds to one mole of double bonds.
Since halogens are more reactive than hydrogen, no
catalyst is needed.
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5. Halogenation
In halogenation of an alkyne, two moles of a halogen (Cl2,
Br2, I2) add to one mole of double bonds.
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5. Halogenation
Draw the structure and write a balanced equation for the
halogenation of each of the following compounds.
3-methyl-1,4-hexadiene
4-bromo-1,3-pentadiene
3-chloro-2,4-hexadiene
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5. Halogenation
A solution of bromine in
water has a reddish-orange
color.
A simple test for the
presence of an alkene or
alkane is to add bromine
water.
If a double or triple bond is
present, the bromine will be
used up in a halogenation
reaction and the color will
disappear.
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5. Hydration
In hydration, one mole of water (H2O) is added to one
mole of double bonds.
A trace of acid is required as a catalyst.
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5. Hydration
Unlike hydrogenation and halogenation, hydration is not
a symmetric addition to a double bond.
If the double bond is not symmetrically located in the
molecule, there are two possible hydration products.
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5. Hydration
The predominant product is determined by
Markovnikov’s rule: The rich get richer.
OR: The carbon that already has more hydrogens will get
the hydrogen from the water.
Hydration of propene:
+ H 2O 
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5. Hydration
Write a balanced equation for the hydration of each of
the following compounds:
2-butene
2-ethyl-3-hexene
2,3-dimethylcyclohexene
Alkynes undergo a much more complicated hydration that you don’t need to
remember at this time!
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5. Hydrohalogenation
Like hydration, hydrohalogenation is an asymmetric
addition to a double bond.
Hydrohalogenation also follows Markovnikov’s rule.
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5. Hydrohalogenation
2-butene + HBr  ?
3-methyl-2-hexene + HCl  ?
cyclopentene + HI  ?
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5. Hydrohalogenation
Here’s your second Journal question!
[Use tag “addition”]
Explain how hydrogenation and halogenation are
different from hydration and hydrohalogenation as
addition reactions. [Hint: There’s a rule involved!]
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6. Aromatic compounds
Consider the following molecular formulas for
unsaturated hydrocarbons:
Hexane (all single bonds): C6H14
Cyclohexane (one ring): C6H12
Hexene (one double bond): C6H12
Hexadiene (two double bonds): C6H10
Cyclohexene (one ring, one double bond): C6H10
Hexatriene (three double bonds): C6H8
Cyclohexadiene (one ring, two double bonds): C6H8
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6. Aromatic compounds
The molecular formula for benzene is C6H6.
The structure must be highly unsaturated.
One ring, three double bonds?
Reactions of benzene:
Benzene does not decolorize bromine solutions.
Benzene does not undergo typical addition reactions.
Benzene reacts mainly by substitution.
The first three items are opposite from what is expected
from unsaturated compounds.
The last item is identical to what is expected for alkanes.
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6. Benzene structure
The benzene ring consists of:
six carbon atoms
joined in a planar hexagonal arrangement
with each carbon bonded to one hydrogen atom.
Two equivalent structures proposed by Kekulé are
recognized today as resonance structures.
The real benzene molecule is a hybrid with each
resonance structure contributing equally to the true
structure.
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6. Benzene structure
Sigma and pi bonding in benzene:
The sharing of six electrons over the entire ring gives the
benzene structure extra stability.
Removing any one of the six electrons would destroy that
stability.
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6. Nomenclature
Most single-substituent compounds are named as
derivatives of benzene.
Bromobenzene
Ethylbenzene
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6. Nomenclature
A few “common” names have been adopted as IUPAC
nomenclature.
toluene
phenol
aniline
xylene (any benzene ring with two methyl groups)
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6. Nomenclature
There are three ways for the methyl groups on xylene to
be arranged.
1,2 [ortho-xylene]
1,3 [meta-xylene]
1,4 [para-xylene]
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6. Nomenclature
The substituent created by removing one hydrogen from
the benzene ring is called phenyl-.
2-phenylhexane
3-phenylcyclopentene
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6. Nomenclature
The substituent consisting of a –CH2 attached to a
benzene ring is called benzyl-.
Benzyl chloride
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6. Polynuclear aromatic hydrocarbons
These consist of rings joined along one side.
Good news! You don’t have to memorize these names!
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6. Reactions of benzene
Because of the stability of benzene’s ring structure, only
substitution reactions are characteristic.
Halogenation: substitution of one or more halogen atoms for
hydrogen atoms.
Cl2 requires FeCl3 catalyst.
Br2 requires FeBr3 catalyst.
Nitration: substitution of one or more nitro- (-NO2) groups for
hydrogen atoms.
Requires nitric acid and concentration sulfuric acid.
Sulfonation: substitution of one sulfonic acid (-SO3H) group for
a hydrogen atom.
SO3 reactant and concentration sulfuric acid.
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7. Heterocyclic aromatic compounds
Heterocyclic aromatic compounds have at least one noncarbon atom incorporated in an aromatic ring or
polynuclear aromatic compound.
Many of these compounds are biologically important.
Components of DNA and RNA
Components of hemoglobin and chlorophyll
Pharmaceuticals
pyridine
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7. Heterocyclic aromatic compounds
Final Journal question for this unit!
[Use tag “common”]
What do DNA, RNA, nicotine, hemoglobin, chlorophyll,
and a drug used to treat ulcers have in common?
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