Nitration of nitrobenzene gives mainly m-dinitro benzene.

Answer

To understand why nitration of nitrobenzene predominantly yields m-dinitrobenzene, we can break down the process step by step.

### Step-by-Step Solution:

1. **Understanding Nitration**:
- Nitration is a chemical reaction that introduces a nitro group (NO₂) into an organic compound. In this case, we are nitrating nitrobenzene using concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) to generate the nitronium ion (NO₂⁺), which is the active electrophile in the reaction.

2. **Formation of the Electrophile**:
- When concentrated HNO₃ and H₂SO₄ are mixed, the nitronium ion (NO₂⁺) is formed. This ion is highly reactive and seeks to attach to the benzene ring.

3. **Electron-Withdrawing Effect of the Nitro Group**:
- The nitro group (–NO₂) is an electron-withdrawing group due to its strong electronegativity. This means it pulls electron density away from the benzene ring, affecting the positions where further electrophilic substitution can occur.

4. **Positioning of the Nitro Group**:
- In nitrobenzene, the nitro group is already present at one of the positions on the benzene ring. The presence of this electron-withdrawing group influences the reactivity of the remaining positions (ortho, meta, and para) for further substitution.
- The ortho (2-position) and para (4-position) positions will experience a partial positive charge due to the electron-withdrawing effect of the nitro group, making them less nucleophilic. Conversely, the meta (3-position) does not have this charge and retains more electron density.

5. **Nucleophilic Attack**:
- Since nitration involves the attack of a nucleophile (the benzene ring) on the electrophile (the nitronium ion), the meta position becomes the most favorable site for this attack. The ortho and para positions are less favorable because they are destabilized by the positive charge resulting from the electron-withdrawing effect of the existing nitro group.

6. **Formation of m-Dinitrobenzene**:
- As a result of the preferential attack at the meta position, the product formed is predominantly m-dinitrobenzene. The reaction can be summarized as:
- Nitrobenzene + NO₂⁺ → m-Dinitrobenzene

### Conclusion:
The nitration of nitrobenzene primarily yields m-dinitrobenzene due to the electron-withdrawing effect of the nitro group, which destabilizes the ortho and para positions while favoring the meta position for electrophilic attack.