Heterocyclic Compounds Part-II (Pyrrole) by Dr Pramod R Padole

Molecular Orbital Structure / Picture / M.O. diagram of Pyrrole,

1. Our Inspiration 1

2. A) Heterocyclic Compounds:

3. a)5-membered Heterocyclic compounds ( i ) Pyrrole or Azole : (Aza-cyclo-penta-2,4-diene) C 4 H 5 N

4. 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) sp 2 (c) sp 3 (b) sp 3 d Molecular Orbital Structure / Picture / M.O. diagram of Pyrrole :

5. Molecular orbital structure of Pyrrole : [email protected] By Dr Pramod R. Padole M.O. of Pyrrole 1 4 2 3 5 Representation 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

6. Molecular orbital structure of Pyrrole : Molecular formula of pyrrole is C 4 H 5 N According to MOT, the sigma (σ) bonds and pi (π) bonds molecular orbital diagram / structure of pyrrole are as,

7. Structure of Pyrrole : In pyrrole , all carbon and nitrogen undergoes sp 2 -hybridisation . 1 All ring atoms in pyrrole ( Four carbon & one nitrogen) contains three sp 2 H.O’s 2 sp 2 hybrid orbital of each atom is half-filled (singly filled) 3 The unhybridised p-orbital (i.e., 2-p z ) of each carbon atom is half-filled & that of nitrogen atom ( 2p z 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 sp 2 -hybridisation (All C, N & H atoms are in one plane). 5

8. E. C. of Carbon:

9. Hybridization: C

10. Hybridization: N

11. Formation of sigma (σ ) bonds M.O. diagram: The sp 2 H.O’s overlap with each other and with s-atomic orbital of the five hydrogen forming three carbon – carbon, two carbon – nitrogen (by sp 2 -sp 2 overlap) , four carbon - hydrogen and one nitrogen – hydrogen (by sp 2 -s overlap ) sigma bonds. So, total ten sigma (σ) bonds are formed. All these sigma bonds lie in one plane and are at 120 from each other.

12. 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.

13. Formation of pi (π) bonds M.O. diagram: The p- orbitals on carbons contain one electron ( 2p z 1 ) each and p-orbital on nitrogen contains two electrons , i.e , lone pair ( 2p z 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.

14. 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.

15. Representation of Pyrrole : A common / general representation of pyrrole is simply pentagon with a circle inside . The circle represents the π-molecular orbital.

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