Chapter 19 Polycyclic & Heterocyclic Aromatic Compounds ...

Chapter 19
Polycyclic & Heterocyclic Aromatic Compounds
Naphthalene
Was first proposed in 1866 by Erlenmeyer:
- Used as mothballs
- Derivatives of it are used in motor fuels &
lubricants
Introduction
Graphite
Top view Side view
Planes of fused benzene rings with an interdistance of 3.5 Ǻ.
This interesting structure enables it to be used as:
- A lubricrant
- An inert electrode

Heterocycles
N
H
Yohimban
A ring having two or more different elements that can
be aromatic.
H
N
H
Nomenclature
- PAHs have individual names
- Numbering is fixed by convention regardless of the position
of the substituent
7
6
8
5
9
10
1
4
2
3
7
6
8
5
1
4
Naphthalene
Anthracene Phenanthrene
2
3
H
3
4
5
6
2 9
1 10
7
8
Nomenclature of PAH
Nomenclature
- Monosubstituted naphthalenes can be designated by Greek
letters too:
b
b
a
a
O O
N
a
a
b
b
1-Nitronaphthalene
Nitronaphthalene 2-Nitronaphthalene
Nitronaphthalene
α-Nitronaphthalene
Nitronaphthalene β-Nitronaphthalene
Nitronaphthalene
7
6
8
5
1
4
2
3
O
N
O

Bonding in PAHs
Resonance energy
Fused aromatics have lower resonance energy than expected
Resonance energy
expected (Kcal/mol): 36
72
Resonance energy
Calculated (Kcal/mol): 36
61
72 108 108
61 84 92
Aromaticity
- Conjugated
- Planar
- Satisfies the Huckel’s Huckel rule (4n+2)
10 π electrons 14 π electrons 14 π electrons
n = 2 n = 3 n = 3
Resonance energy
All carbon bonds in benzene have the same bond length
The carbon bond lengths in naphthalene aren’t aren t all the same:
7
6
8
5
1
4
2
3
Two out of the three resonance structures show C1-C2 C1 C2
double bond

Resonance energy
Phenanthrene shows similar differences among the C9-C10 C9 C10
bond that shows an alkene character:
Br Br
Oxidation
Benzene is difficult to oxidize whereas PAHs are oxidized
more easily but they retain at least one phenyl group to keep
the aromaticity
V 2 O 5 , air, heat
CrO 3 , AcOH
heat
O
O
O
Br
OH
OH
O
1,4-naphthoquinone
Br
- H 2O
O
O
O
Oxidation of PAHs
Reduction of PAHs

Reduction
PAHs can be reduced with sodium and ethanol:
Na, EtOH
Na, EtOH
Na, EtOH
No reaction
tetralin
9,10-dihydroanthracene
EAS of Naphthalenes
Reduction
Hydrogenation of PAHs can be carried out under high T and
Pressure of hydrogen:
+
+
5 H 2
3 H 2
Pt, 35 atm
255 C
Naphthalene
H
1
H
2
H
H H
two sites possible for electrophilic
aromatic substitution
H
Decalin
all other sites at which substitution can occur
are equivalent to 1 and 2
H
H

EAS of Naphthalene
Electrophilic Aromatic Substitution reactions of naphthalenes
occur on C-1 C 1 predominantly:
Br
predominantly:
+
Br 2
HNO 3, H2SO4
H 2SO4 conc.
AcCl, AlCl 3
O
NO 2
SO 3H
CCH 3
EAS of Naphthalene
E
H
when attack is at C-2 C
in order for carbocation to be stabilized by allylic
resonance, the benzenoid character of the other
ring is sacrificed
+
E
H
EAS of Naphthalene
E H E H
+
when attack is at C-1 C
carbocation is stabilized by allylic resonance
benzenoid character of other ring is maintained
EAS of naphthalene
Sulfonation reaction of naphthalene is more complex and is T
controlled:
SO3H At 80 C:
At 160 C:
+ SO 3
fast
slow
+
91% Kinetic
15% Thermodynamic
SO 3H
9% Kinetic
85% Thermodynamic

EAS of naphthalene
2-substituted substituted naphthalene is more stable than1-naphthalene
than1 naphthalene
sulfonic acid:
H
H
H
H H
SO 3H
More repulsion
Less stable
H
H
H
H
Nomenclature
H H
H
H
Less Repulsion
More stable
Aromatic heterocycles have individual names
5
6
4
N 1
3
2
Pyridine
4
5
S
1
N
3
2
4
5
N
H
1
N
3
2
Thiazole Imidazole
SO 3H
Aromatic heterocycles with one heteroatom can be designated
by Greek letters
α
γ
β β
β β
N
Pyridine
α
H
α
N
H
Pyrrole
α
N
Nomenclature of Aromatic Heterocyclic
Compounds
Some Important Aromatic Heterocycles
N
N
H
Pyrrole Furan Thiophene
H
N
N
Pyrimidine Indole
Quinoline Isoquinoline
N
O S
H
N
N
Purine
N
N

Pyridine, a Six-Membered Six Membered Aromatic
Heterocycle
Pyridine
Pyridine is very unreactive; it resembles
nitrobenzene in its reactivity.
Presence of electronegative atom (N) in ring
causes ππ electrons to be held more strongly than
in benzene.
N
There is none.
Generalization
There are so many different kinds of heterocyclic
aromatic compounds that no generalization
is possible.
Some heterocyclic aromatic compounds
are very reactive toward electrophilic
aromatic substitution, others are very unreactive.
δ +
N δ ¯
Pyridine doesn’t doesn t undergo any Friedel-Crafts Friedel Crafts reaction
(alkylation alkylation, , acylation). acylation).
Pyridine
If reacted with a Lewis Acid
the ring becomes more e-deficient e deficient
Pyridine doesn’t doesn t make any coupling with diazonium salts.
Bromination only proceeds at high T. Substitution occurs at
the 3-position 3 position
δ ++
N +
¯FeBr3 FeBr

N
Pyridine can be sulfonated at high temperature.
EAS takes place at C-3. C 3.
Pyridine
SO 3, , H 2SO SO4 HgSO 4, , 230°C 230
N
71%
SO 3H Like benzene, pyridine resists oxidation reactions
on its ring
N
CH 3
Pyridine
KMnO 4, H 3O +
N
CO 2H
Pyridine
Though it is less basic than alkyl amines, pyridine
undergoes many reactions typical of amines
N
HCl
CH 3 I
N
H Cl -
+
N
CH 3 +
When benzene bears EWG, it undergoes a nucleophilic
substitution reaction on its ring:
O 2 N
Nucleophilic Substitution on Pyridine
NO 2
NO 2
Cl
NH 3
O 2 N
I -
NO 2
NO 2
NH 2

Nucleophilic Substitution on Pyridine
Nitrogen in pyridine withdraws electrons from the ring making
it electron deficient:
N
Loss of H -
NH 3, heat
N Br N NH2 Cl NH 2
NH 3, heat
N N
Substitution occurs at C-2 C 2 and C-4 C 4 but not on C-3 C
NH 2 -
N NH 2
N
H -
Mechanism
H
NH 2
N
N NH
H
NH 2
H 2O
N
H
NH 2
N NH 2
Nucleophilic Substitution on Pyridine
Pyridine can undergo substitution if a very strong base is used:
NH 2 - , heat
-H2 N N
N
+
Li
Heat
NH -
N
H 2O
Quinoline and Isoquinoline
N NH 2

N
7
6
8
5
Nomenclature
N 1
4
2
3
Quinoline Isoquinoline
- Both compounds behave like pyridine
- They are weak bases (pK ( pKb = 9.1 & 8.6)
- They undergo EAS more easily than pyridine but on C-5 C 5 & C-8 C
Nucleophilic Substitution Reactions
Like Pyridine, they undergo substitution if a very strong base
is used:
N
N
1- NH 2 -
2- H 2O
1- CH 3Li
2- H 2O
N
N
CH 3
7
6
N
8
5
NH 2
1
4
N 2
3
N
N
EAS Reactions
HNO 3
H 2SO 4
HNO 3
H 2SO 4
NO 2
NO 2
N
N
NO 2
NO 2
Pyrrole, a Five-Membered Five Membered Aromatic
Heterocycle
N
N

N
H
Pyrrole, Furan, and Thiophene
••
N
H
••
O
••
••
S
••
Have 1 less ring atom than benzene or
pyridine to hold same number of ππ electrons
(6).
ππ electrons are held less strongly.
These compounds are relatively reactive
toward EAS..
EAS of Pyrrole
O O
BF 3
O
+ CH 3COCCH COCCH3 N CCH
H
75-92% 75 92%
undergoes EAS readily
C-2 2 is most reactive position
CCH 3
Reactions
Unlike pyridine, pyrrole is not basic under usual conditions (pK ( pKb ~ 14)
N
H
N
H
H 3O +
No stable cation
In Pyrrole, the unshared pair of electrons of the
nitrogen atom participate in conjugation which leaves
no more additional electrons to react with a proton
δ +
N δ ¯
δ ¯
••
N
H
Porphyrins
Four pyrrole units joined by =CH- =CH groups
N
H
N N
H
N
Fully aromatic
Important biological unit found in heme, chlorophyll, and in the cytochrome
A metal ion can penetrate inside the macrocycle
δ +

N
N Fe N
N
Porphyrins
HOOC
H 3C
HOOC
H 2C
N
N
Fe
N
Heme
CH 3
CH 3
N
CH 3
CH 2