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Which electrons from nitrogen take part in the aromatic system of pyrrole?

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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.

In the pyrrole molecule, the nitrogen atom contributes its unshared electron pair (lone pair) to the aromatic system.1

The nitrogen atom in pyrrole is bonded to two carbon atoms and one hydrogen atom via single covalent bonds. Because the nitrogen atom possesses an outermost shell of five electrons, after forming these three covalent bonds, it retains an unshared pair of electrons. These electrons reside in an orbital that is perpendicular to the plane of the ring, allowing them to engage in cyclic conjugation.1

The aromatic sextet in pyrrole is consequently composed of:
* Four electrons from the two carbon-carbon double bonds.
* The two electrons from the nitrogen atom's unshared electron pair.1

This participation of the lone pair is a defining characteristic of "pyrrole-like" heteroatoms, which tend to donate electron density into the conjugated system of the ring.1