Pyrrole, Furan, Thiophene, Imidazole, Oxazole, Thiazole, five membered heterocycles

1. Five Membered
Heterocyclic compounds
Mr. Shriram Bairagi M.Pharm
Assistant Professor & HOD
Department of Pharmaceutical Chemistry
Ideal College of Pharmacy and Research, Kalyan
By

2. Five-Membered Heterocycles
Furan Pyrrole Thiophene
➢ These are common five Membered unsaturated heterocyclic compounds.
➢ Each has two double bonds and O, N, and S heteroatoms respectively.
➢ The numbering starts from the heteroatoms towards the substituent
present according to the priorities.

3. Furan

4. Synthesis of Furan

9. Pyrrole

21. Pyrrole, thiophene, and furan gives electrophilic aromatic substitution reaction. These
compounds are more reactive compared to benzene. Electrophiles majorly attack on
2nd position rather than 3rd position in these heterocyclic compounds. The reason
behind it is the more number of resonating intermediate structure are possible to
accommodate the positive charge when electrophile attacks on 2nd position (three
resonating intermediate structures) that makes the intermediate carbocation more
stable while if electrophile attacks on the 3rd position then only two resonating
intermediate structures are possible as shown in the figure which is comparitively less
stable. That’s why the attack of electrophiles takes place at 2nd position in pyrrole,
thiophene, and furan.

23. Thiophene
Resonance structure of Thiophene

24. Synthesis

26. Paal-Knorr synthesis of thiophene

31. Resonance structure

46. Thiazole

54. THANK YOU