Compound A reacts with `"PCl"_(5)` to give B which on treatment with KCN followed by hydrolysis gave propionic acid. What is A and B respectively?

Options

A. `C_(3)H_(8)and C_(3)H_(7)Cl`
B. `C_(2)H_(6) and C_(2)H_(5)Cl`
C. `C_(2) H_(5)Cl` and `C_(2)H_(5)Cl`
D. `CH_(3)CH_(2) OH and C_(2) H_(5)Cl`

Answer

To solve the problem step by step, we need to identify the compounds A and B based on the given reactions.

### Step 1: Identify Compound A
The question states that compound A reacts with PCl5. In organic chemistry, PCl5 is commonly used to convert alcohols into alkyl chlorides. Therefore, we can deduce that compound A is likely an alcohol.

### Step 2: Determine the Structure of Compound A
Since the final product after hydrolysis is propionic acid (CH3CH2COOH), we can infer that the alcohol must have a structure that can lead to this product. The alcohol must have a two-carbon chain (ethyl) attached to the hydroxyl group (OH). Thus, compound A is ethyl alcohol (C2H5OH or CH3CH2OH).

### Step 3: Reaction of A with PCl5
When ethyl alcohol (A) reacts with PCl5, it forms ethyl chloride (B). The reaction can be represented as:
\[ \text{C2H5OH} + \text{PCl5} \rightarrow \text{C2H5Cl} + \text{POCl3} + \text{HCl} \]
So, compound B is ethyl chloride (C2H5Cl).

### Step 4: Reaction of B with KCN
Next, compound B (ethyl chloride) reacts with KCN. In this reaction, the chloride ion (Cl) is replaced by the cyanide ion (CN):
\[ \text{C2H5Cl} + \text{KCN} \rightarrow \text{C2H5CN} + \text{KCl} \]
This gives us a new compound, ethyl cyanide (C2H5CN).

### Step 5: Hydrolysis of Ethyl Cyanide
The final step involves hydrolysis of ethyl cyanide. Hydrolysis of nitriles (like C2H5CN) in the presence of water and an acid (or base) yields a carboxylic acid:
\[ \text{C2H5CN} + \text{H2O} \rightarrow \text{C2H5COOH} \]
This product is propionic acid (C2H5COOH).

### Conclusion
Thus, we have identified:
- Compound A: Ethyl alcohol (C2H5OH or CH3CH2OH)
- Compound B: Ethyl chloride (C2H5Cl)

### Final Answer
A = Ethyl alcohol (C2H5OH), B = Ethyl chloride (C2H5Cl)

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