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), let's analyze the options provided systematically.

### Step 1: Understand the Reactions
We need to evaluate the different reactions given in the options to see if they can lead to the formation of aromatic nitriles.

### Step 2: Analyze Each Reaction
1. **Reaction 1: ArSO2 + SOCl2 → ArCN + SO2 + 2SCN**
- This reaction produces ArCN, so it can be used to prepare aromatic nitriles.

2. **Reaction 2: Ar-N2+ + Cu-CN**
- This reaction also leads to the formation of ArCN, thus it can be used to prepare aromatic nitriles.

3. **Reaction 3: ArSO2 + P2O5 + S2O → ArCN**
- This reaction results in the formation of ArCN, so it is also a valid method for preparing aromatic nitriles.

4. **Reaction 4: Ar-X + KCN**
- In this reaction, Ar-X (where X is a halogen) does not undergo nucleophilic substitution due to the strong C-X bond, which is stabilized by resonance. Therefore, this reaction does not produce ArCN.

### Conclusion
The reaction that cannot be used to prepare aromatic nitriles is **Reaction 4: Ar-X + KCN**.

### Final Answer
**Option 4: Ar-X + KCN cannot be used to prepare aromatic nitriles (ArCN).**

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