Which of the following amines forms a yellow oil with `NaNO_2//conc.HCl` ?

Options

A. `C_6H_5-NH-CH_3`
B. `C_6H_5-NH-C_6H_5`
C. `C_2H_5-NH-C_2H_5`
D. All of these

Answer

To determine which amines form a yellow oil with NaNO2 and concentrated HCl, we need to understand the reaction that occurs between secondary amines and nitrous acid (HNO2), which is generated in situ from NaNO2 and HCl.

### Step-by-Step Solution:

1. **Identify the Type of Amines**:
- Amines can be classified into primary, secondary, and tertiary amines.
- Only secondary amines react with nitrous acid to form nitroso compounds that are typically yellow oils.

2. **Reaction with NaNO2 and HCl**:
- When a secondary amine reacts with NaNO2 in the presence of concentrated HCl, it forms a nitroso compound.
- The general reaction can be represented as:
\[ R_2NH + HNO_2 \rightarrow R_2N-N=O + H_2O \]
- Here, \( R_2NH \) is a secondary amine, and \( R_2N-N=O \) is the corresponding nitroso compound.

3. **Formation of Yellow Oil**:
- The nitroso compound formed from secondary amines is often a yellow oil.
- For example, dimethylamine (a secondary amine) reacts to form N-nitrosodimethylamine, which is a yellow oil.

4. **Examples of Secondary Amines**:
- Dimethylamine (CH3)2NH
- Aniline (C6H5NH2) is a primary amine, so it does not form a yellow oil with NaNO2 and HCl.
- Other examples of secondary amines include N-methyl aniline, N,N-dimethyl aniline, etc.

5. **Conclusion**:
- The amines that will form a yellow oil with NaNO2 and concentrated HCl are secondary amines.
- Therefore, any secondary amine, whether aliphatic or aromatic, will yield a yellow oil upon treatment with NaNO2 and concentrated HCl.

### Final Answer:
The correct options are secondary amines, such as dimethylamine or any other secondary amine.