Transcript Chapter 1

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Chapter 11
The Unsaturated Hydrocarbons
Denniston
Topping
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5th Edition
11.1 Alkenes and Alkynes:
Structure and Physical Properties
• 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
Aliphatic Hydrocarbon Structure
Comparison
Bonding and Geometry
of Two-Carbon Molecules
Structural Comparison of Five
Carbon Molecules
Basic tetrahedral
zig-zag shape
Planar around the
double bond
Linear at the
triple bond
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
11.2 IUPAC Names
• 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
Comparison of Names
Basic Naming Practice
CH3
CH2 CH3
CH3 CH CH2 CH C
CH2 CH3
3-ethyl-6-methyl-3-heptene
Name
CH3CH C
Br
C CH2CH3
2-bromo-3-hexyne
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
Naming Cycloalkenes
• 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
Name
:
Cl
CH
CH2
CH2
CH3
CH
CH
CH
5-chloro-3-methylcyclohexene
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
11.3 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
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
CH3
CH3 CH3
C C
C C
H
H
CH3
H
H
trans-2-butene
cis-2-butene
Identifying cis/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
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
i. A = C
Isomer!
CH3A
CH3
C
C C
D CH CH
HB
2
3
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
Cl
CH2 CH3
C C
H
Cl
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
CH3
C
CH3 B
Cl
CH CH3 CH3 CH2
CH2 CH3
CH3
CH
B
C C
C C
C C
A
CH CH3
H
H C
H
CH3
CH3
CH3
A
CH3
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
CH3
CH2 C CH CH2
Isoprenoids – Distinctive Aromas
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
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
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
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
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
•Chlorine and bromine are most often the halogen added
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
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)
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 alcohol
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
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
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
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
Alkene Reactions
• Predict the major product in each of the following
reactions
• Name the alkene reactant and the product using IUPAC
nomenclature
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
Some Important Addition
Polymers of Alkenes
11.6 Aromatic Hydrocarbons
Benzene’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
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
H
H
HC
HC
C
C
H
CH
HC
CH
HC
C
C
H
CH
CH
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
Pi Cloud Formation in Benzene
The current model of bonding in benzene
IUPAC Names: Benzenes
• Most simple aromatic compounds are
named as derivatives of benzene
• For monosubstituted benzenes, name the
group and add “benzene”
NO2
Cl
CH2 CH3
nitrobenzene
chlorobenzene
ethylbenzene
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
IUPAC Names of Substituted
Benzenes
CH2 CH3
CH3
Br
1-bromo-2-ethylbenzene
o-bromoethylbenzene
Cl
Cl
1,4-dichlorobenzene
p-dichlorobenzene
NO2
3-nitrotoluene
m-nitrotoluene
Historical Nomenclature
• Some members of the benzene family have
unique names acquired before the IUPAC
system was adopted that are still frequently
used today
CH3
NH2
COOH
OH
Toluene
Aniline
Phenol
Benzoic
acid
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)
CH2 CH CH2
CH CH3
4-phenyl-1-pentene
Polynuclear Aromatic
Hydrocarbons
Polynuclear aromatic hydrocarbons (PAH) are
composed of two or more aromatic rings joined
together
– Many have been shown to cause cancer
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
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
Benzene Nitration
•Nitration places the nitro group on the ring
•Sulfuric acid is needed as a catalyst
Benzene Sulfonation
• Sulfonation places an SO3H group on the ring
– Concentrated sulfuric acid is required as a catalyst
– This is also a substitution reaction
11.7 Heterocyclic Aromatic
Compounds
• 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
N
S
O
N
N
pyridine pyrimidine furan thiophene
Heterocyclic Aromatics
• Heterocyclic aromatics are similar to benzene in
stability and chemical behavior
• Many are significant biologically
N
N
N
N
N
pyrimidine purine
H
N
pyrrole
N
Found in
DNA and RNA
H
Found in hemoglobin
and chlorophyll
Reaction Schematic
Alkene
+ H2O
+ H2
acidic
Hydrohalogenation
Pt, Pd, or Ni
Hydration
Hydrogenation
+ HX
+ X2
adds easily
Halogenatio
n
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
Diagrammatic Summary of
Reactions