What is the order of reactivity of the following towards HCN ?
`(I ) C_6 H_5 CHO`
`(ii ) C_8 H_5 COC_6 H_5`
` (iii ) C_3 CHO`
`(iv ) HCHO`

Explanation

To determine the order of reactivity of the given carbonyl compounds towards HCN, we need to analyze the structure of each compound and how substituents affect the electrophilicity of the carbonyl carbon. The more electrophilic the carbonyl carbon, the more reactive it will be towards nucleophiles like HCN.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- (i) C₆H₅CHO (Benzaldehyde)
- (ii) C₈H₅COC₆H₅ (Benzophenone)
- (iii) C₃CHO (Acetaldehyde)
...