Why does NO₂ group show its effect only at ortho?

The NO₂ group, or nitro group, is a powerful electron-withdrawing group that influences the reactivity of aromatic compounds. Its effects are particularly pronounced at the ortho position due to several factors.

Resonance and Inductive Effects

The nitro group can stabilize the negative charge that develops during electrophilic aromatic substitution reactions through resonance. This stabilization is most effective when the nitro group is positioned ortho to the site of substitution.

Ortho Position Advantages

• Resonance Stabilization: The ortho position allows for better overlap of p-orbitals, enhancing resonance interactions.
• Inductive Effect: The strong electron-withdrawing nature of the nitro group pulls electron density away from the aromatic ring, making the ortho position more reactive.

Steric Considerations

While the para position is also influenced by the nitro group, steric hindrance can limit the effectiveness of substitution reactions at this position compared to the ortho position. The close proximity of the nitro group in the ortho position facilitates more favorable interactions during the reaction process.

Summary of Effects

In summary, the ortho position is favored for reactions involving the NO₂ group due to enhanced resonance stabilization and the significant inductive effect, combined with less steric hindrance compared to the para position. This unique combination makes ortho substitution more favorable in aromatic compounds with a nitro group.