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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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.

References[edit | edit source]

Pyrrole Resources
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