Identify A and B in the given reaction
`(A) overset("Dil. "H_(2)SO_(4)//HgSO_(4))rarr underset("(unstable)")underset("vinyl alcohol")({:(CH_(2)),("||"),("CHOH"):})rarr(B)`

Options

A. `C_(2)H_(2) and CH_(3)CHO`
B. `CH_(2) and HCOOH`
C. `C_(2)H_(4) and CH_(3)COOH`
D. `C_(2)H_(2) and CH_(3)COOH`

Answer

To solve the question, we need to identify compounds A and B in the given reaction sequence. The reaction involves the treatment of a compound A with dilute H₂SO₄ and HgSO₄, leading to an unstable enol (vinyl alcohol), which then tautomerizes to form compound B.

### Step-by-Step Solution:

1. **Identify the Starting Compound (A)**:
- The reaction begins with a compound that, when treated with dilute H₂SO₄ and HgSO₄, produces an enol.
- This suggests that compound A is likely an alkyne, as alkynes can undergo hydration in the presence of these reagents to form enols.
- The simplest alkyne is acetylene (C₂H₂).

**Hint**: Look for a compound that can form an enol upon hydration, typically an alkyne.

2. **Reaction with Dilute H₂SO₄ and HgSO₄**:
- When acetylene (C₂H₂) is treated with dilute H₂SO₄ and HgSO₄, the reaction involves the addition of water across the triple bond.
- This results in the formation of an enol, specifically vinyl alcohol (CH₂=CHOH).

**Hint**: Remember that the presence of HgSO₄ facilitates the addition of H and OH across the triple bond of the alkyne.

3. **Identify the Enol (Vinyl Alcohol)**:
- The unstable enol formed is CH₂=CHOH, which is known as vinyl alcohol.
- This compound is unstable and will undergo tautomerization.

**Hint**: Tautomerization is a process where an enol converts to a more stable form, typically a carbonyl compound.

4. **Tautomerization to Form Compound B**:
- Vinyl alcohol (CH₂=CHOH) undergoes tautomerization to form acetaldehyde (CH₃CHO).
- This process involves the migration of a hydrogen atom and the conversion of the double bond from the carbon-oxygen bond to a carbon-carbon bond.

**Hint**: Tautomerization often results in the formation of a more stable compound, such as an aldehyde or ketone.

5. **Final Identification**:
- Therefore, we conclude that:
- Compound A is acetylene (C₂H₂).
- Compound B is acetaldehyde (CH₃CHO).

### Summary:
- **A**: Acetylene (C₂H₂)
- **B**: Acetaldehyde (CH₃CHO)