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Pyrrole
Chemical name	Pyrrole Pyrrol
Chemical formula	C4H5N
Molecular mass	67.09 g/mol
CAS number	[109-97-7]
Density	0.967 g/cm3
Melting point	-23 °C
Boiling point	129-131 °C
SMILES	C1=CC=CN1
Disclaimer and references

Pyrrole, or pyrrol, is a heterocyclic aromatic organic compound, a five-membered ring with the formula C₄H₅N.

Pyrroles are components of larger aromatic rings, including the porphyrins of heme, the chlorins and bacteriochlorins of chlorophyll, and the corrin ring of vitamin B12.

Contents 1 Reactivity 2 Commercial Uses 3 See also 4 External links 5 References

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 is because the lone pair of electrons of the nitrogen atom becomes delocalized in the aromatic ring. Pyrrole is very weak base (pK_aH about -4). Protonation results in loss of aromaticity and is therefore unfavorable.

Both NH and CH protons in pyrroles are moderately acidic and can be deprotonated with strong bases such as butyllithium and the metal hydrides, rendering the pyrroles 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 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, both of which are compatible with a variety of pyrrole substrates.

Pyrrole compounds can also participate in cycloaddition (Diels-Alder) reactions under certain conditions, such as Lewis acid catalysis, heating, or high pressure.

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. [1]

• Arsole, a moderately-aromatic arsenic analog.
• Furan, an analog with an oxygen instead of the nitrogen.
• Indole, a derivative with a fused benzene ring.
• Phosphole, a non-aromatic phosphorus analog.
• Polypyrrole
• Pyroluria
• Pyrrolidine, the saturated hydrogenated derivative
• Simple aromatic rings
• Thiophene, an analog with a sulfur instead of the nitrogen atom.

• General Synthesis and Reactivity of Pyrrole
• Synthesis of pyrroles (overview of recent methods)
• Substitution reaction mechanisms of nitrogen-containing heteroaromatics

• Links to external chemical sources

1. Loudon, Marc G. (2002). "Chemistry of Naphalene and the Aromatic Heterocycles.", Organic Chemistry, Fourth Edition, New York: Oxford University Press, 1135-1136. ISBN 0-19-511999-1.