Reactivity
The NH proton in pyrroles is moderately acidic with a pKa of 16.5. Pyrrole can be deprotonated with strong bases such as butyllithium and sodium hydride. The resulting alkali pyrrolide is nucleophilic. Treating this conjugate base with an electrophile such as methyl iodide gives N-methylpyrrole.
The resonance contributors of pyrrole provide insight to the reactivity of the compound. Like furan and thiophene, pyrrole is more reactive than benzene towards electrophilic aromatic substitution because it is able to stabilize the positive charge of the intermediate carbocation.
Pyrrole undergoes electrophilic aromatic substitution predominantly at the 2 and 5 positions. Two such reactions that are especially significant for producing functionalized pyrroles are the Mannich reaction and the Vilsmeier-Haack reaction (depicted below), both of which compatible with a variety of pyrrole substrates.
Pyrroles react with aldehydes to form porphyrins. For example, benzaldehyde condenses with pyrrole to give tetraphenylporphyrin. Pyrrole compounds can also participate in cycloaddition (Diels-Alder) reactions under certain conditions, such as under Lewis acid catalysis, heating, or high pressure.
Pyrrole polymerizes in light. An oxidizing agent, such as ammonium persulfate, can also be used, typically at 0 ºC and in darkness to control the polymerization.
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