Which of the following will be most stable diazonium salt `RN_(2)^(+)X^(-)` ?

Options

A. `CH_(3)N_(2)^(+)X^(-)`
B. `C_(6)H_(5)N_(2)^(+)X^(-)`
C. `CH_(3)CH_(2)N_(2)^(+)X^(-)`
D. `C_(6)H_(5)CH_(2)N_(2)^(+)X^(-)`

Explanation

To determine which diazonium salt \( RN_2^+X^- \) is the most stable, we need to analyze the stability of the diazonium salts formed from different groups (alkyl, phenyl, ethyl, and benzyl). The stability of diazonium salts is significantly influenced by the ability of the substituent group (R) to stabilize the positive charge on the nitrogen atom through resonance or inductive effects.

### Step-by-Step Solution:

1. **Identify the Structure of Diazonium Salts**:
The general structure of a diazonium salt is \( R-N_2^+X^- \), where \( R \) can be an alkyl group, phenyl group, or other substituents, and \( X \) is typically a halide ion (like Cl^-).

2. **Consider the Groups**:
The groups we are considering are:
- Alkyl group (e.g., methyl, ethyl)
- Phenyl group (C6H5)
- Benzyl group (C6H5-CH2)
- Ethyl group (C2H5)

3. **Analyze Resonance Stabilization**:
- **Alkyl Groups**: Alkyl groups (like methyl or ethyl) can only provide +I (inductive) effects, which do not stabilize the positive charge effectively.
- **Phenyl Group (C6H5)**: The phenyl group can stabilize the positive charge through resonance. The positive charge on the nitrogen can delocalize into the aromatic ring, providing significant stabilization.
- **Benzyl Group (C6H5-CH2)**: The benzyl group can also stabilize the positive charge through resonance, similar to the phenyl group, but the resonance is slightly less effective than that of the phenyl group alone.
- **Ethyl Group (C2H5)**: Similar to other alkyl groups, the ethyl group provides only +I effects, which are not sufficient for stabilization.

4. **Evaluate Stability**:
- The phenyl diazonium salt \( C6H5-N_2^+Cl^- \) is highly stable due to resonance.
- The benzyl diazonium salt \( C6H5-CH2-N_2^+Cl^- \) is also stable but not as stable as the phenyl diazonium salt.
- Alkyl diazonium salts (like methyl or ethyl) are the least stable due to the lack of resonance.

5. **Conclusion**:
Based on the resonance stabilization, the most stable diazonium salt is the one derived from the phenyl group, \( C6H5-N_2^+Cl^- \).

### Final Answer:
The most stable diazonium salt is the one derived from the phenyl group, \( C6H5-N_2^+Cl^- \).