Pyrrole

"Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C₄H₄NH.^[1] It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C₄H₄NCH₃. Porphobilinogen, a trisubstituted pyrrole, is the biosynthetic precursor to many natural products such as heme."

"Pyrroles are components of more complex macrocycles, including the porphyrins of heme, the chlorins, bacteriochlorins, chlorophyll, porphyrinogens."

"Pyrrole has very low basicity compared to conventional amines and some other aromatic compounds like pyridine. This decreased basicity is attributed to the delocalization of the lone pair of electrons of the nitrogen atom in the aromatic ring. Pyrrole is a very weak base with a pK_aH of about −4. Protonation results in loss of aromaticity, and is, therefore, unfavorable."