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 solve the question regarding which molecule makes electrophilic aromatic substitution difficult, we need to analyze the given compounds based on their substituents and their effects on the benzene ring.

### Step-by-Step Solution:

1. **Identify the Common Structure**:
All the compounds have a common structure of a benzene ring (C6H4) and a carboxylic acid group (COOH).

2. **List the Substituents**:
We need to look at the substituents attached to the benzene ring in each compound:
- Compound 1: NH2 (amino group) at the para position.
- Compound 2: OCH3 (methoxy group) at the para position.
- Compound 3: NO2 (nitro group) at the para position.
- Compound 4: Cl (chloro group) at the para position.

3. **Determine the Effect of Each Substituent**:
- **NH2 (Amino Group)**: This group has a +R (resonance donating) effect, which increases the electron density in the benzene ring, making electrophilic substitution easier.
- **OCH3 (Methoxy Group)**: Similar to the amino group, the methoxy group also has a +R effect, increasing electron density and facilitating electrophilic substitution.
- **NO2 (Nitro Group)**: This group has a -R (resonance withdrawing) effect, which decreases the electron density in the benzene ring, making electrophilic substitution difficult.
- **Cl (Chloro Group)**: The chloro group has a -I (inductive withdrawing) effect, which also decreases electron density but is less effective than the -R effect of the nitro group.

4. **Conclusion**:
Since the nitro group (NO2) is a strong electron-withdrawing group, it significantly decreases the electron density of the benzene ring. This makes electrophilic aromatic substitution very difficult in the presence of the nitro group compared to the other substituents.

5. **Final Answer**:
The compound where electrophilic aromatic substitution is most difficult is **para-nitrobenzoic acid** (option C).