Question

In which of the following molecules lone pair of nitrogen atom is not participating in resonance? (a) C=C(N)c1ccccc1 (b) c1ccncc1 (c) Cc1cccc(C)c1N(C)C (d) NCc1ccccc1

Short answer

The lone pair of nitrogen in molecule (c) does not participate in resonance.

Step-by-step solution

Understand Resonance

Resonance occurs when electrons in a molecule can be distributed over two or more atoms, typically involving lone pairs and  bonds. For nitrogen, it generally must be adjacent to a  system to participate in resonance.

Examine Molecule (a)

Molecule (a) has a structure where the nitrogen atom is directly connected to a carbon-carbon double bond, allowing the lone pair to participate in resonance with the  system.

Examine Molecule (b)

In molecule (b), the nitrogen atom is part of a pyridine ring, where its lone pair participates in the aromaticity and resonance of the ring structure.

Examine Molecule (c)

For molecule (c), the nitrogen atom is part of a tertiary amine and does not have adjacent  bonds to interact with; therefore, the nitrogen lone pair does not participate in resonance.

Examine Molecule (d)

In molecule (d), the nitrogen is attached to a carbon that is part of a benzene ring. However, the nitrogen is sp3 hybridized, and its lone pair generally does not participate in resonance with the benzene ring unless specific conditions allow it, which doesn't apply here.

Key concepts

Lone Pair Participation

In organic chemistry, understanding how lone pairs of electrons participate in resonance is crucial for predicting the stability and reactivity of molecules. Lone pairs, which are pairs of valence electrons not shared in bonds, can delocalize and contribute to resonance structures when the conditions are right. For nitrogen atoms, which often have lone pairs, participation in resonance usually requires a suitable alignment with adjacent pi (π) systems, such as double bonds or aromatic rings.

When a nitrogen atom is adjacent to a π system, its lone pair can overlap with the adjacent π orbitals, allowing for electron delocalization across the structure. This creates a system of interconnected p orbitals that enhances the molecule's stability through resonance. However, if the nitrogen is not directly adjacent to a π bond or if other geometric or electronic factors prevent proper overlap, its lone pair may not participate in resonance.

In molecule (c) from the exercise, the nitrogen is part of a tertiary amine and isolated from adjacent π bonds, meaning its lone pair is not involved in resonance. Similarly, in molecule (d), the nitrogen is attached to a benzene ring, but due to its sp3 hybridization, the lone pair does not typically participate in resonance unless specific conditions are met.

Nitrogen Hybridization

Hybridization of nitrogen atoms significantly influences their geometry and ability to participate in resonance. Hybridization refers to the mixing of atomic orbitals to form new hybrid orbitals that accommodate electron pairs. These hybrid types include sp, sp2, and sp3, each providing different geometries and angular arrangements.

For resonance to occur, an sp2 hybridized nitrogen is ideal because it retains one unhybridized p orbital. This p orbital can overlap with adjacent p orbitals in π systems, facilitating electron delocalization for resonance. An sp hybridized nitrogen would also have this beneficial configuration, while sp3 hybridized nitrogen typically does not participate in resonance, as all its p orbitals are hybridized, leaving none for overlap with a π system.

In molecule (d) from the exercise, the nitrogen is sp3 hybridized. This limits its involvement in resonance with the benzene ring, as its geometry and orbital alignment are not conducive to the necessary overlap for resonance.

Benzene Ring Structure

The benzene ring is a classic example of an aromatic system with significant resonance stability. Its structure consists of six carbon atoms linked in a planar, cyclic arrangement, where adjacent carbon-carbon bonds share electron density between them. This creates a continuous ring of p orbitals overlapping above and below the plane of the ring.

This delocalization of electrons within the aromatic ring leads to increased stability and is why benzene is less reactive than other alkenes. Substituents attached to the benzene ring, such as a nitrogen atom, can influence the overall resonance of the molecule based on their hybridization and connectivity.

Nitrogen's lone pair can participate in the resonance of a benzene ring only if the nitrogen is properly aligned with the π system. If the nitrogen is sp2 hybridized, it can contribute its lone pair to the aromaticity of the ring. In molecule (b), for example, the nitrogen atom participates in the aromatic and resonance stability of the pyridine ring, since its lone pair is delocalized as part of the ring system.