An organic compound A reacts with `PCl_(5)` to give compound B. Compound B reacts with Na/ether to give n-butane. What are compounds A and B ?

Answer

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

### Step 1: Identify the final product
The final product mentioned in the question is n-butane (C₄H₁₀). This indicates that the reaction leading to butane must involve a precursor that can yield butane through a specific reaction mechanism.

**Hint:** Look for reactions that can produce butane from smaller alkyl halides.

### Step 2: Understand the reaction with Na/ether
The reaction of compound B with sodium (Na) in dry ether suggests a Wurtz reaction. In the Wurtz reaction, two alkyl halides react with sodium to form a higher alkane.

Since the final product is butane (C₄H₁₀), we can infer that compound B must be a bromo- or chloro-alkane with two carbon atoms (C₂H₅Cl or C₂H₅Br) because two of these would combine to form butane.

**Hint:** Recall the Wurtz reaction and how it combines two alkyl halides.

### Step 3: Determine compound B
Based on the above reasoning, compound B must be chloroethane (C₂H₅Cl). This is because when two molecules of chloroethane react with sodium in dry ether, they will produce butane:

\[ 2 \text{C}_2\text{H}_5\text{Cl} + 2 \text{Na} \rightarrow \text{C}_4\text{H}_{10} + 2 \text{NaCl} \]

**Hint:** Identify the structure of chloroethane and how it can lead to butane.

### Step 4: Identify compound A
Now, we need to find compound A, which reacts with PCl₅ to form compound B (C₂H₅Cl). PCl₅ is known to convert alcohols into alkyl chlorides. Therefore, compound A must be an alcohol that, when treated with PCl₅, yields chloroethane.

The alcohol that corresponds to chloroethane is ethanol (C₂H₅OH). The reaction can be represented as follows:

\[ \text{C}_2\text{H}_5\text{OH} + \text{PCl}_5 \rightarrow \text{C}_2\text{H}_5\text{Cl} + \text{by-products} \]

**Hint:** Think about common alcohols that can be converted to alkyl halides.

### Final Answer
Thus, the compounds are:
- Compound A: Ethanol (C₂H₅OH)
- Compound B: Chloroethane (C₂H₅Cl)