electrons from nitrogen. In pyrrole, there are four p electrons, two short of the
Hu ¨ckel criteria for aromaticity. The nitrogen atom is sp
2
-hybridized, formally
containing a lone pair of electrons in the p orbital at right angles to the ring.
However, the system delocalizes and pushes the lone pair of electrons into the
ring to complete the sextet required for aromaticity. The nonbonding electrons
on the nitrogen atom become a part of the aromatic sextet. A small number of
simple pyrroles occur in nature. However, biologically more significant
natural pyrroles are rather less simple; they are tetrameric pyrrole derivatives,
known as porphyrins, e.g. chlorophyll-a and haem.
Furan, also known as furane and furfuran, is an oxygen-containing fivemembered aromatic heterocyclic compound that is usually produced when
wood, especially pine wood, is distilled. The highly electronegative oxygen
holds on the electron density tightly. Although it has a lone pair of electrons,
these electrons cannot delocalize easily, and so the system is generally
considered to be almost nonaromatic or weakly aromatic.
Thiophene is a sulphur-containing five-membered unsaturated heterocycle. The lone pair electrons of the sulphur are in the 3s orbital, and are less
able to interact with the p electrons of the double bonds. Therefore,
thiophene is considered weakly aromatic. Acetylenic thiophene is found
in some higher plant species. However, the thiophene ring is present in many
important pharmaceutical products.
S
Acetylenic thiophene
Physical properties of pyrrole, furan and thiophene
Pyrrole is a weakly basic compound. However, as the nonbonding electrons
on the nitrogen atom are part of the aromatic sextet, and no longer available
for protonation, it has an extremely low basicity (pK a ¼ $ 15). Pyrrole
accepts a proton on one of the carbon atoms adjacent to the nitrogen atom,
whereas the proton on the nitrogen atom can be removed by hydroxide ion
to yield its conjugate base.
N
H
N
N
H
..
Base
Acid
..
..
_
Base
Acid
+
Conjugate base
Pyrrole
pK a = ~ 15
Conjugate acid
pK a = − 3.80
Salts containing the pyrrole anion can easily be prepared by this way. The
pair of nonbonding electrons on N in pyrrole is much less available for
protonation than the pair on ammonia. Thus, pyrrole is much less basic than
NH 3 (pK a ¼ 36), i.e. a much stronger acid than NH 3 .
4.7.4 PYRROLE, FURAN AND THIOPHENE:
147
Hu ¨ckel criteria for aromaticity. The nitrogen atom is sp
2
-hybridized, formally
containing a lone pair of electrons in the p orbital at right angles to the ring.
However, the system delocalizes and pushes the lone pair of electrons into the
ring to complete the sextet required for aromaticity. The nonbonding electrons
on the nitrogen atom become a part of the aromatic sextet. A small number of
simple pyrroles occur in nature. However, biologically more significant
natural pyrroles are rather less simple; they are tetrameric pyrrole derivatives,
known as porphyrins, e.g. chlorophyll-a and haem.
Furan, also known as furane and furfuran, is an oxygen-containing fivemembered aromatic heterocyclic compound that is usually produced when
wood, especially pine wood, is distilled. The highly electronegative oxygen
holds on the electron density tightly. Although it has a lone pair of electrons,
these electrons cannot delocalize easily, and so the system is generally
considered to be almost nonaromatic or weakly aromatic.
Thiophene is a sulphur-containing five-membered unsaturated heterocycle. The lone pair electrons of the sulphur are in the 3s orbital, and are less
able to interact with the p electrons of the double bonds. Therefore,
thiophene is considered weakly aromatic. Acetylenic thiophene is found
in some higher plant species. However, the thiophene ring is present in many
important pharmaceutical products.
S
Acetylenic thiophene
Physical properties of pyrrole, furan and thiophene
Pyrrole is a weakly basic compound. However, as the nonbonding electrons
on the nitrogen atom are part of the aromatic sextet, and no longer available
for protonation, it has an extremely low basicity (pK a ¼ $ 15). Pyrrole
accepts a proton on one of the carbon atoms adjacent to the nitrogen atom,
whereas the proton on the nitrogen atom can be removed by hydroxide ion
to yield its conjugate base.
N
H
N
N
H
..
Base
Acid
..
..
_
Base
Acid
+
Conjugate base
Pyrrole
pK a = ~ 15
Conjugate acid
pK a = − 3.80
Salts containing the pyrrole anion can easily be prepared by this way. The
pair of nonbonding electrons on N in pyrrole is much less available for
protonation than the pair on ammonia. Thus, pyrrole is much less basic than
NH 3 (pK a ¼ 36), i.e. a much stronger acid than NH 3 .
4.7.4 PYRROLE, FURAN AND THIOPHENE:
147