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Pyrroles are five-membered nitrogen containing heterocyclic aromatic compounds. The high resonance stabilization of the pyrrole system makes it more reactive than benzene. Hence pyrroles undergo electrophilic substitution reactions such as halogenations, nitration, and sulfonation, with mild electrophiles. The electrophilic reactions occur much more readily at 2 and 5 positions. Although pyrrole is an amine, it is not available for bonding to a proton and hence it is an extremely non-basic one. On the other hand, pyrrole is weakly acidic, and forms salts with potassium hydroxide because the resulting anion is resonance stabilized. Certain N-substitued pyrroles undergo cycloaddition reactions such as (4+2), (2+2), (2+1) cyclizations.
Pyrrole and its derivatives are widely used as intermediates in the synthesis of pharmaceuticals, medicines, agrochemicals, dyes, photographic chemicals, perfumes and other organic compounds. Pyrroles gained pharmacological importance due to their role in the anti-inflammatory, analgesic, antimicrobial, antitumor, antiepileptic, antiviral, antidiabetic, and antihypertensive properties of various pharmaceuticals (Pyrroles and Fused Pyrroles Synthesis and Therapeutic Activities. Mohamed, S. S. et al. Mini-Reviews in Organic Chemistry, 2014, 11(4).). Their structures form part of a vast variety of many natural products including hemoglobin, chlorophyll and alkaloids. In addition, pyrroles can be used as catalysts for polymerization, corrosion inhibitors, preservatives, and as solvents.