The following reaction constituts :
`RNH_2 + S=C=S overset (H_gCl_2) (rarr) underset ("Alkyl isothicoyanate") (R-N=C=S) +HgS`

Options

A. Mustard oil reaction
B. Test for `3^@` amine
C. Test for `2^@` amine
D. Test for `CS_2`

Answer

To solve the given question regarding the reaction of a primary amine with carbon disulfide in the presence of mercuric chloride, we will break down the process step by step.

### Step-by-Step Solution:

1. **Identify the Reactants**:
- The reactants in the reaction are a primary amine (RNH₂) and carbon disulfide (S=C=S).

2. **Reaction with Carbon Disulfide**:
- The primary amine reacts with carbon disulfide to form dithiocarbamic acid. The reaction can be represented as:
\[ RNH_2 + CS_2 \rightarrow RNHCS_2H \]
- This intermediate is known as dithiocarbamic acid.

3. **Addition of Mercuric Chloride**:
- When mercuric chloride (HgCl₂) is added to the reaction mixture, it facilitates the conversion of dithiocarbamic acid to isothiocyanate. The reaction can be represented as:
\[ RNHCS_2H + HgCl_2 \rightarrow R-N=C=S + HgS + 2HCl \]
- Here, R-N=C=S is the alkyl isothiocyanate formed in the reaction.

4. **Formation of Mercury Sulfide**:
- Along with the formation of alkyl isothiocyanate, mercuric sulfide (HgS) is precipitated out as a byproduct.

5. **Characteristics of Isothiocyanates**:
- Alkyl isothiocyanates have a characteristic mustard-like odor, which is significant in identifying the reaction.

6. **Conclusion**:
- The overall reaction constitutes the formation of alkyl isothiocyanate and mercuric sulfide, and it is known as the mustard oil reaction, which specifically tests for primary amines.

### Final Answer:
The reaction constitutes a test for primary amines known as the mustard oil reaction, producing alkyl isothiocyanate (R-N=C=S) and mercuric sulfide (HgS).

---