Chapter 16
Aromatic Compounds
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Chapter 16 2
Discovery of Benzene
Isolated in 1825 by Michael Faraday
who determined C:H ratio to be 1:1.
Synthesized in 1834 by Eilhard
Mitscherlich who determined molecular
formula to be C6H6.
Other related compounds with low C:H
ratios had a pleasant smell,so they
were classified as aromatic,
=>
Chapter 16 3
Kekulé Structure
Proposed in 1866 by Friedrich Kekulé,shortly
after multiple bonds were suggested.
Failed to explain existence of only one isomer
of 1,2-dichlorobenzene.C
C
C
C
C
C
H
H
H
H
H
H
=>
Chapter 16 4
Resonance Structure
Each sp2 hybridized C in the ring has an
unhybridized p orbital perpendicular to
the ring which overlaps around the ring.
=>
Chapter 16 5
Unusual Reactions
Alkene + KMnO4? diol (addition)
Benzene + KMnO4? no reaction.
Alkene + Br2/CCl4? dibromide (addition)
Benzene + Br2/CCl4? no reaction.
With FeCl3 catalyst,Br2 reacts with
benzene to form bromobenzene + HBr
(substitution!),Double bonds remain.
=>
Chapter 16 6
Unusual Stability
Hydrogenation of just one double
bond in benzene is endothermic!
=>
Chapter 16 7
Annulenes
All cyclic conjugated
hydrocarbons were
proposed to be aromatic.
However,cyclobutadiene
is so reactive that it
dimerizes before it can
be isolated.
And cyclooctatetraene
adds Br2 readily.
Look at MO’s to explain
aromaticity,=>
Chapter 16 8
MO Rules for Benzene
Six overlapping p orbitals must form six
molecular orbitals.
Three will be bonding,three antibonding.
Lowest energy MO will have all bonding
interactions,no nodes.
As energy of MO increases,the number of
nodes increases,=>
Chapter 16 9
MO’s for Benzene
=>
Chapter 16 10
Energy Diagram for
Benzene
The six electrons fill three bonding pi orbitals.
All bonding orbitals are filled (“closed shell”),
an extremely stable arrangement,
=>
Chapter 16 11
MO’s for Cyclobutadiene
=>
Chapter 16 12
Energy Diagram for
Cyclobutadiene
Following Hund’s
rule,two electrons
are in separate
orbitals.
This diradical would
be very reactive,
=>
Chapter 16 13
Polygon Rule
The energy diagram for an annulene has
the same shape as the cyclic compound
with one vertex at the bottom.
=>
Chapter 16 14
Aromatic Requirements
Structure must be cyclic with conjugated
pi bonds.
Each atom in the ring must have an
unhybridized p orbital.
The p orbitals must overlap continuously
around the ring,(Usually planar structure)
Compound is more stable than its open-
chain counterpart,=>
Chapter 16 15
Anti- and Nonaromatic
Antiaromatic compounds are cyclic,
conjugated,with overlapping p orbitals
around the ring,but the energy of the
compound is greater than its open-chain
counterpart.
Nonaromatic compounds do not have a
continuous ring of overlapping p orbitals
and may be nonplanar,=>
Chapter 16 16
Hückel’s Rule
If the compound has a continuous ring
of overlapping p orbitals and has 4N + 2
electrons,it is aromatic,
If the compound has a continuous ring
of overlapping p orbitals and has 4N
electrons,it is antiaromatic,
=>
Chapter 16 17
[N]Annulenes
[4]Annulene is antiaromatic (4N e-’s)
[8]Annulene would be antiaromatic,but
it’s not planar,so it’s nonaromatic.
[10]Annulene is aromatic except for the
isomers that are not planar.
Larger 4N annulenes are not
antiaromatic because they are flexible
enough to become nonplanar,=>
Chapter 16 18
MO Derivation of
Hückel’s Rule
Lowest energy MO has 2 electrons.
Each filled shell has 4 electrons.
=>
Chapter 16 19
Cyclopentadienyl Ions
The cation has an empty p orbital,4 electrons,
so antiaromatic.
The anion has a nonbonding pair of electrons
in a p orbital,6 e-’s,aromatic,
=>
Chapter 16 20
Acidity of Cyclopentadiene
pKa of cyclopentadiene is 16,much more
acidic than other hydrocarbons.
=>
p K a = 1 9p K a = 1 6
H O C ( C H 3 ) 3+
H
O C ( C H 3 ) 3
_
+
H H
Chapter 16 21
Tropylium Ion
The cycloheptatrienyl cation has 6 p
electrons and an empty p orbital.
Aromatic,more stable than open chain ion
=>
H O H
H
+
,H 2 O
H
+
Chapter 16 22
Dianion of [8]Annulene
Cyclooctatetraene easily forms a -2 ion.
Ten electrons,continuous overlapping p
orbitals,so it is aromatic,
=>
+ 2 K + 2 K
+
Chapter 16 23
Pyridine
Heterocyclic aromatic compound.
Nonbonding pair of electrons in sp2
orbital,so weak base,pKb = 8.8.
=>
Chapter 16 24
Pyrrole
Also aromatic,but lone pair of electrons is
delocalized,so much weaker base.
=>
Chapter 16 25
Basic or Nonbasic?
NN
Pyrimidine has two basic
nitrogens.
N N H
Imidazole has one basic
nitrogen and one nonbasic.
N
N
N
N
H
Purine? =>
Chapter 16 26
Other Heterocyclics
=>
Chapter 16 27
Fused Ring Hydrocarbons
Naphthalene
Anthracene
Phenanthrene
=>
Chapter 16 28
Reactivity of
Polynuclear Hydrocarbons
As the number of aromatic rings increases,
the resonance energy per ring decreases,
so larger PAH’s will add Br2.
H B r
H B r H B r
B r
H
(mixture of cis and trans isomers) =>
Chapter 16 29
Fused Heterocyclic
Compounds
Common in nature,synthesized for drugs.
=>
Chapter 16 30
Allotropes of Carbon
Amorphous,small particles of graphite;
charcoal,soot,coal,carbon black.
Diamond,a lattice of tetrahedral C’s.
Graphite,layers of fused aromatic rings.
=>
Chapter 16 31
Some New Allotropes
Fullerenes,5- and 6-membered rings
arranged to form a,soccer ball” structure,
Nanotubes,half of a C60 sphere fused to a
cylinder of fused aromatic rings.
=>
Chapter 16 32
Common Names of
Benzene Derivatives
O H O C H 3N H
2
C H 3
p h e n o l t o lu e n e a n i lin e a n i s o le
C
H
C H 2 C
O
C H 3
C
O
H
C
O
O H
s t y r e n e a c e t o p h e n o n e b e n z a ld e h y d e b e n z o ic a c id
=>
Chapter 16 33
Disubstituted Benzenes
The prefixes ortho-,meta-,and para- are
commonly used for the 1,2-,1,3-,and 1,4-
positions,respectively.B r
B r
o - d ib r o m o b e n z e n e o r
1,2 - d i b r o m o b e n z e n e
H O
N O 2
p - n i t r o p h e n o l o r
4 - n i t r o p h e n o l
=>
Chapter 16 34
3 or More Substituents
Use the smallest possible numbers,but
the carbon with a functional group is #1.
N O 2
N O 2
O 2 N
1,3,5 - t r i n i t r o b e n z e n e
N O 2
N O 2
O 2 N
O H
2,4,6 - t r i n i t r o p h e n o l
=>
Chapter 16 35
Common Names for
Disubstituted Benzenes
C H 3
C H 3
C H 3
C H 3H 3 C
C H 3
C
O
O H
O H
H 3 C
m - x y l e n e m e s i t y l e n e o - t o lu ic a c id p - c r e s o l
=>
Chapter 16 36
Phenyl and Benzyl
B r
p h e n y l b r o m i d e
C H 2 B r
b e n z y l b r o m i d e
Phenyl indicates the benzene ring
attachment,The benzyl group has
an additional carbon.
=>
Chapter 16 37
Physical Properties
Melting points,More symmetrical than
corresponding alkane,pack better into
crystals,so higher melting points.
Boiling points,Dependent on dipole
moment,so ortho > meta > para,for
disubstituted benzenes.
Density,More dense than nonaromatics,
less dense than water.
Solubility,Generally insoluble in water,=>
Chapter 16 38
IR and NMR Spectroscopy
C=C stretch absorption at 1600 cm-1.
sp2 C-H stretch just above 3000 cm-1.
1H NMR at?7-?8 for H’s on aromatic
ring.
13C NMR at?120-?150,similar to alkene
carbons,
=>
Chapter 16 39
Mass Spectrometry
=> =>
Chapter 16 40
UV Spectroscopy
=>
Chapter 16 41
End of Chapter 16
Aromatic Compounds
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Chapter 16 2
Discovery of Benzene
Isolated in 1825 by Michael Faraday
who determined C:H ratio to be 1:1.
Synthesized in 1834 by Eilhard
Mitscherlich who determined molecular
formula to be C6H6.
Other related compounds with low C:H
ratios had a pleasant smell,so they
were classified as aromatic,
=>
Chapter 16 3
Kekulé Structure
Proposed in 1866 by Friedrich Kekulé,shortly
after multiple bonds were suggested.
Failed to explain existence of only one isomer
of 1,2-dichlorobenzene.C
C
C
C
C
C
H
H
H
H
H
H
=>
Chapter 16 4
Resonance Structure
Each sp2 hybridized C in the ring has an
unhybridized p orbital perpendicular to
the ring which overlaps around the ring.
=>
Chapter 16 5
Unusual Reactions
Alkene + KMnO4? diol (addition)
Benzene + KMnO4? no reaction.
Alkene + Br2/CCl4? dibromide (addition)
Benzene + Br2/CCl4? no reaction.
With FeCl3 catalyst,Br2 reacts with
benzene to form bromobenzene + HBr
(substitution!),Double bonds remain.
=>
Chapter 16 6
Unusual Stability
Hydrogenation of just one double
bond in benzene is endothermic!
=>
Chapter 16 7
Annulenes
All cyclic conjugated
hydrocarbons were
proposed to be aromatic.
However,cyclobutadiene
is so reactive that it
dimerizes before it can
be isolated.
And cyclooctatetraene
adds Br2 readily.
Look at MO’s to explain
aromaticity,=>
Chapter 16 8
MO Rules for Benzene
Six overlapping p orbitals must form six
molecular orbitals.
Three will be bonding,three antibonding.
Lowest energy MO will have all bonding
interactions,no nodes.
As energy of MO increases,the number of
nodes increases,=>
Chapter 16 9
MO’s for Benzene
=>
Chapter 16 10
Energy Diagram for
Benzene
The six electrons fill three bonding pi orbitals.
All bonding orbitals are filled (“closed shell”),
an extremely stable arrangement,
=>
Chapter 16 11
MO’s for Cyclobutadiene
=>
Chapter 16 12
Energy Diagram for
Cyclobutadiene
Following Hund’s
rule,two electrons
are in separate
orbitals.
This diradical would
be very reactive,
=>
Chapter 16 13
Polygon Rule
The energy diagram for an annulene has
the same shape as the cyclic compound
with one vertex at the bottom.
=>
Chapter 16 14
Aromatic Requirements
Structure must be cyclic with conjugated
pi bonds.
Each atom in the ring must have an
unhybridized p orbital.
The p orbitals must overlap continuously
around the ring,(Usually planar structure)
Compound is more stable than its open-
chain counterpart,=>
Chapter 16 15
Anti- and Nonaromatic
Antiaromatic compounds are cyclic,
conjugated,with overlapping p orbitals
around the ring,but the energy of the
compound is greater than its open-chain
counterpart.
Nonaromatic compounds do not have a
continuous ring of overlapping p orbitals
and may be nonplanar,=>
Chapter 16 16
Hückel’s Rule
If the compound has a continuous ring
of overlapping p orbitals and has 4N + 2
electrons,it is aromatic,
If the compound has a continuous ring
of overlapping p orbitals and has 4N
electrons,it is antiaromatic,
=>
Chapter 16 17
[N]Annulenes
[4]Annulene is antiaromatic (4N e-’s)
[8]Annulene would be antiaromatic,but
it’s not planar,so it’s nonaromatic.
[10]Annulene is aromatic except for the
isomers that are not planar.
Larger 4N annulenes are not
antiaromatic because they are flexible
enough to become nonplanar,=>
Chapter 16 18
MO Derivation of
Hückel’s Rule
Lowest energy MO has 2 electrons.
Each filled shell has 4 electrons.
=>
Chapter 16 19
Cyclopentadienyl Ions
The cation has an empty p orbital,4 electrons,
so antiaromatic.
The anion has a nonbonding pair of electrons
in a p orbital,6 e-’s,aromatic,
=>
Chapter 16 20
Acidity of Cyclopentadiene
pKa of cyclopentadiene is 16,much more
acidic than other hydrocarbons.
=>
p K a = 1 9p K a = 1 6
H O C ( C H 3 ) 3+
H
O C ( C H 3 ) 3
_
+
H H
Chapter 16 21
Tropylium Ion
The cycloheptatrienyl cation has 6 p
electrons and an empty p orbital.
Aromatic,more stable than open chain ion
=>
H O H
H
+
,H 2 O
H
+
Chapter 16 22
Dianion of [8]Annulene
Cyclooctatetraene easily forms a -2 ion.
Ten electrons,continuous overlapping p
orbitals,so it is aromatic,
=>
+ 2 K + 2 K
+
Chapter 16 23
Pyridine
Heterocyclic aromatic compound.
Nonbonding pair of electrons in sp2
orbital,so weak base,pKb = 8.8.
=>
Chapter 16 24
Pyrrole
Also aromatic,but lone pair of electrons is
delocalized,so much weaker base.
=>
Chapter 16 25
Basic or Nonbasic?
NN
Pyrimidine has two basic
nitrogens.
N N H
Imidazole has one basic
nitrogen and one nonbasic.
N
N
N
N
H
Purine? =>
Chapter 16 26
Other Heterocyclics
=>
Chapter 16 27
Fused Ring Hydrocarbons
Naphthalene
Anthracene
Phenanthrene
=>
Chapter 16 28
Reactivity of
Polynuclear Hydrocarbons
As the number of aromatic rings increases,
the resonance energy per ring decreases,
so larger PAH’s will add Br2.
H B r
H B r H B r
B r
H
(mixture of cis and trans isomers) =>
Chapter 16 29
Fused Heterocyclic
Compounds
Common in nature,synthesized for drugs.
=>
Chapter 16 30
Allotropes of Carbon
Amorphous,small particles of graphite;
charcoal,soot,coal,carbon black.
Diamond,a lattice of tetrahedral C’s.
Graphite,layers of fused aromatic rings.
=>
Chapter 16 31
Some New Allotropes
Fullerenes,5- and 6-membered rings
arranged to form a,soccer ball” structure,
Nanotubes,half of a C60 sphere fused to a
cylinder of fused aromatic rings.
=>
Chapter 16 32
Common Names of
Benzene Derivatives
O H O C H 3N H
2
C H 3
p h e n o l t o lu e n e a n i lin e a n i s o le
C
H
C H 2 C
O
C H 3
C
O
H
C
O
O H
s t y r e n e a c e t o p h e n o n e b e n z a ld e h y d e b e n z o ic a c id
=>
Chapter 16 33
Disubstituted Benzenes
The prefixes ortho-,meta-,and para- are
commonly used for the 1,2-,1,3-,and 1,4-
positions,respectively.B r
B r
o - d ib r o m o b e n z e n e o r
1,2 - d i b r o m o b e n z e n e
H O
N O 2
p - n i t r o p h e n o l o r
4 - n i t r o p h e n o l
=>
Chapter 16 34
3 or More Substituents
Use the smallest possible numbers,but
the carbon with a functional group is #1.
N O 2
N O 2
O 2 N
1,3,5 - t r i n i t r o b e n z e n e
N O 2
N O 2
O 2 N
O H
2,4,6 - t r i n i t r o p h e n o l
=>
Chapter 16 35
Common Names for
Disubstituted Benzenes
C H 3
C H 3
C H 3
C H 3H 3 C
C H 3
C
O
O H
O H
H 3 C
m - x y l e n e m e s i t y l e n e o - t o lu ic a c id p - c r e s o l
=>
Chapter 16 36
Phenyl and Benzyl
B r
p h e n y l b r o m i d e
C H 2 B r
b e n z y l b r o m i d e
Phenyl indicates the benzene ring
attachment,The benzyl group has
an additional carbon.
=>
Chapter 16 37
Physical Properties
Melting points,More symmetrical than
corresponding alkane,pack better into
crystals,so higher melting points.
Boiling points,Dependent on dipole
moment,so ortho > meta > para,for
disubstituted benzenes.
Density,More dense than nonaromatics,
less dense than water.
Solubility,Generally insoluble in water,=>
Chapter 16 38
IR and NMR Spectroscopy
C=C stretch absorption at 1600 cm-1.
sp2 C-H stretch just above 3000 cm-1.
1H NMR at?7-?8 for H’s on aromatic
ring.
13C NMR at?120-?150,similar to alkene
carbons,
=>
Chapter 16 39
Mass Spectrometry
=> =>
Chapter 16 40
UV Spectroscopy
=>
Chapter 16 41
End of Chapter 16
