Post on 13-Mar-2018
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Chapter 13
Alkenes and
Alkynes
Based on Material Prepared by
Andrea D. LeonardUniversity of Louisiana at Lafayette
Alkenes and Alkynes
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Alkenes and alkynes are two families of
hydrocarbons that contain multiple bonds.
• Alkenes are compounds that contain a
carbon−carbon double bond.
• Alkenes have the general formula CnH2n.
Alkenes and Alkynes
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• Alkynes are compounds that contain a
carbon−carbon triple bond.
• Alkynes have the general formula CnH2n–2.
Alkenes and Alkynes
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• Alkenes and alkynes have low melting and boiling
points, and are insoluble in water.
• Alkenes and alkynes are composed of nonpolar
bonds.
• Their physical properties are like those of alkanes.
• They are called unsaturated hydrocarbons
because they contain fewer than the maximum
number of H atoms per C.
Alkenes and Alkynes
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• The multiple bond is always drawn in a
condensed structure.
Nomenclature of Alkenes and Alkynes
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HOW TO Name an Alkene or Alkyne
ExampleGive the IUPAC name of each alkene
and alkyne.
Step [1] Find the longest chain that contains both
C atoms of the double or triple bond.
Nomenclature of Alkenes and Alkynes
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HOW TO Name an Alkene or Alkyne
4 C’s in longest chain
• Since the compound is
an alkene, change the
−ane ending to −ene.
butane butene
6 C’s in longest chain
hexane hexyne
• Since the compound is
an alkyne, change the
−ane ending to −yne.
Nomenclature of Alkenes and Alkynes
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HOW TO Name an Alkene or Alkyne
Step [2]Number the carbon chain from the end that
gives the multiple bond the lower number.
• Name the compound using the first number
assigned to the multiple bond.
1-butene 2-hexyne
Nomenclature of Alkenes and Alkynes
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HOW TO Name an Alkene or Alkyne
Step [3] Number and name the substituents,
and write the name.
Nomenclature of Alkenes and Alkynes
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HOW TO Name an Alkene or Alkyne
Step [3] Number and name the substituents,
and write the name.
Nomenclature of Alkenes and Alkynes
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• Compounds with two double bonds are called
dienes.
• Dienes are named by changing the −ane
ending of the parent alkane to −adiene.
• Each double bond gets its own number.
1 2 3 4 6 5 4 3 2 1
1,3-butadiene 5-methyl-1,4-hexadiene
Nomenclature of Alkenes and Alkynes
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• When naming cycloalkenes, the double bond is
located between C1 and C2.
• The “1” is usually omitted in the name.
• The ring is numbered to give the first
substituent the lower number.
CH3
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1-methylcyclopentene
CH3
CH3
12
6
1,6-dimethylcyclohexene
Cis–Trans IsomersA. Stereoisomers—A New Class of Isomer
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• There is no rotation around the C atoms of a
double bond.
2-butene
two CH3 groups
on the same side
two CH3 groups
on opposite sides
• Therefore, 2-butene has two possible arrangements:
cis isomer trans isomer
Cis–Trans IsomersA. Stereoisomers—A New Class of Isomer
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Cis–Trans IsomersA. Stereoisomers—A New Class of Isomer
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• When the two groups on one end of the
double bond are identical (e.g., both H or both
CH3), no cis and trans isomers are possible.
Cis–Trans IsomersA. Stereoisomers—A New Class of Isomer
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• Stereoisomers are isomers that differ only in the
spatial arrangement of atoms.
• Constitutional isomers differ in the way the
atoms are bonded to each other.
Cis–Trans IsomersB. Saturated and Unsaturated Fatty Acids
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• Fatty acids are carboxylic acids (RCOOH) with
long carbon chains of 12–20 carbon atoms.
• Naturally occurring animal fats and vegetable
oils are formed from fatty acids.
• Saturated fatty acids have no double bonds in
their long hydrocarbon chains.
• Unsaturated fatty acids have one or more double
bonds in their long hydrocarbon chains.
Cis–Trans IsomersB. Saturated and Unsaturated Fatty Acids
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Interesting Alkenes in Food and Medicine
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• Lycopene, the red pigment in tomatoes and
watermelons, has 13 double bonds.
• Lycopene is an antioxidant, a compound that
prevents unwanted oxidation from occurring.
• Diets containing high levels of antioxidants result
in decreased risk of heart disease and cancer.
Reactions of Alkenes
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• Alkenes undergo addition reactions wherein new
groups X and Y are added to the alkene.
• One bond of the double bond is broken and two
new single bonds are formed.
Reactions of Alkenes
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Reactions of AlkenesA. Addition of Hydrogen—Hydrogenation
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• Hydrogenation is the addition of H2 to an alkene.
• The metal catalyst (usually palladium—Pd)
speeds up the rate of the reaction.
• The product of hydrogenation is an alkane.
Reactions of AlkenesA. Addition of Hydrogen—Hydrogenation
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• Example of hydrogenation:
Reactions of AlkenesB. Addition of Halogen—Halogenation
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• Halogenation is the addition of halogen (X2) to
an alkene.
• X2 is usually Cl2 or Br2.
• Halogenation occurs readily and does not
require a catalyst.
• The product of halogenation is a dihalide.
Reactions of AlkenesB. Addition of Halogen—Halogenation
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• Examples of halogenation:
Reactions of AlkenesC. Addition of Hydrogen Halides—
Hydrohalogenation
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• Hydrohalogenation is the addition of HX (HCl or
HBr) to an alkene.
• The product of hydrohalogenation is an alkyl
halide.
Reactions of AlkenesC. Addition of Hydrogen Halides—
Hydrohalogenation
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• Examples of hydrohalogenation:
Reactions of AlkenesC. Addition of Hydrogen Halides—
Hydrohalogenation
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• If the reactant is an asymmetrical alkene, its is
possible to form two products.
• These two potential products are constitutional
isomers.
Reactions of AlkenesC. Addition of Hydrogen Halides—
Hydrohalogenation
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• However, only one of the two products will
actually form in more abundance.
• Markovnikov’s rule states that the H atom of
H–X will bond to the less substituted C atom
in the C═C double bond.
• It means that the C in the double bond with the
most H’s will bond to the H atom of H–X.
Reactions of AlkenesC. Addition of Hydrogen Halides—
Hydrohalogenation
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• Looking at the reaction again:
Reactions of AlkenesC. Addition of Hydrogen Halides—
Hydrohalogenation
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• Looking at the reaction again:
Reactions of AlkenesD. Addition of Water—Hydration
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• Hydration is the addition of water to an alkene.
• Hydration requires a strong acid, H2SO4.
• The product formed by hydration is an alcohol.
Reactions of AlkenesD. Addition of Water—Hydration
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• An example of hydration:
Reactions of AlkenesD. Addition of Water—Hydration
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• If the reactant is an asymmetrical alkene, the
product will be determined by Markovnikov’s
rule.
Focus on Health and MedicineMargarine or Butter?
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• Butter is made up of saturated fatty acid chains.
• A diet rich in saturated fatty acids stimulate an
excessive production of cholesterol.
• Scientists have attempted to produce
alternative versions of butter (margarine) with
similar taste and properties, but with some C═C
double bonds (i.e., unsaturated fatty acid
chains).
Focus on Health and MedicineMargarine or Butter?
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Focus on Health and MedicineMargarine or Butter?
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• Unfortunately, some partial hydrogenations leave
trans double bonds on the fatty acid chain.
• Trans fatty acids are very similar in shape to
saturated fatty acids.
• Trans fatty acids have the same effects as saturated
fatty acids: stimulate cholesterol production.
Polymers
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• Polymers are large molecules made up of
repeating units of smaller molecules (monomers)
covalently bonded together.
PolymersA. Synthetic Polymers
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• In polymerization, the monomer C═C double bonds
are broken and single bonds linking the monomers
together are formed.
PolymersA. Synthetic Polymers
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Aromatic Compounds
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• Aromatic compounds are compounds that
contain a benzene ring.
• Each C is trigonal planar (i.e., 120° bond
angles), making benzene a planar molecule.
Aromatic Compounds
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• Each of these representations has the same arrangement
of atoms, but different locations of electrons.
• These are resonance structures, and the true structure is
with all three electron pairs in the double bonds
delocalized. It is usually represented as follow:
Aromatic Compounds
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• Aromatic hydrocarbons do not undergo the
addition reactions that characterize alkenes.
Nomenclature of Benzene Derivatives
A. Monosubstituted Benzenes
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To name a benzene ring with one substituent:
• Name the substituent first
• Then add the word benzene at the end
Nomenclature of Benzene Derivatives
A. Monosubstituted Benzenes
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• Some monosubstituted benzenes have
common names that you must learn.
Nomenclature of Benzene Derivatives
B. Disubstituted Benzenes
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Nomenclature of Benzene Derivatives
B. Disubstituted Benzenes
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• If there are two groups on the benzene ring and
they are different, alphabetize the two
substituent names.
Nomenclature of Benzene Derivatives
B. Disubstituted Benzenes
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• If one of the two substituents is part of a
common root, then name the molecule as a
derivative of that monosubstituted benzene.
Nomenclature of Benzene Derivatives
C. Polysubstituted Benzenes
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1. Number to give the lowest possible numbers
around the ring.
2. Alphabetize the substituent names.
3. When the substituents are part of common roots:
• Put the common root substituent at C1, but
omit the “1” from the name
• Name the molecule as a derivative of that
monosubstituted benzene
Nomenclature of Benzene Derivatives
C. Polysubstituted Benzenes
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• Assign the lowest set of numbers.
• Alphabetize the names of all the substituents.
4-chloro-1-ethyl-2-propylbenzene
Nomenclature of Benzene Derivatives
C. Polysubstituted Benzenes
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• Name the molecule as a derivative of the common
root aniline.
• Assign the NH2 group to position 1 and then assign
the lowest possible set of numbers to the other
groups.2,5-dichloroaniline
Nomenclature of Benzene DerivativesD. Aromatic Compounds with More than One Ring
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Focus on Health and MedicineAromatic Drugs
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• Some common drugs that contain benzene rings
are:
Focus on Health and MedicinePhenols as Antioxidants
Reactions of Aromatic Compounds
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• Aromatic compounds undergo substitution
reactions primarily.
• Substitution is a reaction in which an atom is
replaced by another atom or group of atoms.
• Substitution of H by X keeps the stable aromatic
ring intact.
Reactions of Aromatic Compounds
A. Chlorination and DDT
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• In chlorination, a Cl atom substitutes for a
hydrogen atom on the benzene ring.
• The pesticide DDT is
formed by a
chlorination reaction.
Reactions of Aromatic Compounds
B. Nitration and Sulfa Drugs
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• Benzene reacts with nitric acid (HNO3) in the
presence of sulfuric acid (H2SO4) to form
nitrobenzene.
• Nitration is a valuable reaction because nitro
gropus are readily converted into amino
groups.
Reactions of Aromatic Compounds
B. Nitration and Sulfa Drugs
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• Sulfa drugs, such as the antibacterial agents
shown below, are formed by the nitration
reaction.
Reactions of Aromatic Compounds
C. Sulfonation and Detergent Synthesis
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• In sulfonation, benzene reacts with SO3 in the
presence of H2SO4 such that a SO3H group
substitutes for a hydrogen atom on the benzene ring.
• The synthetic detergent
shown is a product of
sulfonation.