`[A] + [B ] to C_2 H_5 NH CONHC_2 H_5 `
`{:( (a ) COCl_2 , (b ) ALCl_3),(( c ) C_2 H_5 NH_2 , (d) CH_2 Br_2 ):}`
here , [A] & [B] are

Options

A. a,b
B. b,c
C. c,d
D. a,c

Answer

To solve the problem, we need to identify the reactants [A] and [B] that lead to the formation of the product \( C_2H_5NHCONHC_2H_5 \). Let's analyze the given options step by step.

### Step 1: Analyze the Product Structure
The product \( C_2H_5NHCONHC_2H_5 \) contains:
- A carbonyl group (C=O)
- Two ethylamine groups (C2H5NH)

This suggests that one reactant must provide the carbonyl group, while the other must provide the amine groups.

### Step 2: Identify the Reactants
Let's look at the given options:
1. **(a) COCl2** - This compound contains a carbonyl group (C=O) and is known as thionyl chloride. It can react to form an amide.
2. **(b) AlCl3** - This is a Lewis acid and does not provide a carbonyl or amine group.
3. **(c) C2H5NH2** - This is ethylamine and contains the amine group.
4. **(d) CH2Br2** - This compound does not contain a carbonyl or amine group.

From this analysis, we can conclude:
- **[A] = COCl2** (provides the carbonyl group)
- **[B] = C2H5NH2** (provides the amine groups)

### Step 3: Reaction Mechanism
1. **Formation of the Intermediate:**
- Ethylamine \( C_2H_5NH_2 \) attacks the carbonyl carbon of \( COCl_2 \), forming an intermediate where the Cl is displaced.
- The structure at this point is \( C_2H_5NH-C(=O)Cl \).

2. **Formation of the Final Product:**
- Another molecule of ethylamine attacks the carbonyl carbon, leading to the formation of the amide bond.
- The final product is \( C_2H_5NHCONHC_2H_5 \).

### Conclusion
Thus, the correct reactants [A] and [B] are:
- **[A] = COCl2**
- **[B] = C2H5NH2**

### Final Answer
The correct option is (a) COCl2 and (c) C2H5NH2.

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