A carbonyl compound reacts with hydrogen cyanide to form cyanohydrin which on hydrolysis forms a recemic mixrture of `alpha`-hydroxy acid. The carbonyl compound `D`.

Options

A. acetaldehyde
B. acetone
C. diethyl ketone
D. formaldehyde

Explanation

To solve the problem of identifying the carbonyl compound \( D \) that reacts with hydrogen cyanide to form a cyanohydrin and subsequently a racemic mixture of an alpha-hydroxy acid, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Reaction**:
- A carbonyl compound (aldehyde or ketone) reacts with hydrogen cyanide (HCN) to form a cyanohydrin. The general structure of a cyanohydrin is \( R_1R_2C(OH)(CN) \), where \( R_1 \) and \( R_2 \) are alkyl or hydrogen groups.

2. **Formation of Racemic Mixture**:
- The cyanohydrin undergoes hydrolysis to form an alpha-hydroxy acid. For a racemic mixture to form, the carbon atom that is bonded to the hydroxyl group (OH) must be chiral, meaning it must have four different substituents.

3. **Choosing the Carbonyl Compound**:
- We need to select a carbonyl compound that can lead to the formation of a chiral center upon reaction with HCN.
- The simplest carbonyl compounds to consider are aldehydes and ketones.

4. **Analyzing Possible Candidates**:
- **Acetaldehyde (Ethanal)**: \( CH_3CHO \)
- Reacts with HCN to form \( CH_3C(OH)(CN)H \). Upon hydrolysis, this will yield \( CH_3C(OH)COOH \) (lactic acid), which is a racemic mixture due to the presence of a chiral center.
- **Acetone (Propanone)**: \( CH_3COCH_3 \)
- Forms a cyanohydrin, but the resulting product does not have a chiral center since both substituents on the carbon are \( CH_3 \).
- **Diethyl Ketone (Butan-2-one)**: \( CH_3C(=O)C_2H_5 \)
- Similar to acetone, it does not form a chiral center upon reaction with HCN.
- **Formaldehyde (Methanal)**: \( HCHO \)
- Also does not yield a chiral center as it has two hydrogen atoms attached to the same carbon.

5. **Conclusion**:
- The only carbonyl compound that meets the criteria of forming a racemic mixture upon hydrolysis is **Acetaldehyde (Ethanal)**. Therefore, the carbonyl compound \( D \) is acetaldehyde.

### Final Answer:
The carbonyl compound \( D \) is **Acetaldehyde (Ethanal)**.

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