Which of the following is an `o-, p-`directing but deactivating substituent in an electrophilic aromatic substitution :

Options

A. `-C Cl_(3)`
B. `-Cl`
C. `-NHCOCH_(3)`
D. `-OCH_(3)`

Answer

To determine which of the following groups is an ortho-para directing but deactivating substituent in an electrophilic aromatic substitution, we need to analyze the properties of the substituents in question.

### Step-by-Step Solution:

1. **Understand Electrophilic Aromatic Substitution**:
- In electrophilic aromatic substitution (EAS), an electrophile attacks the aromatic ring. The nature of the substituent already present on the ring influences the reactivity and the position where the electrophile will attack.

2. **Identify Directing Effects**:
- Substituents can be classified as either ortho-para directing or meta directing:
- **Ortho-para directing**: These groups donate electron density to the aromatic ring, making it more reactive towards electrophiles. Examples include -OH, -OCH3, -NH2.
- **Meta directing**: These groups withdraw electron density, making the ring less reactive. Examples include -NO2, -CN, -COOH.

3. **Classify Substituents**:
- Substituents can also be classified as activating (increase reactivity) or deactivating (decrease reactivity):
- **Activating groups**: Increase the reactivity of the aromatic ring (e.g., -OH, -OCH3).
- **Deactivating groups**: Decrease the reactivity of the aromatic ring (e.g., -NO2, -CF3).

4. **Analyze the Given Substituents**:
- For the question, we need to find a group that is ortho-para directing but also deactivating.
- **Chlorine (Cl)**: Chlorine has a -I (inductive) effect which is electron-withdrawing, making it a deactivator. However, it also has a +M (mesomeric) effect, which makes it ortho-para directing. Therefore, Cl is ortho-para directing but deactivating.
- Other groups like -NH2 or -OCH3 are activating and ortho-para directing, while groups like -NO2 are deactivating and meta directing.

5. **Conclusion**:
- The only substituent that is both ortho-para directing and deactivating is chlorine (Cl).

### Final Answer:
Chlorine (Cl) is an ortho-para directing but deactivating substituent in electrophilic aromatic substitution.