Question

Write a resonance form for pyrrole in which nitrogen has a formal charge of 1. Are comparable resonance forms possible for pyridine

Answer 1

While pyrrole has a resonance system involving the nitrogen's lone pair, drawing a resonance form with a positive charge on nitrogen is not favorable. Pyridine's lone pair is not in the aromatic system, allowing for resonance structures with a protonated nitrogen, such as the pyridinium ion. Understanding resonance helps to explain electron distribution in molecules like these.

Step-by-step explanation:

The question is asking to write a resonance form for pyrrole where nitrogen has a formal charge of +1 and to explore if comparable resonance forms are possible for pyridine. However, in pyrrole, the nitrogen atom is already part of a resonance system and typically does not bear a formal positive charge in the preferred resonance structures due to the aromatic nature of pyrrole. The question essentially tests our understanding of resonance structures in heterocyclic aromatic compounds and the difference between pyrrole and pyridine concerning their nitrogen lone pairs and basicity.

For pyridine, the nitrogen atom's lone pair is not involved in the aromatic π-system and lies in an sp² orbital, which is perpendicular to the plane of the aromatic ring. This means it can be protonated to form the pyridinium ion (C5H5NH+), which has different resonance structures. For pyrrole, the nitrogen's lone pair is part of the aromatic π-system, and while one could hypothetically draw a resonance structure with nitrogen carrying a positive charge, this would not be a major contributor to the overall resonance hybrid due to the disruption of its aromatic stability.

The concept of resonance is crucial here, as it describes the delocalization of electrons in molecules that cannot be accurately depicted with a single Lewis structure. Resonance structures are drawn to represent all possible arrangements of electrons that conform to the molecular geometry, and the true structure is a hybrid of these resonance forms. In general, resonance structures involving minimal formal charges and maintaining aromatic stability are more favorable.

Answer 2

See explanation and image attached

Step-by-step explanation:

Here attached are resonance forms of pyrrole and pyridine. The images were obtained from quora and researchgate respectively.

Now, we can see that in the resonance forms of pyrrole, the nitrogen atom in the heterocycle has a formal charge of +1. However, in the six membered pyridine hetrocycle, the nitrogen atom may have a formal charge of -1 or +1 as shown in the canonical structures attached. The structures in which nitrogen has a +1 formal charge in pyridine are comparable to structures obtained from pyrrole. These structures have less contribution to the structure of pyridine.