Aldol addition can be

Options

A. Acid catalysed
B. Base catalysed
C. Acid promoted
D. A and B both

Answer

To solve the question regarding aldol addition, we will analyze the mechanisms involved in both acid-catalyzed and base-catalyzed reactions.

### Step-by-Step Solution:

1. **Understanding Aldol Addition**:
Aldol addition involves the reaction of two carbonyl compounds, typically aldehydes or ketones, that contain alpha-hydrogens. The presence of alpha-hydrogens is crucial as they can be removed to form an enolate ion or an enol, depending on the reaction conditions.

2. **Base-Catalyzed Mechanism**:
- In a basic medium (e.g., using NaOH), the base abstracts an alpha-hydrogen from the carbonyl compound (e.g., acetaldehyde, CH3CHO).
- This results in the formation of an enolate ion.
- The enolate ion then acts as a nucleophile and attacks another molecule of the carbonyl compound, leading to the formation of a β-hydroxy aldehyde or ketone.
- Upon hydrolysis, the enolate is converted back to the aldol product.

3. **Acid-Catalyzed Mechanism**:
- In an acidic medium, the carbonyl compound reacts with an acid (e.g., H+) to form an enol.
- The enol then acts as a nucleophile and attacks another molecule of the carbonyl compound.
- This also leads to the formation of a β-hydroxy aldehyde or ketone.
- The reaction can further proceed to dehydration to form an α,β-unsaturated carbonyl compound.

4. **Conclusion**:
- Both mechanisms (acid-catalyzed and base-catalyzed) can lead to aldol addition. Therefore, the correct answer to the question is that aldol addition can occur through both mechanisms.

### Final Answer:
The correct option is D: Aldol addition can be both acid-catalyzed and base-catalyzed.