Which hydrocarbon on warming with mercuric sulphate and dilute sulphuric acid forms carbonyl compounds ?

Question

Accepted Answer

Option 3, "Alkyne," is the correct answer because alkynes, when treated with mercuric sulfate (HgSO₄) and dilute sulfuric acid (H₂SO₄), undergo a hydration reaction that leads to the formation of carbonyl compounds, specifically ketones or aldehydes. This reaction is known as the "oxymercuration-demercuration" process.

### Explanation for Option 3 (Alkyne):

1. **Reaction Mechanism**: 
   - Alkynes, which are hydrocarbons with at least one carbon-carbon triple bond, react with mercuric sulfate and dilute sulfuric acid in an aqueous medium.
   - The reaction involves the addition of water (H₂O) across the triple bond.
   - Initially, an enol is formed, which is an unstable intermediate.
   - The enol rapidly tautomerizes to form a more stable carbonyl compound, typically a ketone. For terminal alkynes, the product can be an aldehyde.

2. **Example**:
   - Consider acetylene (ethyne, C₂H₂) as an example of an alkyne. When it reacts with HgSO₄ and H₂SO₄, it forms acetaldehyde (CH₃CHO).

### Explanation for Why Other Options are Incorrect:

1. **Option 1 (Alkane)**:
   - Alkanes are saturated hydrocarbons with single carbon-carbon bonds. They are generally unreactive and do not undergo addition reactions like hydration. Therefore, they do not form carbonyl compounds under these conditions.

2. **Option 2 (Alkene)**:
   - Alkenes, which contain carbon-carbon double bonds, can undergo hydration reactions, but they typically form alcohols rather than carbonyl compounds when treated with dilute sulfuric acid. The presence of mercuric sulfate is not necessary for their hydration, and they do not form carbonyl compounds directly through this process.

3. **Option 4 (Benzene)**:
   - Benzene is an aromatic hydrocarbon with a stable ring structure. It does not react with mercuric sulfate and dilute sulfuric acid to form carbonyl compounds. Benzene's stability and resonance make it resistant to such addition reactions.

In conclusion, the unique reactivity of alkynes with mercuric sulfate and dilute sulfuric acid, leading to the formation of carbonyl compounds, justifies why Option 3 is the correct answer.