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How does pyrrole differ structurally from pyridine?
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heterocyclic compound
There are many different sulfur-containing heterocycles. One of the best known is thiophene, C4H4S, derivatives of which occur as plant pigments and as other natural products such as biotin.
Pyrrole and pyridine differ primarily in their ring size and the electronic contribution of their nitrogen atoms to their aromatic systems.
Structural Differences
- Ring Size: Pyrrole is a five-membered ring composed of four carbon atoms and one nitrogen atom. Pyridine is a six-membered ring composed of five carbon atoms and one nitrogen atom.12
- Nitrogen Bonding: In pyrrole, the nitrogen atom is bonded to two carbon atoms and one hydrogen atom. In pyridine, the nitrogen atom is bonded to only two carbon atoms.3
Electronic and Aromatic Properties
The two compounds also differ in how their nitrogen atoms participate in the aromatic -electron sextet:
- Pyrrole (Pyrrole-like): The nitrogen atom contributes its unshared pair of electrons to the aromatic sextet. Because these electrons are drawn into the ring, there is a net flow of electron density from the nitrogen atom to the carbon atoms.3
- Pyridine (Pyridine-like): The nitrogen atom contributes only one electron to the aromatic sextet, while its unshared electron pair remains uninvolved. Due to nitrogen's higher electronegativity compared to carbon, electrons in the pyridine ring tend to move toward the nitrogen atom.3