Which gives lactic acid on hydrolysis after reacting with HCN in KCN ?

Options

A. HCHO
B. `CH_(3)`CHO
C. `C_(6)H_(5)`CHO
D. `CH_(3)"COCH"_(3)`

Explanation

To determine which compound gives lactic acid upon hydrolysis after reacting with HCN in the presence of KCN, we need to analyze the possible candidates. Lactic acid has the structure CH3-CHOH-COOH, which contains three carbon atoms.

### Step-by-Step Solution:

1. **Identify the Structure of Lactic Acid:**
- Lactic acid has the molecular formula C3H6O3. Its structure can be represented as:
```
CH3-CHOH-COOH
```

2. **Consider the Reaction with HCN and KCN:**
- The reaction of carbonyl compounds (aldehydes or ketones) with HCN in the presence of KCN leads to the formation of cyanohydrins. The nucleophilic cyanide ion (CN-) attacks the electrophilic carbonyl carbon.

3. **Evaluate Each Compound:**
- **Option 1: Formaldehyde (HCHO)**
- Reaction:
```
HCHO + HCN → CH2(OH)CN
```
- Hydrolysis:
```
CH2(OH)CN + H2O → CH3-CHOH-COOH (not possible, only 2 carbons)
```
- Result: Does not yield lactic acid.

- **Option 2: Acetaldehyde (CH3CHO)**
- Reaction:
```
CH3CHO + HCN → CH3-CH(OH)CN
```
- Hydrolysis:
```
CH3-CH(OH)CN + H2O → CH3-CHOH-COOH
```
- Result: Yields lactic acid.

- **Option 3: Benzaldehyde (C6H5CHO)**
- Reaction:
```
C6H5CHO + HCN → C6H5-CH(OH)CN
```
- Hydrolysis:
```
C6H5-CH(OH)CN + H2O → C6H5-CHOH-COOH (not possible, only 2 carbons)
```
- Result: Does not yield lactic acid.

- **Option 4: Acetone (CH3COCH3)**
- Reaction:
```
CH3COCH3 + HCN → CH3-C(OH)(CN)-CH3
```
- Hydrolysis:
```
CH3-C(OH)(CN)-CH3 + H2O → CH3-CO-CH3 (not possible, only 3 carbons)
```
- Result: Does not yield lactic acid.

4. **Conclusion:**
- The only compound that yields lactic acid upon hydrolysis after reacting with HCN and KCN is **acetaldehyde (CH3CHO)**.

### Final Answer:
**Acetaldehyde (CH3CHO) gives lactic acid on hydrolysis after reacting with HCN in KCN.**