Which of the following is most reactive towards electrophillic aromatic substitution for halogen ?

Options

A.
B.
C.
D.

Answer

To determine which compound is most reactive towards electrophilic aromatic substitution for halogen, we need to analyze the substituents on the aromatic rings in the given options. The reactivity towards electrophilic aromatic substitution is influenced by whether the substituents are electron-donating or electron-withdrawing.

### Step-by-Step Solution:

1. **Identify the Nature of Substituents**:
- For electrophilic aromatic substitution, we need to identify whether the substituents on the aromatic ring are electron-donating groups (EDGs) or electron-withdrawing groups (EWGs).
- EDGs increase the electron density of the aromatic ring, making it more reactive towards electrophiles, while EWGs decrease the electron density, making it less reactive.

2. **Analyze Each Option**:
- **Option A: Toluene (C6H5-CH3)**: The methyl group (–CH3) is an electron-donating group through hyperconjugation and inductive effect (+I). This increases the electron density on the benzene ring, making it more reactive.
- **Option B: Nitrobenzene (C6H5-NO2)**: The nitro group (–NO2) is a strong electron-withdrawing group (-I and -R). This decreases the electron density on the benzene ring, making it less reactive.
- **Option C: Ethylbenzene (C6H5-C2H5)**: The ethyl group (–C2H5) is also an electron-donating group (+I and +H), similar to toluene, but it has fewer alpha hydrogens compared to toluene.
- **Option D: Chlorobenzene (C6H5-Cl)**: The chlorine atom (–Cl) has both -I (inductive effect) and +R (resonance effect) properties, but the -I effect is stronger, making it an electron-withdrawing group overall.

3. **Compare the Electron Density**:
- Toluene has the highest electron density due to the presence of three alpha hydrogens contributing to hyperconjugation.
- Ethylbenzene has two alpha hydrogens, which means it has less hyperconjugation compared to toluene.
- Nitrobenzene and chlorobenzene are less reactive due to their electron-withdrawing groups.

4. **Conclusion**:
- Among the options, toluene (Option A) is the most reactive towards electrophilic aromatic substitution because it has the highest electron density due to the electron-donating effect of the methyl group.

### Final Answer:
**Option A: Toluene (C6H5-CH3)** is the most reactive towards electrophilic aromatic substitution for halogen.