The document discusses electrophilic substitution reactions of pyrrole. It explains that electrophilic substitution in pyrrole occurs preferentially at the 2-position and 5-position. This is because attack of an electrophile at the 2-position forms a more stable carbocation intermediate due to greater delocalization of positive charge through three resonance structures compared to only two structures for attack at the 3-position. Specific electrophilic substitution reactions of pyrrole discussed include halogenation, nitration, sulphonation, and Friedel-Crafts acetylation. Reaction conditions and products are provided for converting pyrrole to halogenated, nitro, and sulfonated derivatives.

1. 1

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

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

5. LOGO
Explaination on the basis of Resonance stability of
intermediate Carbonium ion (σ-complex)
formed by the attack of
electrophile (E+):
Orientation of Electrophilic
Substitution in Pyrrole:
N
H
Pyrrole
1
2 / 5
3 / 44 / 3
5 / 2
N
H
Pyrrole
1
2 / 5
3 / 44 / 3
5 / 2

6. N
H
Pyrrole
1

7. N
H
Pyrrole
1
N
H
Pyrrole
1
N
H
Pyrrole
1
2 / 5
3 / 44 / 3
5 / 2

8. Attack of E+ at ?
N
H
Pyrrole
1
2 / 5
3 / 44 / 3
5 / 2

9. N
H
Pyrrole
1
2 / 5
3 / 44 / 3
5 / 2
Orientation of Electrophilic Substitution in Pyrrole:
pramodpadole@gmail.com Prepared by Dr Pramod R Padole
Attack of E+ at 2-position (at C-2):2
Attack of E+ at 3-position (at C-3):1
In pyrrole, electrophilic substitution occur at

10. LOGOOrientation of Electrophilic Substitution
in Pyrrole:
Q.1) Discuss / Describe the orientation of electrophilic substitution
in pyrrole. (W-09, S-14 & W-18, 3- 4 Mark)
Q.2) Why Electrophilic substitution in pyrrole takes place at two (2)
and five (5) positions. (S-13 & W-14, 3 Mark)
Q.3) On the basis of resonance structure of pyrrole, explain why it
undergoes electrophilic substitution at 2-position and not at
3-position. (S-13, 4 Mark)
Q.4) Why pyrrole undergoes electrophilic substitutions reactions
preferably at C2 and C3 positions? (W-13, 3 Mark)
Q.5) Explain the orientation of electrophilic substitution in case of
pyrrole. (S-15, 4 Mark)
Q.6) Substitution of an electrophile in pyrrole mainly occurs at position
number: (S-17 & W-18, ½ Mark)
(a) 1 (b) 2 (c) 3 (d) 4
Q.7) At what positions pyrrole undergoes electrophilic substitution?
Give reasons. (S-17, 4 Mark)

11. Orientation of Electrophilic Substitution in Pyrrole:
 Resonating structures of an intermediate carbonium ion (σ-
complex) formed by the attack of electrophile (E+) at 2-position and
3-position are given below:
(1) Attack of the electrophile at 3-position in pyrrole leads to an
intermediate with only two resonance structures (less stable).
1 1 1
2
3
5
2
4 3
2
N
E
Electrophile
Only two resonating structure
L e s s S t a b l e
N
H
N
H
III
- H
H
N
H
3-Substituted
Pyrrole
H H
1) Electrophilic Attack at 3-position (or 4-position):
E E E
attack
at C3
H
3
3

12. 1
3
2
Resonating structure
N
H
I
H
E
1
25
4
N
E
Electrophile
H
1) Electrophilic Attack at 3-position (or 4-position):
attack
at C3
H
3
Orientation of Electrophilic Substitution in Pyrrole:
(1) Attack of the electrophile at 3-position in pyrrole leads to an
intermediate with only two resonance structures (less stable).

13. 1
3
2
N
H
II
H
E
Resonating structureResonating structure
1
3
2
N
H
I
H
E
Orientation of Electrophilic Substitution in Pyrrole:
(1) Attack of the electrophile at 3-position in pyrrole leads to an
intermediate with only two resonance structures (less stable).

14. Orientation of Electrophilic Substitution in Pyrrole:
(1) Attack of the electrophile at 3-position in pyrrole leads to an
intermediate with only two resonance structures (less stable).
1
3
2
N
H
II
H
E
Resonating structure
- H
N
H
3-Substituted
Pyrrole
E
3

15. Orientation of Electrophilic Substitution in Pyrrole:
 Resonating structures of an intermediate carbonium ion (σ-
complex) formed by the attack of electrophile (E+) at 2-position and
3-position are given below:
(1) Attack of the electrophile at 3-position in pyrrole leads to an
intermediate with only two resonance structures (less stable).
1 1 1
2
3
5
2
4 3
2
N
E
Electrophile
Only two resonating structure
L e s s S t a b l e
N
H
N
H
III
- H
H
N
H
3-Substituted
Pyrrole
H H
1) Electrophilic Attack at 3-position (or 4-position):
E E E
attack
at C3
H
3
3

16. Orientation of Electrophilic Substitution in Pyrrole:
2) Electrophilic Attack at 2-position (or 5-position):
 Pyrrole undergoes electrophilic substitution at the 2-position.
 Attack of the electrophile at 2-position in pyrrole leads to an intermediate
with three resonance structures. That is, the intermediates produced by
attack at 2-position is more stable.
1 1 1
3
5
2
4 33
N
E
Electrophile
Three resonating structure
M o r e S t a b l e
N
E
H
H
N
E
H
H
III
N
E
H
H
III
- H
H
N
H
E
2-Substituted Pyrrole
2) Electrophilic Attack at 2-position (or 5-position):
H
2
attack
at C2
Favoured Product
2
1 1
2
2
344 4
5
5
5

17. Orientation of Electrophilic Substitution in Pyrrole:
.
1 1 1
3
5
2
4 33
N
E
Electrophile
Three resonating structure
M o r e S t a b l e
N
E
H
H
N
E
H
H
III
N
E
H
H
III
- H
H
N
H
E
2-Substituted Pyrrole
2) Electrophilic Attack at 2-position (or 5-position):
H
2
attack
at C2
Favoured Product
2
1 1
2
2
344 4
5
5
5
1 1 1
2
3
5
2
4 3
2
N
E
Electrophile
Only two resonating structure
L e s s S t a b l e
N
H
N
H
III
- H
H
N
H
3-Substituted
Pyrrole
H H
1) Electrophilic Attack at 3-position (or 4-position):
E E E
attack
at C3
H
3
3
2-electrophilic (or 5-electrophilic) attacked intermediate carbocation which is
more stable due to more number of resonance structures and greater
delocalization of positive charge than 3-electrophilic (or 4-electrophilic)
attacked carbonium ion.
That is the reason that, in pyrrole electrophilic attack occurs at 2-position
(or 5-position) is more stable rather at 3-position (or 4-position).

18. LOGO
Chemical Reactions of
Pyrrole:
Or Electrophilic Substitution Reactions:
N
H
Pyrrole
1
2 / 5
3 / 44 / 3
5 / 2
Pyrrole undergoes electrophilic substitution
reactions at 2-position.

19. Chemical Reactions of Pyrrole:
First Second Third Fourth
Halogenation:
Pyrrole
undergoes
halogenation
rapidly due to
activation of
pyrrole ring.
Nitration:
Reaction with
cold solution of
HNO3 in Acetic
anhydride:
Or
Preparation of
2-nitro-pyrrole
from Pyrrole:
Sulphonation:
Reaction with
SO3
in
Pyridine:
Friedel –Craft
Acylation
(Acetylation):
or
Reaction with
Pyrrole &
Acetic
anhydride
at 250oC:

20. Chemical Reactions of Pyrrole:
(a) Chlorination:
(c) Iodination:
Reaction with
Iodine
in presence of
aqueous
KI solution:
1) Halogenation:
(b) Bromination:
Reaction with
Bromine in
presence of
ethanol at 0oC:
Reaction
with
Chlorine:
Reaction
with
sulphuryl
chloride
(SO2Cl2)
in ether
at 0oC
(273 K):

21. Chemical Reactions of Pyrrole:
(a) Chlorination:
(i) Reaction with
Chlorine:
(ii) Reaction with
sulphuryl chloride (SO2Cl2)
in ether at 0oC (273 K):
When pyrrole is reacted or treated or heated with chlorine
(5 equivalents of Cl2); to form penta-chloro-pyrrole.
N
H
N
Cl
Cl Cl
Cl25 Cl Cl 5 HCl
Excess
Pyrrole Chlorine
Penta-chloro-pyrrole

22. Chemical Reactions of Pyrrole:
(a) Chlorination:
(ii) Reaction with sulphuryl chloride (SO2Cl2) in ether at 0oC (273 K):
When pyrrole is reacted or treated or heated with sulphuryl
chloride (SO2Cl2) in ether at 0oC (273 K) ; to form 2, 3, 4, 5 –
tetra chloro-pyrrole.
Q.1) Complete the following reaction. (W-18, 2 Mark)
N
H
Ether
2 SO2Cl2
273K
(O0
C)
?
N
H
N
H
Cl
Ether
ClCl
Cl2 SO2Cl2
0o
C
or
273KPyrrole
Sulphuryl
chloride
2,3,4,5-tetra-chloro-pyrrole
( sulfuryl chloride, SO2Cl2, is a source of chlorine whereas
thionyl chloride, SOCl2 is a source of chloride ions)

23. Chemical Reactions of Pyrrole:
(b) Bromination:
Reaction with Bromine in presence of ethanol at 0oC:
Q.1) How will you convert: Pyrrole to tetra-bromo pyrrole? (W-13, 2 Mark)
When pyrrole is reacted or treated with bromine (4 equivalents)
in presence of ethanol at 0oC (or 273K); to form
2,3,4,5 –tetra bromo-pyrrole.
0o
C
or
273K
N
H
N
H
4 Br2
C2H5OH
Br Br
BrBr 4 HBr
Pyrrole 2,3,4,5-tetra-bromo-pyrrole

24. Chemical Reactions of Pyrrole:
(c) Iodination:
Reaction with Iodine in presence of aqueous KI solution:
Q.1) How will you obtain: 2,3,4,5 – tetra-iodo-pyrrole from pyrrole? (S-16, 2 Mark)
When pyrrole is reacted or treated with iodine (4 moles) in
presence of in aqueous potassium iodide (KI) solution; to form
2,3,4,5 – tetra-iodo-pyrrole (or iodole).
N
H
N
H
II
II4 I2
4 HI
aq. KI
Pyrrole 2,3,4,5-tetra-iodo-pyrrole

25. LOGO
Prepared by Dr Pramod R Padole
Reaction with
cold solution of HNO3 in Acetic anhydride:
Or
Preparation of
2-nitro-pyrrole from Pyrrole:
(2) Nitration:

26. LOGO
Reaction with cold solution of HNO3 in Acetic anhydride:
 When pyrrole is reacted or treated with cold solution of
nitric acid (HNO3) in acetic anhydride at about 0-5oC; to
form 2- nitro-pyrrole
Q.1) Complete the following reaction. (S-10, S-12 & W-14, 2 Mark)
Q.2) What happen when pyrrole reacts with Nitric acid? (W-12, 2 Mark)
N
H
HNO3 (CH3CO)2O
N
H
NO2 2 CH3COOH
283K
0-5 C
o
Pyrrole
Nitric acid
Acetic anhydride
2-nitro-pyrrole
Acetic acid
HO-NO2
or
H

27. LOGO
Reaction with cold solution of HNO3 in Acetic anhydride:
 When pyrrole is reacted or treated with cold solution of
nitric acid (HNO3) in acetic anhydride at about 0-5oC; to
form 2- nitro-pyrrole
CH3-C-O-C-CH3 + HO-NO2
O O
CH3-C-O-NO2 + CH3COOH
O
CH3
-C-O-NO2
O
N H
H
N NO2
H
CH3
COOH
Acetic anhydride Acetyl nitrate Acetic acid
+ 0-5 C
o
cold
+
Acetic acid
Pyrrole 2-nitro-pyrrole
Acetyl nitrate
1)
2)

28. LOGO
Sulphonation:
Reaction with SO3 in Pyridine:
Q.1) What happen when, Pyrrole is heated with sulphur trioxide in
pyridine? (S-10, 2 Mark)
Q.2) Complete the chemical equation or reaction. (C2H4Cl2)
(S-11, W-13, W-16 & W-17, 2 Mark)
Q.3) What is the action of sulphur trioxide in pyridine on pyrrole?
(W-12, 2 Mark)
Q.4) How will you convert: Pyrrole to pyrrole-2-sulphonic acid.
(W-13 & W-14, 2 Mark)
Q.5) Complete the following reaction. (W-13, 2 Mark)
N
H
Pyrrole
1
2 / 5
3 / 44 / 3
5 / 2

29. Sulphonation:
Reaction with SO3 in Pyridine:
 When pyrrole is heated with sulphur trioxide (SO3) in
pyridine, as a solvent, at about 90-100oC (363-373 K);
to from pyrrole – 2-sulphonic acid (or 2-pyrrole-
sulphonic acid).

30. Friedel –Craft Acylation
(Acetylation):
or Reaction with Pyrrole &
Acetic anhydride at 250oC:
Q.1) What happen when: Pyrrole is heated with acetic anhydride at 523 K ?
(W-09 & S-15, 2 Mark)
Q.2) Complete the following reaction: (S-18, 2 Mark)
N
H
(CH3CO)2O
523K
250 C
0
H
?

31. LOGOReaction with Pyrrole &
Acetic anhydride at 250oC:
 When pyrrole is heated with acetic anhydride
at 250oC (523K), pyrrole undergoes acylation;
to form 2-acetyl pyrrole.
 No catalyst is required in this reaction.
Note: Pyrrole does not give acetylation reaction with acetyl chloride;
because pyrrole contain imino hydrogen (>NH).
N
H
(CH3CO)2O
N
H
CH3COOHC
O
CH3
523K
250 C
0
Pyrrole
Aceteic anhydride
2-acetyl-pyrrole
H

32. LOGO

33. LOGO
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Heterocyclic Compounds by Dr Pramod R. Padole
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