The only aldehyde which undergoes haloform reaction is

Options

A. Formaldehyde
B. Acetaldehyde
C. Benzaldehyde
D. Propionaldehyde

Explanation

**Step-by-Step Solution:**

1. **Understanding the Haloform Reaction**: The haloform reaction is a chemical reaction that occurs when a methyl ketone or an aldehyde with an alpha methyl group reacts with halogens (like iodine) in the presence of a base. The reaction leads to the formation of a haloform (like iodoform) and a carboxylic acid.

2. **Identifying the Aldehyde**: Among the aldehydes, we need to identify which one has an alpha methyl group (a methyl group attached to the carbon adjacent to the carbonyl carbon). The only aldehyde that fits this criterion is acetaldehyde (ethanal), which has the structure CH3CHO.

3. **Structure of Acetaldehyde**: The structure of acetaldehyde can be represented as:
\[ \text{CH}_3\text{C}(=O)\text{H} \]
Here, the methyl group (CH3) is the alpha group attached to the carbonyl carbon.

4. **Reaction Mechanism**: When acetaldehyde reacts with iodine (I2) in the presence of a base (like NaOH), it undergoes halogenation at the alpha position. The reaction proceeds as follows:
- The methyl group (CH3) is halogenated to form iodoform (CHI3).
- The remaining part of the molecule converts into a carboxylic acid (in this case, acetic acid).

5. **Conclusion**: Since acetaldehyde is the only aldehyde that has an alpha methyl group and can undergo the haloform reaction, we conclude that the answer to the question is:
- **Acetaldehyde (Ethanal)**.

**Final Answer**: The only aldehyde which undergoes the haloform reaction is **Acetaldehyde (Ethanal)**.

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