Pyrrole, Furan and Thiophene Electrophilic Substitution Reactions | Chemistry Optional Notes for UPSC PDF Download

 Page 1


Heterocyclic Chemistry
CH-2-I
Five-membered Heterocycles
Pyrrole, Furan and Thiophene
Page 2


Heterocyclic Chemistry
CH-2-I
Five-membered Heterocycles
Pyrrole, Furan and Thiophene
Five Membered Heterocycles-Introduction
• The main reason for the study of pyrrole came from the work on the structure of
haem; the blood respiratory pigment, and the chlorophyll; the green photosynthetic
pigment of plants.
• Thiophen does occur in plants in association with polyacetylenes with which they
are biogenetically closely linked.
• Furan occurs widely in secondary plant metabolites, especially in terpenoids.
• Unsubstituted pyrrole, furan, and thiophene are usually obtained from petroleum.
Page 3


Heterocyclic Chemistry
CH-2-I
Five-membered Heterocycles
Pyrrole, Furan and Thiophene
Five Membered Heterocycles-Introduction
• The main reason for the study of pyrrole came from the work on the structure of
haem; the blood respiratory pigment, and the chlorophyll; the green photosynthetic
pigment of plants.
• Thiophen does occur in plants in association with polyacetylenes with which they
are biogenetically closely linked.
• Furan occurs widely in secondary plant metabolites, especially in terpenoids.
• Unsubstituted pyrrole, furan, and thiophene are usually obtained from petroleum.
General Characteristics
• Pyrrole, thiophene and furan are colorless liquids of boiling points 131
o
, 84
o
and
32
o
respectively.
• Pyrrole has a relatively high boiling point as compared to furan and thiophene,
this is due to the presence of intermolecular hydrogen bonding in pyrrole.
N
N N
H
H H
Page 4


Heterocyclic Chemistry
CH-2-I
Five-membered Heterocycles
Pyrrole, Furan and Thiophene
Five Membered Heterocycles-Introduction
• The main reason for the study of pyrrole came from the work on the structure of
haem; the blood respiratory pigment, and the chlorophyll; the green photosynthetic
pigment of plants.
• Thiophen does occur in plants in association with polyacetylenes with which they
are biogenetically closely linked.
• Furan occurs widely in secondary plant metabolites, especially in terpenoids.
• Unsubstituted pyrrole, furan, and thiophene are usually obtained from petroleum.
General Characteristics
• Pyrrole, thiophene and furan are colorless liquids of boiling points 131
o
, 84
o
and
32
o
respectively.
• Pyrrole has a relatively high boiling point as compared to furan and thiophene,
this is due to the presence of intermolecular hydrogen bonding in pyrrole.
N
N N
H
H H
Structure  and Aromaticity
• Pyrrole, furan and thiophene are aromatic because:
1) they fulfill the criteria for aromaticity, the extent of delocalization of the
nonbonding electron pair is decisive for the aromaticity, thus the grading of
aromaticity is in the order of: furan < pyrrole < thiophene < benzene this order is
consistent with the order of electronegativity values for oxygen (3.44), nitrogen
(3.04) and thiophene (2.56).
2) They tend to react by electrophilic substitution due appearance of –ve charge on
carbon atoms due to delocalization as shown in the following resonance structures.
Page 5


Heterocyclic Chemistry
CH-2-I
Five-membered Heterocycles
Pyrrole, Furan and Thiophene
Five Membered Heterocycles-Introduction
• The main reason for the study of pyrrole came from the work on the structure of
haem; the blood respiratory pigment, and the chlorophyll; the green photosynthetic
pigment of plants.
• Thiophen does occur in plants in association with polyacetylenes with which they
are biogenetically closely linked.
• Furan occurs widely in secondary plant metabolites, especially in terpenoids.
• Unsubstituted pyrrole, furan, and thiophene are usually obtained from petroleum.
General Characteristics
• Pyrrole, thiophene and furan are colorless liquids of boiling points 131
o
, 84
o
and
32
o
respectively.
• Pyrrole has a relatively high boiling point as compared to furan and thiophene,
this is due to the presence of intermolecular hydrogen bonding in pyrrole.
N
N N
H
H H
Structure  and Aromaticity
• Pyrrole, furan and thiophene are aromatic because:
1) they fulfill the criteria for aromaticity, the extent of delocalization of the
nonbonding electron pair is decisive for the aromaticity, thus the grading of
aromaticity is in the order of: furan < pyrrole < thiophene < benzene this order is
consistent with the order of electronegativity values for oxygen (3.44), nitrogen
(3.04) and thiophene (2.56).
2) They tend to react by electrophilic substitution due appearance of –ve charge on
carbon atoms due to delocalization as shown in the following resonance structures.
• The order of aromatic character of these three heterocycles compared to benzene is
as follows:
Benzene > Thiophene > pyrrole > Furan
This order is consistent with the resonance Energies and order of the     
electronegativity values.
• In comparison to benzene the order of reactivity in electrophilic substitution is as
follows:
Pyrrole > Furan > Thiophene > Benzene
• Eelectrophilic substitution on furan requires very mild non acidic conditions (acids 
may induce polymerization or ring opening), however, for thiophene the acidity is 
less critical since it is stable to aqueous minral acids but not to 100 % strong acids 
or Lewis acids such as AlCl
3
.
• Regioselectivity: The 2 & 5 (a) positions are more reactive than 3 & 4 ( ß ) 
Positions, as in pyrrole the intermediate  results from electrophilic attack at C2 can 
be stabillized by three resonance structure while the intermaediate results from the 
attack C3 is only satbilized by two resonance structures. 
Structure  and Aromaticity