`Aoverset("KCN")toBoverset(H_(3)O+)underset(NH_(3)//Delta)toCoverset(Br_(2))underset(KOH)toCH_(3)-CH_(2)-NH_(2)`
reactant A in given reaction sequence?

Options

A. `CH_(3)-CH_(2)-CH_(2)-Cl`
B. `CH_(3)-CH_(2)-Cl`
C. `CH_(3)-Cl`
D. None of these

Answer

To determine the reactant A in the given reaction sequence, we will analyze the reactions step by step.

### Step-by-Step Solution:

1. **Identify the final product (D)**: The final product of the reaction sequence is given as CH3-CH2-NH2 (ethylamine).

2. **Work backward from the final product**: The last step in the sequence involves the reaction of an amide with Br2 in the presence of KOH, which is characteristic of the Hofmann degradation reaction. This reaction converts an amide to a primary amine.

3. **Determine the amide (C)**: Since the final product is CH3-CH2-NH2, the amide (C) must be CH3-CH2-C(=O)-NH2 (ethylamide).

4. **Identify the hydrolysis step (B)**: The step before the formation of the amide involves hydrolysis of a nitrile (C). The nitrile that hydrolyzes to form the amide is CH3-CH2-CN (propionitrile).

5. **Determine the nitrile formation step (A)**: The formation of the nitrile (B) from alkyl halide (A) occurs when KCN is used. The alkyl halide that would yield propionitrile upon reaction with KCN is CH3-CH2-Cl (ethyl chloride).

6. **Conclusion**: Therefore, the reactant A in the given reaction sequence is CH3-CH2-Cl (ethyl chloride).

### Final Answer:
The reactant A in the given reaction sequence is **CH3-CH2-Cl (ethyl chloride)**.