M.Sc. II S.M. Joshi Reactivity of Five Membered Heterocyclic Rings

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Presentation transcript:

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

Aromatic Five-Membered Heterocycles

Pyrrole is an extremely weak base:

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

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

Electrophilic aromatic substitution reactions:

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

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

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

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

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:

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

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

Thank You