`CH_(3)CHO+HCHOoverset("dil. NaOH")underset("Heat")rarrAoverset("HCN)underset(H_(3)O^(+))rarrB`
the structure of compound B is

Options

A. `CH_(2)=CH-underset(OH)underset("| ")"CH"-COOH`
B. `CH_(2)=CH-underset(CN)underset("| ")"CH"-OH`
C. `CH_(3)CH_(2)-underset(OH)underset("| ")"CH"-COOH`
D. `CH_(3)-underset(OH)underset("| ")"CH"-COOH`

Explanation

To solve the given problem step by step, we will analyze the reaction of acetaldehyde (CH₃CHO) with formaldehyde (HCHO) in the presence of dilute NaOH and heat, followed by a reaction with HCN and hydrolysis to find the structure of compound B.

### Step 1: Aldol Condensation
1. **Reactants**: We start with acetaldehyde (CH₃CHO) and formaldehyde (HCHO).
2. **Base Catalysis**: In the presence of dilute NaOH, acetaldehyde undergoes deprotonation. The alpha hydrogen from CH₃CHO is abstracted by NaOH, forming a nucleophile (CH₂=CHO⁻).
3. **Nucleophilic Attack**: The nucleophile (CH₂=CHO⁻) attacks the carbonyl carbon of formaldehyde (HCHO), leading to the formation of an aldol product.

**Intermediate Structure**: The product at this stage is 3-hydroxybutanal (CH₂(OH)CH₂CHO).

### Step 2: Dehydration
4. **Heating**: Upon heating, the aldol product undergoes dehydration (loss of water). The hydroxyl group (OH) and a hydrogen atom are removed, resulting in the formation of a double bond.

**Final Structure of A**: The resulting compound A is crotonaldehyde (CH₂=CHCHO).

### Step 3: Reaction with HCN
5. **Nucleophilic Addition of HCN**: The compound A (crotonaldehyde) reacts with hydrogen cyanide (HCN). The carbonyl carbon of crotonaldehyde is attacked by the cyanide ion (CN⁻), leading to the formation of a cyanohydrin.

**Intermediate Structure**: The product formed is 2-hydroxy-3-cyanobutanal (CH₂=CH(OH)C(CN)CHO).

### Step 4: Hydrolysis
6. **Hydrolysis of Cyanohydrin**: The cyanohydrin undergoes hydrolysis in the presence of water and an acid (H₃O⁺), converting the cyanide group (CN) into a carboxylic acid (COOH).

**Final Structure of B**: The final product B is 2-hydroxy-3-butenoic acid (CH₂=CH(OH)C(OH)COOH).

### Conclusion
The structure of compound B is 2-hydroxy-3-butenoic acid.

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