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 identify compounds A and B in the given reaction, we can follow these steps:

### Step 1: Analyze the Reaction Conditions
The reaction involves dilute sulfuric acid (H₂SO₄) and mercuric sulfate (HgSO₄). This combination is typically used in the hydration of alkynes.

**Hint:** Look for reactions involving alkynes and hydration.

### Step 2: Identify Compound A
Given that the reaction starts with an alkyne, we need to determine which alkyne is being used. The reaction leads to the formation of vinyl alcohol (which is unstable). Since vinyl alcohol is derived from a two-carbon alkyne, we can deduce that compound A must be ethyne (C₂H₂), which has the structure HC≡CH.

**Hint:** Remember that alkynes have a triple bond, and for two carbons, the simplest alkyne is ethyne.

### Step 3: Understand the Formation of Vinyl Alcohol
When ethyne (A) reacts with dilute H₂SO₄ and HgSO₄, it undergoes mercuric ion-catalyzed hydration. This reaction results in the formation of vinyl alcohol (enol), which has the structure CH₂=CHOH.

**Hint:** Enols are formed from the addition of water across the triple bond of alkynes.

### Step 4: Identify Compound B
Vinyl alcohol (enol) is unstable and can undergo tautomerization to form a more stable compound, which is a ketone or aldehyde. In this case, vinyl alcohol tautomerizes to acetaldehyde (B), which has the structure CH₃CHO.

**Hint:** Tautomerization typically involves the conversion of an enol to a carbonyl compound (ketone or aldehyde).

### Final Answer
- **Compound A:** Ethyne (C₂H₂ or HC≡CH)
- **Compound B:** Acetaldehyde (C₂H₄O or CH₃CHO)

### Summary
The reaction starts with ethyne (A), which is hydrated to form vinyl alcohol (unstable), and then tautomerizes to form acetaldehyde (B).