1. M.Sc. II S.M. Joshi Reactivity of Five Membered Heterocyclic Rings
Prof Pawar A. A.

2. Aromatic Five-Membered Heterocycles

3. Pyrrole is an extremely weak base:

4. The dipole moment in pyrrolidine (left) is attributed to
the electron-withdrawing property of the nitrogen atom:

6. Pyrrole, furan, and thiophene undergo electrophilic
substitution, preferentially at C-2:

7. Electrophilic aromatic substitution reactions:

8. Three resonance structures: more stable
Structures of the intermediates that can be formed from
the reaction of an electrophile with pyrrole at C-2 and C-3
Three resonance structures: more stable
Two resonance structures: less stable

9. If both positions adjacent to the heteroatom are occupied, electrophilic substitution occurs at C-3:

10. Why? Because of cation stabilization by lone-pair resonance release:

11. The relative reactivities of the five-membered
heterocycles in Friedel–Crafts reaction:

12. The resonance hybrid of pyrrole indicates that there is a
partial positive charge on the nitrogen:
Pyrrole is unstable in strongly acid solution because the
protonated pyrrole polymerizes:

13. Five resonance structures for the anion
Pyrrole is more acidic than pyrrolidine because of stabilization of its conjugated base by resonance
Five resonance structures for the anion
Localized anion

15. Indole, benzofuran, and benzothiophene all contain a five-membered aromatic ring fused to a benzene ring:

16. Thank You