Question

Why does the presence of nitro groups at ortho- and para- positions in haloarenes increase their reactivity towards nucleophilic substitution reactions?

Hint:

In \( \text{S}_\text{N}2 \) reactions, primary halides react faster than secondary or tertiary halides due to less steric hindrance. In haloarenes, nitro groups at ortho- and para- positions increase reactivity by stabilizing the transition state through their electron-withdrawing nature.

Answer 1

The presence of nitro groups at the ortho- and para- positions in haloarenes increases their reactivity towards nucleophilic substitution reactions due to the following reasons:
1. Electron-Withdrawing Effect:
- Nitro groups (\( \text{NO}_2 \)) are strong electron-withdrawing groups due to their resonance and inductive effects. They pull electron density away from the aromatic ring, particularly at the ortho- and para- positions, making the carbon attached to the halogen more electrophilic. 2. Stabilization of the Transition State:
- In nucleophilic substitution reactions, the intermediate or transition state is stabilized by electron-withdrawing groups. The nitro group at the ortho- or para- position stabilizes the negative charge on the transition state, making it easier for the nucleophile to attack the electrophilic carbon. 3. Increased Reactivity:
- This increased electrophilicity of the carbon to which the halogen is attached makes the molecule more reactive towards nucleophilic substitution reactions, especially in the \( \text{S}_\text{N}2 \) mechanism. Thus, nitro groups at ortho- and para- positions in haloarenes increase their reactivity towards nucleophilic substitution reactions by making the carbon more electrophilic and stabilizing the transition state.