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Wednesday, January 1, 2020

Reactivity order of Pyrrole, Furan and Thiophene towards Electrophilic substitution :

Pyrrole, furan and thiophene undergo electrophilic substitution reactions like nitration, sulphonation, halogenation etc. characteristic of aromatic rings. That carbons in 5-membered heterocyclic rings have higher electron density compared to benzene and hence undergo electrophilic substitution more readily than benzene.The electrophilic substitution takes place preferentially at 2-position (C-2).
The attack of an electrophile on pyrrole, for example, will lead to formation of 2- and 3-substitution products by way of carbocations (1) and (2) respectively.The substitution occurs preferably at C-2 position because the intermediate obtained by attack at this position is more stable than the intermediate obtained by attack at C-3. The positive charge in intermediate (1) is more delocalized than intermediate (2) and hence is more stable and preferred intermediate.
The electrophilic substitution at C-2 in furan and thiophene can also be accounted in the same manner. Furan is not as reactive as pyrrole in electrophilic substitution reactions because the oxygen in furan is more electronegative than nitrogen in pyrrole and therefore does not enhance the electron density of carbons as much as pyrrole. Thiophene is less reactive than furan towards electrophilic substitution because the p-electrons of sulphur are in 3p orbital which overlaps less effectively than the 2p orbital of nitrogen or oxygen with 2p orbitals of carbon. The relative reactivities towards electrophilic substitution follows the order:

Bredt's rule:

SIR effect:
Dancing Resonance:
Inversion of Amines: