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Pyrrole, or pyrrol, is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. Substituted derivatives are also called pyrroles. For example, C4H4NCH3 is N-methylpyrrole. Porphobilinogen is a trisubstituted pyrrole, which is the biosynthetic precursor to many natural products.
Additional recommended knowledge
Pyrrole has very low basicity compared to amines and other aromatic compounds like pyridine where the ring nitrogen is not bonded to a hydrogen atom. 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 -4. Protonation results in loss of aromaticity and is therefore unfavorable.
Many methods exist for the organic synthesis of pyrrole and its derivatives. Classic named reactions are the Knorr pyrrole synthesis, the Hantzch pyrrole synthesis and the Paal-Knorr synthesis.
The starting materials in the Piloty-Robinson pyrrole synthesis are 2 equivalents of an aldehyde and hydrazine. The product is a pyrrole with specific substituents in the 3 and 4 positions. The aldehyde reacts with the diamine to an intermediate di-imine (R-C=N-N=C-R) which with added hydrochloric acid, gives ring-closure and loss of ammonia to the pyrrole.
In the second step a [3,3]sigmatropic reaction takes place between two intermediates.
Both NH and CH protons in pyrroles are moderately acidic and can be deprotonated with strong bases such as butyllithium and the metal hydrides. The resulting "pyrrolides" are nucleophilic. Trapping of the conjugate base with an electrophile (e.g. an alkyl or acyl halide) reveals which sites were deprotonated based on which ring positions actually react as nucleophiles. The product distribution of such a reaction can often be complex and depends on the base used (especially the counterion, such as lithium from butyllithium or sodium from sodium hydride), existing substitution of the pyrrole, and the electrophile.
Pyrrole undergoes electrophilic aromatic substitution predominantly at the 2 and 5 positions, though the substitution product at positions 3 and 4 is obtained in low yields. 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 are compatible with a variety of pyrrole substrates. Reaction of pyrroles with formaldehyde form porphyrins.
In a 1994 report released by five top cigarette companies, pyrrole is one of the 599 additives to cigarettes. Its use or purpose, however, is unknown, like most cigarette additives. 
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pyrrole". A list of authors is available in Wikipedia.