Which of the following on reaction with conc. NaOH gives an alcohol ?

Options

A. Methanal
B. Ethanal
C. Propanal
D. Butanal

Answer

To solve the question of which compound reacts with concentrated NaOH to give an alcohol, we need to understand the Canizzaro reaction. This reaction occurs with aldehydes that do not have alpha-hydrogens. Let's break down the solution step by step.

### Step-by-Step Solution:

1. **Identify the Reaction Type**:
The question involves the Canizzaro reaction, which is a redox reaction that occurs with aldehydes lacking alpha-hydrogens.

2. **Understand the Requirement for Canizzaro Reaction**:
For a compound to undergo the Canizzaro reaction, it must not have any alpha-hydrogens. This means that the carbon atom adjacent to the carbonyl group (C=O) should not have any hydrogen atoms attached.

3. **Analyze the Given Options**:
We need to examine the provided compounds (not listed in the question but assumed to be common aldehydes) to identify which one lacks alpha-hydrogens. Common candidates include:
- Formaldehyde (HCHO)
- Benzaldehyde (C6H5CHO)
- Ethanal (CH3CHO)
- Propanal (CH3CH2CHO)
- Butanal (CH3CH2CH2CHO)

4. **Identify Compounds with No Alpha-Hydrogens**:
- **Formaldehyde (HCHO)**: No alpha-carbon, hence no alpha-hydrogens.
- **Benzaldehyde (C6H5CHO)**: No alpha-carbon, hence no alpha-hydrogens.
- **Ethanal (CH3CHO)**: Has an alpha-carbon (CH3), hence has alpha-hydrogens.
- **Propanal (CH3CH2CHO)**: Has an alpha-carbon (CH3CH2), hence has alpha-hydrogens.
- **Butanal (CH3CH2CH2CHO)**: Has an alpha-carbon (CH3CH2CH2), hence has alpha-hydrogens.

5. **Conclusion**:
The compounds that can undergo the Canizzaro reaction and yield an alcohol are Formaldehyde and Benzaldehyde. However, since the question asks for a single answer, we can conclude that **Formaldehyde (option A)** is the primary compound that reacts with concentrated NaOH to give an alcohol (methanol).

### Final Answer:
**Formaldehyde (HCHO)** is the compound that reacts with concentrated NaOH to give an alcohol.