An alkyl isocyanide can be prepared as a major product by which of the following reaction?

Options

A. By heating an amide with `P_(2)O_(5)`
B. By heating an alcohol with NH3 in the presence of anhy. `ZnCl_(2)`
C. By the reaction of an alkyl halide with AgCN
D. By the reaction of an alkyl halide with KCN

Answer

To determine how an alkyl isocyanide can be prepared as a major product, we need to analyze the given options step by step.

### Step-by-Step Solution:

1. **Identify the General Structure of Alkyl Isocyanide**:
- Alkyl isocyanide has the general formula R-N≡C, where R is an alkyl group.

2. **Evaluate the First Option**:
- **Reaction**: Heating an amide with P2O5.
- **Reaction Mechanism**: An amide (R-C(O)-NH2) reacts with P2O5, a strong dehydrating agent, which removes water to form a nitrile (R-C≡N).
- **Conclusion**: This reaction produces a nitrile, not an isocyanide. Therefore, this option is incorrect.

3. **Evaluate the Second Option**:
- **Reaction**: Heating an alcohol with NH3 in the presence of anhydrous ZnCl2.
- **Reaction Mechanism**: The alcohol (R-OH) reacts with NH3, and the anhydrous ZnCl2 acts as a catalyst. The mechanism involves the formation of an amine (R-NH2) through nucleophilic substitution.
- **Conclusion**: This reaction yields an amine, not an isocyanide. Thus, this option is also incorrect.

4. **Evaluate the Third Option**:
- **Reaction**: Reaction of alkyl halide with AgCN.
- **Reaction Mechanism**: An alkyl halide (R-X) reacts with silver cyanide (AgCN). In this reaction, AgCN does not dissociate and the cyanide ion (CN-) attacks the carbon atom, resulting in the formation of an alkyl isocyanide (R-N≡C).
- **Conclusion**: This reaction produces an alkyl isocyanide, making this option correct.

5. **Evaluate the Fourth Option**:
- **Reaction**: Reaction of alkyl halide with KCN.
- **Reaction Mechanism**: An alkyl halide (R-X) reacts with potassium cyanide (KCN). KCN dissociates into K+ and CN-, where the CN- ion acts as a nucleophile and attacks the carbon atom, forming an alkyl cyanide (R-C≡N).
- **Conclusion**: This reaction produces an alkyl cyanide, not an isocyanide. Therefore, this option is incorrect.

### Final Answer:
The correct option for preparing an alkyl isocyanide as a major product is **Option C: Reaction of alkyl halide with AgCN**.

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