Properties
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 pKaH of about -1 to -2. Protonation results in loss of aromaticity, and is, therefore, unfavorable.
Like many amines, pyrrole slowly decomposes on exposure to air and light. It turns brown over time due to accumulation of impurities such as polypyrrole and various amine oxides. It is usually purified by distillation immediately before use.
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Impure pyrrole being distilled to separate it from colored impurities
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Pure pyrrole collected from the still is now colorless and transparent to all wavelengths of visible light due to the removal of impurities
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