The presence of -NO2 group at ortho or para position increases the reactivity of haloarenes towards nucleophilic substitution reactions. Give reason to explain the above statement.
Ans.
The presence of a nitro group (-NO2) at the ortho or para position in haloarenes increases the reactivity of these compounds towards nucleophilic substitution reactions due to the electron-withdrawing nature of the nitro group.
The nitro group is a strong electron-withdrawing group because of the strong electronegativity of the nitrogen and oxygen atoms. The nitrogen atom is more electronegative than carbon, causing it to withdraw electron density from the benzene ring through resonance and inductive effects.
Here’s how the presence of the nitro group increases reactivity towards nucleophilic substitution:
-
Resonance effects: The nitro group can delocalize the negative charge across the nitrogen and oxygen atoms through resonance. This resonance stabilization allows the negative charge to be distributed over a larger area of the molecule, making the carbon atoms in the benzene ring more electron-deficient. This increased positive character at the ortho and para positions makes these positions more susceptible to attack by nucleophiles.
-
Inductive effects: The electronegative nitrogen atom of the nitro group withdraws electron density through inductive effects, which further deactivates the benzene ring towards electrophilic aromatic substitution. However, this withdrawal of electron density enhances the positive character at the ortho and para positions, making these positions more favorable for nucleophilic attack.
