Pyrrole

Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4H4NH. It is a colorless 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.

Structure and bonding

Pyrroles are aromatic. Like furan and thiophene, the pyrrole ring is aromatic because it contains 4 π-electrons in the two double bonds plus one lone pair of electrons on the nitrogen. The lone pair on the nitrogen is not part of the aromatic π-system. Pyrroles are weakly basic, with a conjugate acid pKa of −3.8. The most basic site is C-2. Pyrroles are also weakly acidic at the N-H position, with a pKa of 17.5.

Preparation

Pyrrole can be prepared by treatment of furan with ammonia in the presence of acid catalysts, such as AlCl3. Pyrrole can also be formed by dehydrogenation of pyrrolidine.

Reactions

Pyrrole undergoes reactions that are characteristic of other aromatic compounds, notably electrophilic aromatic substitution. It is particularly susceptible to such reactions because of the electron-donating effect of the nitrogen. Hence, electrophiles attack pyrrole at the 2 position, then rearrangement of the intermediate σ-complex leads to substitution at the 3 position.

Applications

Pyrrole is a constituent of tobacco smoke and not as an ingredient. Pyrrole is used as a precursor to the drug tolmetin. Polypyrrole is of some commercial value.

See also

• Furan
• Thiophene
• Porphobilinogen
• Tolmetin
• Polypyrrole

References

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