Which of the following compounds is most reactive in electrophilic aromatic substitution?

Options

A.
B.
C.
D.

Answer

To determine which compound is most reactive in electrophilic aromatic substitution, we need to analyze the substituents on each compound and their effects on the electron density of the benzene ring.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds given are:
- A) Chlorobenzene
- B) Nitrobenzene
- C) Benzene
- D) Phenol

2. **Understand Electrophilic Aromatic Substitution**: In this reaction, an electrophile attacks the aromatic compound. The reactivity depends on the electron density of the aromatic ring; higher electron density means higher reactivity.

3. **Analyze Chlorobenzene**:
- Chlorobenzene has a chlorine atom as a substituent.
- Chlorine has a -I (inductive) effect, which withdraws electron density from the ring.
- Chlorine also has a +R (resonance) effect, where it can donate electron density through resonance.
- However, the -I effect is stronger than the +R effect, leading to a net decrease in electron density.
- **Conclusion**: Chlorobenzene is not very reactive.

4. **Analyze Nitrobenzene**:
- Nitrobenzene has a nitro group (-NO2) as a substituent.
- The nitro group has both -I and -R effects, both of which withdraw electron density from the ring.
- This makes the electron density significantly lower.
- **Conclusion**: Nitrobenzene is very unreactive towards electrophilic aromatic substitution.

5. **Analyze Benzene**:
- Benzene has no substituents affecting its electron density.
- It has a moderate level of electron density, but without any substituents to enhance it.
- **Conclusion**: Benzene has average reactivity.

6. **Analyze Phenol**:
- Phenol has a hydroxyl group (-OH) as a substituent.
- The -OH group has a -I effect, which withdraws some electron density.
- However, it has a strong +R effect, which significantly increases electron density due to resonance.
- The resonance structures show that the ortho and para positions gain electron density, making phenol highly reactive.
- **Conclusion**: Phenol is the most reactive compound towards electrophilic aromatic substitution.

7. **Final Comparison**:
- Chlorobenzene: Low reactivity (due to -I effect dominating).
- Nitrobenzene: Very low reactivity (due to both -I and -R effects).
- Benzene: Moderate reactivity.
- Phenol: High reactivity (due to strong +R effect).

8. **Answer**: The compound most reactive in electrophilic aromatic substitution is **Phenol**.