Arrange the following in the increasing order of their reactivity towards addition of HCN :
Acetone , acetaldehyde , benzaldehyde , acetophenone

Explanation

To arrange the given compounds (acetone, acetaldehyde, benzaldehyde, and acetophenone) in increasing order of their reactivity towards the addition of HCN, we need to consider two main factors: steric hindrance and charge density on the carbonyl carbon.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Acetone: CH₃COCH₃
- Acetaldehyde: CH₃CHO
- Benzaldehyde: C₆H₅CHO
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