Cyanohydrin of which compound on hydrolysis will give lactic acid?

Options

A. `C_(6)H_(5)`CHO
B. HCHO
C. `CH_(3)`CHO
D. `CH_(3)-CH_(2)-CHO`

Answer

To determine which compound's cyanohydrin will yield lactic acid upon hydrolysis, we need to analyze the structure of lactic acid and the possible cyanohydrins formed from the given compounds.

### Step-by-Step Solution:

1. **Identify the Structure of Lactic Acid**:
- Lactic acid has the chemical formula \( \text{C}_3\text{H}_6\text{O}_3 \) or can be represented as \( \text{CH}_3\text{CH(OH)COOH} \).
- This indicates that lactic acid contains a hydroxyl group (-OH) and a carboxylic acid group (-COOH).

2. **Consider the Cyanohydrin Formation**:
- Cyanohydrins are formed from carbonyl compounds (aldehydes or ketones) when they react with hydrogen cyanide (HCN).
- The reaction involves the nucleophilic attack of the cyanide ion (CN⁻) on the carbonyl carbon, leading to the formation of a cyanohydrin.

3. **Evaluate Each Compound**:
- **Option 1: Benzaldehyde (C₆H₅CHO)**:
- Upon reaction with HCN, it forms \( \text{C}_6\text{H}_5\text{CH(OH)CN} \).
- Hydrolysis of this cyanohydrin yields \( \text{C}_6\text{H}_5\text{CH(OH)COOH} \), which is not lactic acid (it has a phenyl group).

- **Option 2: Formaldehyde (HCHO)**:
- Reaction with HCN gives \( \text{H}_2\text{C(OH)CN} \).
- Hydrolysis leads to \( \text{H}_2\text{C(OH)COOH} \), which is not lactic acid.

- **Option 3: Acetaldehyde (CH₃CHO)**:
- Reaction with HCN results in \( \text{CH}_3\text{CH(OH)CN} \).
- Hydrolysis converts the cyanide to a carboxylic acid, yielding \( \text{CH}_3\text{CH(OH)COOH} \), which is indeed lactic acid.

- **Option 4: Propionaldehyde (CH₃CH₂CHO)**:
- Reaction with HCN gives \( \text{CH}_3\text{CH}_2\text{CH(OH)CN} \).
- Hydrolysis results in \( \text{CH}_3\text{CH}_2\text{CH(OH)COOH} \), which is not lactic acid.

4. **Conclusion**:
- The only compound that yields lactic acid upon hydrolysis of its cyanohydrin is **Acetaldehyde (CH₃CHO)**.
- Therefore, the correct answer is **Option 3: CH₃CHO**.