Which alkene on heating with hot and conc. `KMnO_(4)` solution gives acetone and a gas, which turns lime water milky -

Options

A. 2–Methyl–2–butene
B. Isobutylene
C. 1–Butene
D. 2–Butene

Answer

To solve the question, we need to identify which alkene, when heated with hot and concentrated KMnO4 solution, produces acetone and a gas that turns lime water milky.

### Step-by-Step Solution:

1. **Identify the Reaction Type**: The reaction involves the oxidation of an alkene using hot and concentrated KMnO4, which is known as a strong oxidizing agent. This reaction typically leads to the formation of carbonyl compounds (aldehydes or ketones) and can also produce carboxylic acids.

2. **Determine the Products**: We need to find an alkene that, upon oxidation, yields acetone (a ketone) and carbon dioxide (the gas that turns lime water milky). The gas that turns lime water milky is carbon dioxide (CO2).

3. **Consider Possible Alkenes**: The alkenes we can consider include:
- 1-butene
- 2-butene
- Isobutylene (2-methylpropene)
- Other alkenes with similar structures.

4. **Analyze the Oxidation of 2-Methylpropene (Isobutylene)**:
- The structure of isobutylene (2-methylpropene) is CH2=C(CH3)2.
- When isobutylene is oxidized by hot and concentrated KMnO4, it undergoes oxidative cleavage.
- This cleavage results in the formation of acetone (CH3COCH3) and carbon dioxide (CO2).

5. **Confirm the Reaction**:
- The reaction can be summarized as:
\[ \text{CH}_2=C(\text{CH}_3)_2 \xrightarrow{\text{KMnO}_4} \text{CH}_3\text{COCH}_3 + \text{CO}_2 \]
- The acetone produced is a ketone, and the carbon dioxide is the gas that turns lime water milky.

6. **Conclusion**: The alkene that fits the criteria of producing acetone and carbon dioxide when treated with hot and concentrated KMnO4 is **isobutylene (2-methylpropene)**.

### Final Answer:
**Isobutylene (2-methylpropene)** is the alkene that gives acetone and carbon dioxide when heated with hot and concentrated KMnO4.