4-hydroxy-4-methyl-2-pentanone is obtained when one of the following is reacted with base. The compound is

Options

A. acetone
B. methanal
C. acetaldehyde
D. ethyl methyl ketone

Explanation

To solve the question regarding the formation of 4-hydroxy-4-methyl-2-pentanone, we need to identify the reactant that, when reacted with a base, will yield this compound. The process involves understanding aldol condensation. Let's break it down step by step.

### Step-by-Step Solution:

1. **Identify the Structure of the Product**:
- The compound 4-hydroxy-4-methyl-2-pentanone has the following structure:
- It is a ketone (due to the presence of the carbonyl group, C=O).
- The longest carbon chain is 5 carbons long (pentanone).
- There is a hydroxyl (–OH) group and a methyl (–CH3) group on the fourth carbon.

2. **Determine the Number of Carbon Atoms**:
- The product has 6 carbon atoms in total (from 4-hydroxy-4-methyl-2-pentanone).
- In aldol condensation, two molecules of a carbonyl compound react to form a β-hydroxy ketone.

3. **Calculate the Reactant**:
- Since the product has 6 carbon atoms, the reactant must have half that number of carbon atoms (3 carbons) because two moles of the reactant will combine.
- Therefore, we need a carbonyl compound with 3 carbon atoms.

4. **Identify Possible Reactants**:
- The possible carbonyl compounds with 3 carbon atoms are:
- Acetone (CH3-CO-CH3) - a ketone.
- Acetaldehyde (CH3CHO) - an aldehyde.
- Propanal (CH3CH2CHO) - an aldehyde.
- Since we are looking for a ketone, acetone is the only suitable reactant.

5. **Aldol Condensation Mechanism**:
- When acetone reacts with a base, it undergoes deprotonation to form an enolate ion.
- The enolate ion then attacks the carbonyl carbon of another acetone molecule, leading to the formation of the β-hydroxy ketone (4-hydroxy-4-methyl-2-pentanone).

6. **Conclusion**:
- The compound that, when reacted with a base, yields 4-hydroxy-4-methyl-2-pentanone is **acetone**.

### Final Answer:
The compound is **acetone**.