Resonance structure of Pyrrole, Basic nature of Pyrrole & Acidic Nature of Pyrrole

1. 1
B.Sc. Final year
MEB Students
for
Today’s
Chemistry Lecture
by
Dr Pramod R Padole

2. 2

4. LOGO
A) Heterocyclic Compounds:
N
H
2
34
5
1
Pyrrole
N
1 2
3
4
5
6
Pyridine

5. LOGO
a)5-membered Heterocyclic compounds
(i) Pyrrole or Azole:
(Aza-cyclo-penta-2,4-diene) C4H5N
N
H
2
34
5
1
Pyrrole

6. LOGO
Resonance:
The double bond keep on changing their
positions and this is called Resonance.
Resonance is also called mesomerism.
I II
Resonance structure of Benzene Resonance hybrid

7. LOGO
Note that:
I) Bond length of C-C bond = 1.54 Ao
II) Bond length of C=C bond = 1.34 Ao
III) Bond length of all C-C bonds in Benzene = 1.39 Ao

8. LOGOwww.themegallery.com
of Pyrrole
Resonance structure

9. pramodpadole@gmail.com By Dr Pramod R Padole
How do you know when to draw
resonance structures?

10. Directing ( or orienting) influence:
A molecule can have resonance structures when it has a lone pair or a double bond on the atom next to a double bond.

11. LOGO
Resonance Structure of Pyrrole:
According to the resonance theory,
pyrrole is a resonance hybrid of the
following resonating structures.
Q.1) Discuss the resonance in Pyrrole. (S-16, 4 Mark)

12. LOGO
N
H
Pyrrole
1
2
34
5
Resonance Structure of Pyrrole:
 According to the resonance theory, pyrrole is a resonance hybrid of the following
resonating structures.
 In the resonating structures, Nitrogen donates electrons (Lone pair) to the
pyrrole ring.
 Therefore, electron density on Carbon atom (position-3) increases, i.e., they have
become negatively charged at C-3 positon .
Q.1) Discuss the resonance in Pyrrole. (S-16, 4 Mark)
Resonating Structures
of Pyrrole
N
H
I
3
1
2
4
5

13. LOGO
Resonating Structures
of Pyrrole
N
H
I
3
1
2
4
5
Resonance Structure of Pyrrole:
 According to the defination of resonance, therefore, electron density on Carbon atom
(position-5) increases, i.e., they have become negatively charged at C-5 positon .
Resonating Structures
of Pyrrole
N
H
II
5
1
2

14. LOGO
N
H
III
2
1
34
Resonating Structures of Pyrrole
Resonating Structures
of Pyrrole
N
H
II
5
1
2
Resonance Structure of Pyrrole:
 According to the defination of resonance, therefore, electron density on Carbon atom
(position-2) increases, i.e., they have become negatively charged at C-2 positon .

15. LOGO
Resonating Structures of Pyrrole
N
H
IV
4
1
5 2
3
N
H
III
2
1
34
Resonating Structures of Pyrrole
Resonance Structure of Pyrrole:
 According to the defination of resonance, therefore, electron density on Carbon atom
(position-4) increases, i.e., they have become negatively charged at C-4 positon .

16. LOGO
Resonance Structure of Pyrrole:
 In the resonating structures, Nitrogen donates electrons
(Lone pair) to the pyrrole ring.
 Therefore, electron density on Carbon atom increases, i.e.,
they have become negatively charged.
 The resonance energy of pyrrole is 21 Kcal/mole.
 It is less than pyridine (31 Kcal/mole) and benzene (36 Kcal/mole).
 So, pyrrole is more reactive than pyridine and benzene
(towards the electrophilic substitution reaction).
N
H
N
H
N
H
N
H
N
H
I II III IVPyrrole
3
5 2
4
1
1 1 11
2
34
5 2
4
5
2
34
5 2
3
Resonating Structures of Pyrrole

17. Company
LOGO
Q.1) Explain, why, pyrrole is weak base. (S-11 & W-15, 2 Mark)
Q.2) Explain the basic character of pyrrole.
Q.3) Explain, why? Pyrrole acts as a very weak base.
Q.4) Explain the basicity of Pyrrole. (W-16, 2 Mark)
Q.5) Compare the basic nature of pyrrole with pyridine. (S-17, 4 Mark)
Q.6) Describe basic nature of pyrrole. (S-19 & W-19, 2-4 Mark)
Basic Nature or
Basic Character of Pyrrole:

18. Basic Nature (or Character) of Pyrrole:
 When pyrrole is reacted with dil. HCl in absence of
oxygen (O2); to form crystalline salt, called as pyrrole
hydrochloride.
 This reaction indicates pyrrole is a base.
C4H4NH + HCl [C4H4NH2] Cl
O2
N
H
O2
H Cl
N
H H
.. in absence of
Base Acid Pyrrole hydrochloride salt (Stable)
(Crystalline salt)
Or
+ in absence of
Acid
Pyrrole hydrochloride salt
Pyrrole
Base
....
Cl
Note:
Pyrrole hydrochloride is stable in absence of Oxygen, otherwise polymerization
rapidly occurs to produce a brown resin.
Any species that donates a pair of electrons to a Lewis acid

19. Basic Nature or Basic Character of Pyrrole:
The reason for the weakly basic character
of pyrrole are -
(1) The lone pair of electrons present on
nitrogen atom of pyrrole is available for
donation / protonation / co-ordination.
So, pyrrole is basic in nature.
N
H
Pyrrole
..
Lone pair of electrons
available for protonation / donation /
coordination.

20. Basic Nature or Basic Character of Pyrrole:
(2) The lone pair of electrons present on nitrogen atom
of pyrrole is involved in the formation of delocalized
π-molecular orbital or involved in resonance.
Because of this involvement, the availability of the lone pair
of electrons of the nitrogen atom for protonation is very much
decreased ( i.e., lone pair of electrons is not available
easily for donation / protonation / co-ordination).
So, pyrrole is very weak basic. (pKb= 8.4).
Lone pair Lone pair involed in the formation of -M.ON
H
N
H

21. Basic Nature or Basic Character of Pyrrole:
According to the resonance theory, pyrrole is
a resonance hybrid of the following
resonating structures.

22. Basic Nature or Basic Character of Pyrrole:
(3) Further if, a proton is added to the nitrogen by reaction
with acid.
So, resulting product (structure) is obtained, which lost their
aromaticity (aromatic character) and resonance energy.
So, pyrrole cation is very unstable.
Hence, pyrrole is very weak base.
N
H
H
N
H H
+
Pyrrole
aromatic in nature
Proton
from acid
..
Lost their aromatic character
Pyrrole cation
Unstable

23. LOGO
Q.1) Explain / Discuss the acidic nature of pyrrole.
(S-14 & W-14, 4 Mark)
Q.2) Explain why, pyrrole can also act as a weak acid.
Q.3) Explain the acidity of Pyrrole. (W-16, 2 Mark)
Q.4) Describe acidic nature of pyrrole. (S-19, 2 Mark)
Acidic Nature of Pyrrole:

24. Acidic Nature of Pyrrole:
Pyrrole is not only acts as a weak base but also act as
a very weak acid (pKa=15).
 The acidity of pyrrole is about same as that of acetylene.
e.g. 1) When pyrrole is reacted with solid KOH; to form
a salt called as potassio pyrrole or pyrryl potassium.
So, pyrrole is acidic in nature.
N
H
K OH
N
H2 O
+
Pyrrole
solid
Base
Acid
+.. ..
Potassio pyrrole /
pyrryl potassium (salt)
K

25. Acidic Nature of Pyrrole:
e.g. 2)
Pyrrole is also reacted with Grignard reagent; to form
hydrocarbon (Alkane) and N-pyrrole magnesium halide.
(Because pyrrole contain active hydrogen atom.
Active H-atom is that H-atom, which is more acidic than the H of
an alkanes, R-H).
N
H
N
R-Mg X
CH3
-
MgBr
R-H+
Pyrrole
+.. ..
or
G.R.
dry Ether
Mg-X
Alkane
N-pyrrole magnesium halide

26. Acidic Nature of Pyrrole:
 The reason for the weakly acidic character of pyrrole is that –
 The N-H bond in pyrrole is a weak bond as the lone pair of
electrons present on N-atom is involved in resonance
(i.e., delocalization of π-molecular orbital), makes it positively
charged and increases the possibility of abstraction of bonding pair
of electron from Hydrogen (removal proton); to form pyrryl anion.
 Furthermore, pyrryl anion is more stabilized than pyrrole by the
delocalization of negative charge over the ring on the basis of
resonance structures. (As phenol and phenoxide ion)
So, pyrrole is acidic in nature.
N
H
- H N
III
N N N
III IV V
N
Stabilization of pyrryl anion
Weak bond
1 1 1 1 1 1
2
3
3
4
5
5
2
4 3
2
4

27. Acidic Nature of Pyrrole:
 The reason for the weakly acidic character of pyrrole is that –
 The N-H bond in pyrrole is a weak bond as the lone pair of
electrons present on N-atom is involved in resonance
(i.e., delocalization of π-molecular orbital), makes it positively
charged and increases the possibility of abstraction of bonding pair
of electron from Hydrogen (removal proton); to form pyrryl anion.
 Furthermore, pyrryl anion is more stabilized than pyrrole by the
delocalization of negative charge over the ring on the basis of
resonance structures. (As phenol and phenoxide ion)
So, pyrrole is acidic in nature.
N
H
- H N
III
N N N
III IV V
N
Stabilization of pyrryl anion
Weak bond
1 1 1 1 1 1
2
3
3
4
5
5
2
4 3
2
4

28. Acidic Nature of Pyrrole:
 The reason for the weakly acidic character of pyrrole is that –
 The N-H bond in pyrrole is a weak bond as the lone pair of
electrons present on N-atom is involved in resonance
(i.e., delocalization of π-molecular orbital), makes it positively
charged and increases the possibility of abstraction of bonding pair
of electron from Hydrogen (removal proton); to form pyrryl anion.
 Furthermore, pyrryl anion is more stabilized than pyrrole by the
delocalization of negative charge over the ring on the basis of
resonance structures. (As phenol and phenoxide ion)
So, pyrrole is acidic in nature.
N
H
- H N
III
N N N
III IV V
N
Stabilization of pyrryl anion
Weak bond
1 1 1 1 1 1
2
3
3
4
5
5
2
4 3
2
4

29. LOGO

30. LOGO
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Heterocyclic Compounds by Dr Pramod R. Padole
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