Why does presence of a nitro group make the benzene ring less reactive in comparison to the unsubstituted benzene ring. Explain.

The presence of a nitro group (-NO2) on a benzene ring makes the ring less reactive compared to an unsubstituted benzene ring due to the following reasons:

1. Electron Withdrawing Nature: The nitro group is a strong electron-withdrawing group. This means it pulls electron density away from the benzene ring towards itself. This reduces the electron density in the benzene ring, making it less likely to participate in reactions that involve the donation of electrons.

2. Resonance Effect: The nitro group has a resonance effect, which further reduces the electron density in the benzene ring. In the resonance structures of nitrobenzene, the positive charge is delocalized over the benzene ring while the negative charge is localized on the oxygen atom of the nitro group. This makes the benzene ring electron-deficient, thus reducing its reactivity.

3. Inductive Effect: The nitro group also has a -I (negative inductive) effect, which means it pulls electrons away from the benzene ring through the sigma bonds. This further reduces the electron density in the benzene ring, making it less reactive.

In summary, the presence of a nitro group on a benzene ring makes the ring less reactive due to its electron-withdrawing nature and its resonance and inductive effects, which all serve to reduce the electron density in the benzene ring.