p-chloroaniline and anilinium hydrochloride cannot be distinguished by

Options

A. Sandmeyer's reaction
B. `NaHCO_(3)`
C. `AgNO_(3)`
D. Carbylamine test.

Explanation

To solve the question regarding the distinction between p-chloroaniline and anilinium hydrochloride, we need to analyze the properties and reactions of both compounds.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- p-Chloroaniline is a primary amine with the structure C6H4ClNH2 (where Cl is at the para position).
- Anilinium hydrochloride is the hydrochloride salt of aniline, represented as C6H5NH3Cl.

2. **Understand the Reactions**:
- **Sandmeyer Reaction**: This reaction involves the conversion of an amine to a haloarene using sodium nitrite (NaNO2) and hydrochloric acid (HCl).
- For p-chloroaniline, it will convert to para-dichlorobenzene.
- For anilinium hydrochloride, it will convert to chlorobenzene.
- The products are different (solid vs. liquid), indicating that they can be distinguished by this reaction.

3. **Sodium Carbonate (Na2CO3) Reaction**:
- Anilinium hydrochloride (being an acidic salt) reacts with Na2CO3 to release CO2 gas.
- p-Chloroaniline does not react with Na2CO3 in the same manner since it is not an acidic salt.
- Therefore, they can be distinguished by this reaction.

4. **Silver Nitrate (AgNO3) Reaction**:
- Anilinium hydrochloride will react with AgNO3 to form a white precipitate of AgCl.
- p-Chloroaniline does not form a precipitate with AgNO3.
- Thus, they can be distinguished by this reaction as well.

5. **Carbylamine Test**:
- This test is specific for primary amines. Both p-chloroaniline and anilinium hydrochloride can undergo this test.
- The carbylamine test involves heating the amine with chloroform and alcoholic KOH, leading to the formation of isocyanides.
- Since both compounds are primary amines, they will give a positive result in this test.

### Conclusion:
The two compounds, p-chloroaniline and anilinium hydrochloride, cannot be distinguished by the **Carbylamine test**. Therefore, the correct answer is option D.