Which of the given reaction cannot be used to prepare aromatic nitriles (ArCN) ?

Options

A. `ArCONH_2+SOCl_2`
B. `ArN_2^(+)+CuCN`
C. `ArCONH_2+P_2O_5`
D. `ArX+KCN`

Answer

To determine which reaction cannot be used to prepare aromatic nitriles (ArCN), we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Option 1: ArCO NH2 + SOCl2**
- Reaction: ArCO NH2 (an aromatic amide) reacts with thionyl chloride (SOCl2).
- Product: The reaction produces ArCN (aromatic nitrile), along with sulfur dioxide (SO2) and hydrochloric acid (HCl).
- Conclusion: This reaction can be used to prepare aromatic nitriles. **(Not the correct answer)**

2. **Option 2: ArN2+ + CuCN**
- Reaction: An aryl diazonium ion (ArN2+) reacts with copper(I) cyanide (CuCN).
- Product: The reaction produces ArCN (aromatic nitrile), nitrogen gas (N2), and copper(I) ion (Cu+).
- Conclusion: This reaction can also be used to prepare aromatic nitriles. **(Not the correct answer)**

3. **Option 3: ArCO NH2 + P2O5**
- Reaction: An aromatic amide (ArCO NH2) reacts with phosphorus pentoxide (P2O5), which is a dehydrating agent.
- Product: The reaction leads to the formation of ArCN (aromatic nitrile) by removing water.
- Conclusion: This reaction can be used to prepare aromatic nitriles. **(Not the correct answer)**

4. **Option 4: ArX + KCN**
- Reaction: An aryl halide (ArX) reacts with potassium cyanide (KCN).
- Explanation: Aryl halides do not undergo nucleophilic substitution reactions due to the strong C-X bond and resonance stabilization, which makes them inert towards nucleophiles like CN-. Therefore, this reaction cannot effectively produce ArCN.
- Conclusion: This reaction cannot be used to prepare aromatic nitriles. **(Correct answer)**

### Final Answer:
The reaction that cannot be used to prepare aromatic nitriles is **Option 4: ArX + KCN**.