Which of the following not only decolorises alkaline potassium permanganate but also gives red precipitate with ammoniacal cuprous chloride solution?

Options

A. Ethane
B. Methane
C. Ethene
D. Acetylene

Answer

To solve the question, we need to identify which compound not only decolorizes alkaline potassium permanganate but also gives a red precipitate with ammoniacal cuprous chloride solution. Let's analyze the options step by step.

### Step 1: Understand the Reaction with Alkaline Potassium Permanganate
- Alkaline potassium permanganate (KMnO4) is a strong oxidizing agent.
- Compounds that can decolorize KMnO4 typically undergo oxidation.
- Among the given options (ethane, methane, ethylene, and acetylene), we need to determine which can be oxidized.

### Step 2: Analyze Each Compound
1. **Methane (CH4)**:
- Methane is a saturated hydrocarbon and does not readily undergo oxidation with KMnO4.
- Therefore, it does not decolorize KMnO4.

2. **Ethane (C2H6)**:
- Similar to methane, ethane is also a saturated hydrocarbon and does not significantly react with KMnO4.
- Thus, it does not decolorize KMnO4.

3. **Ethylene (C2H4)**:
- Ethylene is an unsaturated hydrocarbon and can be oxidized, but it does not form a stable product that would decolorize KMnO4 significantly.

4. **Acetylene (C2H2)**:
- Acetylene is an alkyne and can be oxidized by KMnO4 to form oxalic acid (C2H2O4).
- This reaction leads to the decolorization of the purple color of KMnO4.

### Step 3: Reaction with Ammoniacal Cuprous Chloride
- Ammoniacal cuprous chloride (CuCl in NH4OH) is used to test for terminal alkynes.
- When acetylene reacts with ammoniacal cuprous chloride, it forms copper acetylide (CuC2H), which is a red precipitate.

### Step 4: Conclusion
- Based on the analysis, acetylene (C2H2) is the only compound that:
1. Decolorizes alkaline potassium permanganate by oxidation to oxalic acid.
2. Forms a red precipitate with ammoniacal cuprous chloride as copper acetylide.

Thus, the answer to the question is **acetylene (C2H2)**.

### Final Answer
**Acetylene (C2H2)** is the correct answer.