Pyrrole

What is the structure and chemistry of pyrrole?

Pyrrole (1H-pyrrole, CAS 109-97-7) is a planar, five-membered ring of four carbon atoms and one nitrogen atom. The nitrogen lone pair forms part of the aromatic π-system, so pyrrole is a very weak base. The N–H proton is weakly acidic. The ring is electron-rich and undergoes electrophilic substitution readily, mainly at the 2-position. Strong acids and oxidants can cause polymerisation. Common laboratory routes to substituted pyrroles include the Paal–Knorr, Knorr and Hantzsch syntheses.

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Where is pyrrole used?

The pyrrole ring occurs widely in nature, including in porphyrins (haem, chlorophyll), corrins (vitamin B12) and bile pigments such as bilirubin. In industry and research, pyrrole is used as an intermediate for active pharmaceutical ingredients, agrochemicals, dyes and fragrances. It is also the monomer for polypyrrole, an electrically conducting polymer used in sensors, coatings and electronic devices.

What should buyers and labs consider when sourcing pyrrole?

Pyrrole is supplied in reagent and synthesis grades, and in higher-purity grades for polymer and electronic applications. Purity is usually confirmed by GC. Check the CoA for assay, water content and colour. Pyrrole is flammable and toxic, so consult the supplier's SDS for its GHS classification, handling requirements and personal protective equipment. Because pyrrole oxidises and darkens on standing, it is normally stored tightly closed, under inert gas, protected from light and kept cool. Older stock may need distilling before use. Confirm the regulatory status for your market, such as REACH registration or TSCA inventory listing.

Related concepts

Pyrrole belongs to the five-membered aromatic heterocycles, alongside furan, thiophene and imidazole. Its saturated analogue is pyrrolidine, and indole is its benzo-fused derivative. Polypyrrole and porphyrins are its most important macromolecular and natural derivatives.