Nitrogen gas is evolved when `HNO_(2)` reacts with .

Options

A. `C_(6)H_(5)COCH_(2)`
B. `C_(6)H_(5)CONH_(2)`
C. `CH_(3)CH_(2)NH_(2)`
D. `NH_(2)CONH_(2)`

Explanation

To determine when nitrogen gas (N₂) is evolved during the reaction of nitrous acid (HNO₂) with various compounds, we can analyze each option provided.

### Step-by-Step Solution:

1. **Understanding the Reaction of HNO₂**:
- HNO₂ reacts with amines to form diazonium salts, which can decompose to release nitrogen gas (N₂). The general reaction can be represented as:
\[ RNH_2 + HNO_2 \rightarrow R-N_2^+ + Cl^- + H_2O \]
- The diazonium salt (R-N₂⁺) is often unstable, leading to the release of nitrogen gas.

2. **Option 1: C₆H₅-C(=O)-CH₃ (Acetophenone)**:
- When HNO₂ reacts with acetophenone, it does not produce nitrogen gas. Instead, it forms an enol and water:
\[ C_6H_5C(=O)CH_3 + HNO_2 \rightarrow C_6H_5C(=NOH)CH_3 + H_2O \]
- **Conclusion**: No nitrogen gas is evolved.

3. **Option 2: C₆H₅CONH₂ (Acetanilide)**:
- The reaction of acetanilide with HNO₂ produces a diazonium salt:
\[ C_6H_5CONH_2 + HNO_2 \rightarrow C_6H_5C(=O)N_2^+ + Cl^- + H_2O \]
- The diazonium salt is unstable and decomposes to release nitrogen gas:
\[ C_6H_5C(=O) + N_2 + H_2O \]
- **Conclusion**: Nitrogen gas is evolved.

4. **Option 3: CH₃CH₂NH₂ (Ethylamine)**:
- Ethylamine reacts with HNO₂ to form a diazonium salt:
\[ CH_3CH_2NH_2 + HNO_2 \rightarrow CH_3CH_2N_2^+ + Cl^- + H_2O \]
- This diazonium salt is also unstable, leading to the release of nitrogen gas:
\[ CH_3CH_2OH + N_2 + H_2O \]
- **Conclusion**: Nitrogen gas is evolved.

5. **Option 4: NH₂CONH₂ (Urea)**:
- Urea reacts with HNO₂, producing two diazonium groups:
\[ NH_2CONH_2 + 2HNO_2 \rightarrow N_2 + 2Cl^- + H_2O \]
- This reaction leads to the release of nitrogen gas:
- **Conclusion**: Nitrogen gas is evolved.

### Final Summary:
- Nitrogen gas is evolved in the reactions of HNO₂ with:
- Acetanilide (Option 2)
- Ethylamine (Option 3)
- Urea (Option 4)