Molecular Orbital diagram of Pyrrole

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
Q.1) Discuss / Explain the molecular orbital picture/structure of
pyrrole.
(W-12, W-13, S-16, W-17, S-18 & W-19, 4 Mark)
Q.2) In Pyrrole, nitrogen atom is in a state of _____ hybridization.
(W-17, ½ Mark)
(a) sp (b) sp2 (c) sp3 (b) sp3d
Molecular Orbital Structure /
Picture / M.O. diagram of
Pyrrole:

6. L o g o
Molecular orbital structure of Pyrrole:
pramodpadole@gmail.com By Dr Pramod R. Padole
M.O. of
Pyrrole
1
4
2
3
5Representation
of pyrrole
Molecular Formula
E.C. &
Hybridization of
C & N
E.C. of H
Structure
Formation of
delocalized pi (π)
bonds M.O. diagram
Formation of sigma (σ)
bonds M.O. diagram

7. LOGOMolecular orbital structure of
Pyrrole:
Molecular formula of pyrrole is C4H5N
 According to MOT, the sigma (σ) bonds and pi (π) bonds
molecular orbital diagram / structure of pyrrole are as,
sp2
hybridization
E.C. of C6
V.S.E.C.
2s2
, 2p2
2s 2px 2py 2pz
G.S.
E.C. of C6
E.S.
E.C. of C6
sp2
H.S.
unhybridised
orbital
3 sp2
H.Os.
sp2
hybridization
E.C. of N7
V.S.E.C.
2s2
, 2p3
2s 2px 2py 2pz
G.S.
E.C. of N7
E.S.
E.C. of N7
sp2
H.S.
unhybridised
orbital
3 sp2
H.Os.
Electronic configuration of Carbon: Electronic configuration of Nitrogen:
E.C. of H1
1s
&

8. LOGO
Structure of Pyrrole:
In pyrrole, all carbon and nitrogen undergoes sp2-hybridisation.1
All ring atoms in pyrrole ( Four carbon & one nitrogen) contains
three sp2 H.O’s2
sp2 hybrid orbital of each atom is half-filled (singly filled)3
The unhybridised p-orbital (i.e., 2-pz) of each carbon atom is half-filled
& that of nitrogen atom (2pz
2) is full-filled having a lone pair of
electrons (i.e., p-orbital on nitrogen contains two electrons).4
According to molecular orbital theory (MOT);
Pyrrole ring is planar due to sp2-hybridisation
(All C, N & H atoms are in one plane).5

9. LOGO
E. C. of Carbon:

10. LOGO
Hybridization:
C

11. LOGO
Hybridization:
N

12. LOGO
Formation of sigma (σ ) bonds M.O. diagram:
 The sp2 H.O’s overlap with each other and with s-atomic orbital of
the five hydrogen forming three carbon – carbon, two carbon –
nitrogen (by sp2-sp2 overlap) , four carbon - hydrogen and one
nitrogen – hydrogen (by sp2-s overlap ) sigma bonds.
 So, total ten sigma (σ) bonds are formed.
 All these sigma bonds lie in one plane and are at 1200 from each
other.

13. LOGO
Formation of pi (π) bonds M.O. diagram:
 Each ring atom (C & N) in pyrrole also has an
unhybridised p-orbital and these are perpendicular to
the plane of sigma bonds and are parallel to each other.

14. LOGO
Formation of pi (π) bonds M.O. diagram:
 The p-orbitals on carbons contain one electron (2pz
1) each and
p-orbital on nitrogen contains two electrons, i.e, lone pair (2pz
2).
Lateral overlap of these five p-orbitals gives rise to the formation
of delocalized π-molecular orbital (or π-electron clouds) above and
below the plane of ring. This π-molecular orbital contains a total of
six electrons, due to this pyrrole shows aromatic properties or
aromaticity by satisfying Huckel’s (4n+2)π e-s, rule.
Lateral
overlapping
unhybridized
p-orbitals unhybridized
p-orbitals

15. LOGO
Formation of pi (π) bonds M.O. diagram:
 Thus, pyrrole shows aromatic character (nature) as it
contains aromatic sextet.
 This aromatic sextet opposes addition reaction and
favours electrophilic substitution reaction.
Pyrrole has Three pairs of delocalized pi (π) electrons.
Two of the pairs are shown as bonds and third pair is shown as
a pair of nonbonding electrons on N-atom.N
H
2
34
5
1
Pyrrole
..

16. LOGO
Representation of Pyrrole:
A common / general representation of
pyrrole is simply pentagon with a circle
inside.
The circle represents the π-molecular
orbital.
N
H
 M.O.

17. LOGO

18. LOGO
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Heterocyclic Compounds by Dr Pramod R. Padole
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