Out of benzene, m–dinitrobenzene and toluene which will undergo nitration most easily and why?

Explanation

To determine which compound among benzene, m-dinitrobenzene, and toluene undergoes nitration most easily, we need to analyze the effects of substituents on the benzene ring and their influence on electrophilic aromatic substitution reactions.

### Step-by-Step Solution:

1. **Understanding Nitration**:
- Nitration is an electrophilic aromatic substitution reaction where a nitro group (NO2) is introduced into the aromatic ring. The nitronium ion (NO2+) acts as the electrophile in this reaction.

2. **Analyzing the Compounds**:
- **Benzene**: Benzene is a simple aromatic compound with no substituents. It has a stable electron cloud due to its resonance structure, which allows for electrophilic attack.
- **Toluene**: Toluene has a methyl group (–CH3) attached to the benzene ring. The methyl group is an electron-donating group due to its +I (inductive) and +R (resonance) effects, which increases the electron density on the benzene ring, making it more reactive towards electrophiles.
- **m-Dinitrobenzene**: This compound has two nitro groups (–NO2) in the meta position. Nitro groups are strong electron-withdrawing groups due to their –I (inductive) and –R (resonance) effects. They decrease the electron density on the benzene ring, making it less reactive towards electrophiles.

3. **Comparing Reactivity**:
- **Toluene**: The electron-donating effect of the methyl group increases the electron density on the ring, facilitating the attack by the nitronium ion. Therefore, toluene will undergo nitration most easily.
- **Benzene**: Benzene has a moderate electron density due to its resonance structure, making it less reactive than toluene but more reactive than m-dinitrobenzene.
- **m-Dinitrobenzene**: The presence of two electron-withdrawing nitro groups significantly reduces the electron density on the ring, making it the least reactive towards nitration.

4. **Conclusion**:
- The order of reactivity towards nitration is: **Toluene > Benzene > m-Dinitrobenzene**. Thus, toluene will undergo nitration most easily due to the electron-donating effect of the methyl group.

### Final Answer:
Toluene will undergo nitration most easily, followed by benzene, and m-dinitrobenzene will undergo it the least due to the electron-donating and electron-withdrawing effects of the substituents.