`A+CS_(2)+HgCl_(2)` give `C_(2)H_(5)-N=C=S`. Thus compound
A is :-

Options

A. `C_(2)H_(5)NH_(2)`
B. `C_(2)H_(5)NHC_(2)H_(5)`
C. `CH_(3)-CH=NOH`
D. `CH_(3)CH_(2)NO_(2)`

Answer

To determine the compound A in the reaction \( A + CS_2 + HgCl_2 \rightarrow C_2H_5 - N = C = S \), we can follow these steps:

### Step 1: Identify the Reaction Type
The reaction described is a part of the Hoffmann mustard oil reaction, which is typically used to test for primary amines. In this reaction, a primary amine reacts with carbon disulfide (CS2) and mercuric chloride (HgCl2) to form a thiourea derivative.

**Hint:** Recognize that the reaction involves a primary amine and a specific reagent combination.

### Step 2: Write the General Reaction
The general reaction for the Hoffmann mustard oil reaction can be represented as:
\[ RNH_2 + CS_2 \rightarrow RNHCS + H_2S \]
\[ RNHCS + HgCl_2 \rightarrow RNC=S + HgS + 2HCl \]

**Hint:** Understand the transformation of the primary amine into a thiourea derivative through the reaction with CS2.

### Step 3: Analyze the Given Product
The product given in the question is \( C_2H_5 - N = C = S \). This indicates that the starting amine (A) must be a primary amine that, when reacted, gives rise to an ethyl group (C2H5) attached to the nitrogen.

**Hint:** Look for a primary amine that can yield an ethyl group upon reaction.

### Step 4: Identify the Structure of Compound A
Since the product contains an ethyl group, we can deduce that compound A must be ethylamine, which has the structure:
\[ C_2H_5NH_2 \]

**Hint:** Consider the simplest primary amine that corresponds to the ethyl group.

### Step 5: Confirm the Reaction
Now, we can confirm the reaction:
1. Ethylamine \( (C_2H_5NH_2) \) reacts with \( CS_2 \) to form \( C_2H_5NCS \).
2. This then reacts with \( HgCl_2 \) to yield \( C_2H_5N=C=S \) along with by-products \( HgS \) and \( 2HCl \).

**Hint:** Verify that the reaction steps align with the expected products.

### Conclusion
Thus, the compound A is ethylamine, represented as:
\[ A = C_2H_5NH_2 \]

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