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Chapter 11
The Unsaturated Hydrocarbons:Alkenes, Alkynes, and Aromatic
Denniston
Topping
Caret
6th Edition
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1
• Both alkenes and alkynes are unsaturated hydrocarbons
• The alkene functional group is the carbon-carbon double bond
• The alkyne functional group is the carbon-carbon triple bond– Simplest alkene: ethene (ethylene)
C2H4– Simplest alkyne: ethyne (acetylene)
C2H2
11.1 Alkenes and Alkynes:Structure and Physical Properties
2
Aliphatic Hydrocarbon Structure Comparison
11.1
Str
uct
ure
and P
hysi
cal
Pro
per
ties
3
Bonding and Geometryof Two-Carbon Molecules
11.1
Str
uct
ure
and P
hysi
cal
Pro
per
ties
4
Structural Comparison of Five Carbon Molecules
Basic tetrahedral Planar around the Linear at the zig-zag shape double bond triple bond
11.1
Str
uct
ure
and P
hysi
cal
Pro
per
ties
5
Physical Properties• Physical properties of the alkenes and
alkynes are quite similar to those of alkanes– Nonpolar
– Not soluble in water
– Highly soluble in nonpolar solvents
• Boiling points rise with molecular weight
11.1
Str
uct
ure
and P
hysi
cal
Pro
per
ties
6
11.2 Alkenes and Alkynes: Nomenclature
• Base name from longest chain containing the multiple bond
• Change from -ane to -ene or -yne
• Number from the end, that will give the first carbon of the multiple bond the lower number
• Prefix the name with the number of the first multiple bond carbon
• Prefix branch/substituent names as for alkanes
7
Comparison of Names
11.2
Nom
encl
ature
8
Basic Naming Practice
3-ethyl-6-methyl-3-heptene
Nam
e
2-bromo-3-hexyne
11.2
Nom
encl
ature
9
Molecules With More Than One Double Bond
• Alkenes having more than one double bond:
– 2 double bonds = alkadiene
– 3 triple bonds = alkatriene
• Same rules for alkynes
11.2
Nom
encl
ature
10
Naming Cycloalkenes
5-chloro-3-methylcyclohexene
Name:
• Cyclic alkenes are named like cyclic alkanes – Prefix name with cyclo
• Numbering must start at one end of the double bond and pass through the bond
• Substituents must have the lower possible numbers– Either number clockwise or counterclockwise
11.2
Nom
encl
ature
11
Naming Haloalkenes
• Double or triple bonds take precedence over a halogen or alkyl group
– 2-Chloro-2-butene
– If 2 or more halogens, indicate the position of each
11.2
Nom
encl
ature
12
11.3 Geometric Isomers: A Consequence of Unsaturation
• Carbon-carbon double bonds are rigid
– Orbital shape restricts the rotation around the bond
– Results in cis-trans isomers
– Requires two different groups on each of the carbon atoms attached by the double bond
13
Naming Geometric Isomers
2-butene is the first example of an alkene which can have two different structures based on restricted rotation about the double bond
trans-2-butene cis-2-butene
11.3
Geo
met
ric
Isom
ers
14
Identifying cis and trans Isomers
• If one end of the C=C has two groups the same, cis-trans isomers are not possible
• Both carbons of the C=C must have two different groups attached
• Find a group common to both ends of the C=C
– If the common group is on the same side of the pi bond, the molecule is cis
– If on the opposite side, the molecule is trans
11.3
Geo
met
ric
Isom
ers
15
Questions to Identify cis-trans Isomers
1. Are both groups on a double-bond carbon the same?
1. A = B? C = D? If no, continue
2. Is one group on each carbon the same?• A = C or D? B = C or D? If either or both is
yes, cis-trans isomer is present
i. A " B
ii. C " D
So continue
A
D
C
B11.3
Geo
met
ric
Isom
ers
16
Distinguishing cis-trans Isomers
•Each carbon has 2 different substituents
–One substituent on each carbon is the same (Cl)
–The 2 chlorine atoms are attached on opposites of the plane of the double bond = trans
•trans-1,2-dichloro-1-butene
11.3
Geo
met
ric
Isom
ers
17
cis-trans Isomers
• Decide whether each compound is
– cis
– trans
– neither
• A: methyls are trans
• B: no cis-trans. Right C has two isopropyls
• C: hydrogens are cis
A B C
11.3
Geo
met
ric
Isom
ers
18
11.4 Alkenes in Nature
• Alkenes are abundant in nature– Ethene is a fruit ripener and promotes plant growth
– Polyenes built from the isoprene skeleton are called isoprenoids
– Isoprene is the basic 5 carbon unit shown here
• The next slide shows some isoprenoids
19
Isoprenoids – Distinctive Aromas
11.4
Alk
enes
in N
ature
20
11.5 Reactions Involving Alkenes and Alkynes
• There are two kinds of reactions typical of alkenes:
– Addition: two molecules combine to give one new molecule
– Redox: oxidation and reduction
• The two classes are not always mutually exclusive
21
Addition: General Reaction
• A small molecule, AB, reacts with the pi electrons of the double bond
• The pi bond breaks and its electrons are used to bond to the A and B pieces
• Some additions require a catalyst
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
22
Types of Addition Reactions
1. Symmetrical: same atom added to each carbon
• Hydrogenation - H2 (Pt, Pd, or Ni as
catalyst)
• Halogenation - Br2, Cl2
2. Unsymmetrical: H and another atom are added to the two carbons
• Hydrohalogenation - HCl, HBr
• Hydration - H2O (requires strong acid
catalyst e.g., H3O+, H2SO4, H3PO4)
3. Self-addition or polymerization
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
23
Hydrogenation: Addition of H2
Hydrogenation is the addition of a molecule of hydrogen (H2) to a carbon-carbon double bond to
produce an alkane
•The double bond is broken
•Two new C-H bonds result
•Platinum, palladium, or nickel is required as a catalyst
•Heat and/or pressure may also be required
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
24
Halogenation: Addition of X2
Halogenation is the addition of a molecule of halogen (X2) to a carbon-carbon double bond to
produce an alkane
•The double bond is broken
•Two new C-X bonds result
•Reaction occurs quite readily and does NOT require a catalyst
•Cl and Br are most often the halogen added
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
25
Bromination of an Alkene
• Left beaker contains bromine, but no
unsaturated hydrocarbon
• Right beaker contains bromine, but reaction
with an unsaturated hydrocarbon results in a
colorless solution
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
26
Unsymmetrical Addition
Two products are possible depending how the 2 groups (as H and OH) add to the ends of the pi bond
• The hydrogen will add to one carbon atom
• The other carbon atom will attach the other piece of the addition reagent
– OH (Hydration)
– Halogen (Hydrohalogenation)
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
27
Hydration
• A water molecule can be added to an alkene
– The addition of a water molecule to an alkene is called hydration
• Presence of strong acid is required as a catalyst
• Product resulting is an alcohol11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
28
Markovnikov’s Observation
• Dimitri Markovnikov (Russian) observed many acid additions to C=C systems
• He noticed that the majority of the hydrogen went to a specific end of the double bond
• He formulated an explanation
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
29
Markovnikov’s Rule
When an acid adds to a double bond -
the H of the acid most often goes to the end of the double bond, which had more hydrogens attached initially
• H-OH
• H-Cl
• H-Br
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
30
Hydration of Alkynes
• Hydration of an alkyne is a more complex process
– The initial product is not stable• Enol produced – both an alkene and an
alcohol
• Product is rapidly isomerized
– Final product is either• Aldehyde
• Ketone
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
31
Hydrohalogenation
• An alkene can be combined with a hydrogen halide such as HBr or HCl
• The reaction product is an alkyl halide
• Markovnikov’s Rule is followed in this reaction
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
32
Alkene Reactions• Predict the major product in each of the following
reactions
• Name the alkene reactant and the product using
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
33
Addition Polymers of Alkenes
• Polymers are macromolecules composed of repeating units called monomers
– Polymers can be made up of thousands of monomers linked together
• Many commercially important materials are addition polymers made from alkenes and substituted alkenes
– Addition polymers are named for the fact that they are made by the sequential addition of the repeating alkene monomer
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
34
Some Important Addition Polymers of Alkenes
11.5
Rea
ctio
ns
Involv
ing
Alk
enes
and A
lkynes
35
11.6 Aromatic HydrocarbonsBenzene’s structure was first proposed 150 years
ago
– A cyclic structure for benzene, C6H6
– Something special about benzene • Although his structures showed double bonds, the
molecule did not react as if it had any unsaturation
– Originally named aromatic compounds for the pleasant smell of resins from tropical trees (early source)
– Now aromatic hydrocarbons are characterized by a much higher degree of chemical stability than predicted by their chemical composition
• Most common group of aromatic compounds is based on the 6-member aromatic ring, benzene
36
Benzene Structure• The benzene ring consists of:
– Six carbon atoms
– Joined in a planar hexagonal arrangement
– Each carbon is 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 to the true structure
11.6
Aro
mat
ic H
ydro
carb
ons
37
Benzene Structure – Modern
Modern concept of benzene structure is based on overlapping orbitals
– Each carbon is bonded to two others by sharing a pair of electrons
– These same carbon atoms also each share a pair of electrons with a hydrogen atom
– Remaining 6 electrons are located in p orbitals that are perpendicular to the plane of the carbon ring
• These p orbitals overlap laterally
• Form a cloud of electrons above and below the ring
11.6
Aro
mat
ic H
ydro
carb
ons
38
Pi Cloud Formation in Benzene
The current model of bonding in benzene
11.6
Aro
mat
ic H
ydro
carb
ons
39
IUPAC Names: Benzenes
• Most simple aromatic compounds are named as derivatives of benzene
• For monosubstituted benzenes, name the group and add “benzene”
chlorobenzene ethylbenzenenitrobenzene
11.6
Aro
mat
ic H
ydro
carb
ons
40
IUPAC Names: Benzenes
• For disubstituted benzenes, name the groups in alphabetical order– The first named group is at position 1
– If a “special group” is present, it must be number 1 on the ring
• An older system of naming indicates groups using – ortho (o) = 1,2 on the ring
– meta (m) = 1,3 on the ring
– para (p) = 1,4 on the ring
11.6
Aro
mat
ic H
ydro
carb
ons
41
IUPAC Names of Substituted Benzenes
1-bromo-2-ethylbenzeneo-bromoethylbenzene
3-nitrotoluenem-nitrotoluene
1,4-dichlorobenzene p-dichlorobenzene1
1.6
Aro
mat
ic H
ydro
carb
ons
42
Historical Nomenclature
• Some members of the benzene family have unique names acquired before the IUPAC system was adopted that are still frequently used today
11.6
Aro
mat
ic H
ydro
carb
ons
43
Benzene As a Substituent
When the benzene ring is a substituent
on a chain (C6H5), it is called a phenyl
group
– Note the difference between
• Phenyl
• Phenol (a functional group)
4-phenyl-1-pentene
11.6
Aro
mat
ic H
ydro
carb
ons
44
Polynuclear Aromatic Hydrocarbons
Polynuclear aromatic hydrocarbons (PAH) are composed of two or more aromatic rings joined together
– Many have been shown to cause cancer
11.6
Aro
mat
ic H
ydro
carb
ons
45
Reactions of Benzene
• Benzene does not readily undergo addition reactions
• Benzene typically undergoes aromatic substitution reactions:
– An atom or group substitutes for an H on the ring
– All benzene reactions we consider require a catalyst
– The reactions are:1. Halogenation
2. Nitration
3. Sulfonation
11.6
Aro
mat
ic H
ydro
carb
ons
46
Benzene Halogenation
Halogenation places a Br or Cl on the ring
– The reagent used is typically Br2 or Cl2
– Fe or FeCl3 are used as catalysts
11.6
Aro
mat
ic H
ydro
carb
ons
47
Benzene Nitration
•Nitration places the nitro group on the ring
•Sulfuric acid is needed as a catalyst
11.6
Aro
mat
ic H
ydro
carb
ons
48
Benzene Sulfonation
Sulfonation places an SO3H group on the ring
– Concentrated sulfuric acid is required as a catalyst
– This is also a substitution reaction
11.6
Aro
mat
ic H
ydro
carb
ons
49
11.7 Heterocyclic Aromatic
• Rings with at least one atom other than carbon as part of the structure of the aromatic ring – This hetero atom is typically O, N, S
– The ring also has delocalized electrons
• The total number of atoms in the ring is typically either: – A six membered ring
– Some have a five membered ring
50
Heterocyclic Aromatics
• Heterocyclic aromatics are similar to benzene in stability and chemical behavior
• Many are significant biologically
Found inDNA and RNA
Found in hemoglobinand chlorophyll
11.7
Het
erocy
clic
Aro
mat
ic
Com
pounds
51
Reaction Schematic
Alkene
Hydrogenation
Hydration
Halogenation
Hydrohalogenation+ H2
Pt, Pd, or Ni
+ H2Oacidic
+ X2
adds easily
+ HX
52
Summary of Reactions
1. Addition Reactions of Alkenes
a. Hydrogenation
b. Hydration
c. Halogenation
d. Hydrohalogenation
2. Addition Polymers of Alkenes
3. Reactions of Benzene
a. Halogenation
b. Nitration
c. Sulfonation
53
Diagrammatic Summary of Reactions
54
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