The compound that decolourises alk. `KMnO_4` is

Options

A. `C_3H_8`
B. `C_2H_4`
C. `CH_4`
D. `"CCl"_4`

Answer

To determine which compound decolorizes alkaline KMnO4, we need to identify which of the given compounds is unsaturated. Alkaline KMnO4 is a strong oxidizing agent that reacts with unsaturated compounds (those containing double or triple bonds) and causes a color change, typically decolorizing the purple color of KMnO4.

### Step-by-Step Solution:

1. **Identify the Compounds**: The options provided are:
- C3H8 (Propane)
- C2H4 (Ethylene)
- CH4 (Methane)
- CCl4 (Carbon Tetrachloride)

2. **Classify the Compounds**:
- C3H8 (Propane) is a saturated hydrocarbon (alkane) with no double or triple bonds.
- C2H4 (Ethylene) is an unsaturated hydrocarbon (alkene) with a double bond.
- CH4 (Methane) is also a saturated hydrocarbon (alkane) with no double or triple bonds.
- CCl4 (Carbon Tetrachloride) is a saturated compound and does not contain any double or triple bonds.

3. **Determine Reactivity with KMnO4**:
- Since KMnO4 can oxidize unsaturated compounds, we check which of the options is unsaturated.
- C3H8, CH4, and CCl4 are all saturated and will not react with KMnO4 to decolorize it.
- C2H4, being an unsaturated compound, will react with KMnO4 and cause it to lose its purple color.

4. **Conclusion**: The compound that decolorizes alkaline KMnO4 is C2H4 (Ethylene).

### Final Answer:
**C2H4 (Ethylene)** is the compound that decolorizes alkaline KMnO4.