Question

The nitroso group, \(-\mathrm{N}=\mathrm{O},\) is one of the few nonhalogens that is an ortho- and para-directing deactivator. Explain this behavior by drawing resonance structures of the carbocation intermediates in ortho, meta, and para electrophilic reaction on nitrosobenzene, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}=\mathrm{O}\).

Short answer

Ortho and para positions stabilize via resonance with the nitroso group.

Step-by-step solution

Understanding Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a reaction in which an aromatic ring undergoes substitution in the presence of an electrophile. The nitroso group, \(-\mathrm{N}=\mathrm{O}\), influences where the electrophile will attach to the benzene ring. The key is to analyze resonance structures to explain ortho- and para-directing deactivation.

Drawing Ortho Attack Resonance Structures

Consider the electrophile attacking the ortho position of nitrosobenzene. This forms a carbocation intermediate where the positive charge is stabilized through resonance. Draw the structure with the electrophile at the ortho position and move benzene ring electrons to show resonance stabilization. You'll see structures with the positive charge delocalized across the ring but also partially on the nitrogen, indicating increased stability despite deactivation.

Drawing Para Attack Resonance Structures

Next, place the electrophile at the para position, creating a para-directed carbocation intermediate. Draw resonance structures similar to the ortho case, showing how the positive charge can be delocalized across the ring and towards the nitrogen. Due to position symmetry, para-intermediates often show increased resonance stability, like the ortho structures.

Drawing Meta Attack Resonance Structures

For a meta attack, place the electrophile at the meta position, forming a carbocation intermediate. Draw all possible resonance structures. As seen, the positive charge on the benzene ring cannot resonate towards the nitrogen of the nitroso group, hence it provides less stability compared to ortho and para intermediates.

Comparing Resonance Stabilization

Evaluate and compare the resonance forms from the ortho, meta, and para attacks. The resonance stabilization is greater in the ortho and para positions because the positive charge from electrophilic attack can be delocalized onto the nitrogen atom of the nitroso group, unlike in the meta position, leading to enhanced stability despite being a deactivator.

Key concepts

Nitroso Group

The nitroso group, represented as a chemical structure of it's unique in its behavior during Electrophilic Aromatic Substitution (EAS) reactions. Instead of typical behavior, where electron-withdrawing groups direct electrophilic attack to meta positions, the nitroso group directs to ortho and para positions. This group consists of nitrogen double-bonded to oxygen, meaning it can both withdraw and donate electrons due to resonance effects.

• With its resonance capabilities, the nitroso group stabilizes positive charges more effectively when the electrophile attacks at ortho or para positions.
• This is somewhat counterintuitive as it also deactivates the benzene ring, making it less reactive because its electron-withdrawing nature still slightly impairs the electron density of the aromatic system.

Ortho- and para-directing effects

In electrophilic aromatic substitution reactions, different substituents on the aromatic ring can influence where the incoming electrophile will add. The directing effects guide whether an electrophile will attach preferentially to the ortho, meta, or para positions.

• The nitroso group, an unusual case for non-halogen deactivators, exerts ortho- and para-directing effects.
• This happens because the electrons can be delocalized into the nitrogen atom of the nitroso group through resonance when an electrophile adds to the ring.

When attacked at the ortho position, the aromatic system can shift its stabilizing the carbocation intermediate. A similar effect is seen at the para position, often resulting in more symmetrical resonance structures due to the para position's location. This leads to stable intermediates, although the general electron-withdrawing nature still slightly deactivates the ring.

Resonance structures

Resonance structures are vital in understanding the behavior of the nitroso group during electrophilic aromatic substitution. They show how electrons can be spread out or delocalized in a molecule.

• When an electrophile attacks at the ortho or para position, resonance structures illustrate the movement of electrons.
• The electron shift creates several resonance forms, distributing the positive charge across different atoms of the aromatic ring.

In the case of the nitroso group, the positive charge appearing at the nitrogen atom. This network of resonance structures explains why ortho and para attacks are more stabilized, as opposed to the meta attack, where such delocalization towards the nitrogen is impossible.

Carbocation intermediates

Carbocation intermediates are a crucial concept in understanding why certain positions on the aromatic ring are favored during substitution reactions. These intermediates form when an electrophile attacks the benzene ring, temporarily creating a positive charge.

• For nitrosobenzene, carbocation intermediates occur when an electrophile attacks the ortho or para positions, establishing a positively charged carbon atom on the ring.
• The nitroso group helps stabilize these intermediates, as the nitrogen atom can accommodate the positive charge, unlike in the meta position.

This stabilization through resonance is why the nitroso group is ortho- and para-directing, even while it generally deactivates the aromatic ring.