Electrophilic aromatic substitution is difficult in

Options

A. `p - NH_(2) C_(6)H_(4) CO_(2)H`
B. `p- CH_(3) OC_(6)H_(4) CO_(2) H`
C. `P- NO_(2) - C_(6) H_(4) CO_(2) H`
D. `p - Cl C_(6) H_(4) CO_(2)H`

Answer

To determine which compound will have the most difficult electrophilic aromatic substitution, we need to analyze the effects of the substituents on the aromatic ring. Electrophilic aromatic substitution reactions are influenced by the nature of the substituents already present on the ring.

### Step-by-Step Solution:

1. **Identify the Substituents:**
The question provides us with several substituents on an aromatic ring:
- NH2 (amino group)
- COOH (carboxylic acid)
- OCH3 (methoxy group)
- NO2 (nitro group)
- Cl (chlorine)

2. **Determine the Nature of Each Substituent:**
- **NH2**: This is an electron-donating group (EDG) due to its lone pair of electrons. It increases the electron density on the aromatic ring, making it more reactive towards electrophiles.
- **COOH**: This is an electron-withdrawing group (EWG). It decreases the electron density on the aromatic ring, making it less reactive.
- **OCH3**: This is also an electron-donating group (EDG) due to the lone pair on oxygen, which can participate in resonance.
- **NO2**: This is a strong electron-withdrawing group (EWG). It significantly decreases the electron density on the ring, making it very difficult for electrophiles to attack.
- **Cl**: Chlorine is a halogen that can both withdraw electrons through -I effect and donate electrons through resonance (+R effect), but its overall effect is weakly electron-withdrawing.

3. **Analyze the Effects on Electrophilic Aromatic Substitution:**
- The presence of electron-donating groups (NH2 and OCH3) increases the reactivity of the aromatic ring towards electrophiles.
- The presence of electron-withdrawing groups (COOH and NO2) decreases the reactivity of the aromatic ring towards electrophiles.
- Among the substituents, NO2 is a very strong electron-withdrawing group, which makes the aromatic ring significantly less reactive towards electrophilic substitution.

4. **Conclusion:**
Based on the analysis, the substituent that would make electrophilic aromatic substitution the most difficult is **NO2**. Therefore, the answer to the question is that electrophilic aromatic substitution is most difficult in the presence of a nitro group (NO2).

### Final Answer:
Electrophilic aromatic substitution is most difficult in the presence of **NO2**.