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]
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]
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]
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]
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[edit]
References[edit]
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Pyrrole[edit]
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Pyrrole 2D full
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Pyrrole 2D numbered
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Pyrrole CRC MW 3D balls A
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Pyrrole CRC MW 3D vdW
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Heme B
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Pyrrolsynthese 1
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Hantzsch Pyrrole Synthesis Scheme
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Knorr Pyrrole Synthesis Scheme
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Paal-Knorr Pyrrole Synthesis
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Van Leusen Mechanism
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Barton-Zard reaction
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Piloty-Robinson reaction
