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In which of the following nitrogen-containing rings is the indicated lone pair on nitrogen delocalized into (participates in resonance with) the ring pi system?

1. Pyridine, where the lone pair lies in an sp2 orbital in the plane of the ring
2. Pyrrole, where the N-H nitrogen's lone pair occupies a p orbital
3. A six-membered aromatic ring with the in-plane nitrogen lone pair
4. A six-membered ring with an in-plane N-H lone pair

Correct answer: Pyrrole, where the N-H nitrogen's lone pair occupies a p orbital

Solution

In pyridine the nitrogen lone pair lies in an sp2 hybrid orbital in the plane of the ring, perpendicular to the aromatic pi system, so it cannot conjugate with the ring. In pyrrole the nitrogen is bonded to H and contributes its lone pair from a p orbital into the pi system, giving the aromatic 6 pi-electron sextet. Therefore the pyrrole-type N-H lone pair participates in resonance.

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