Which one the following has maximum reactivity towards HCN ?

Options

A. HCHO
B. `CH_3COCH_3`
C. `CH_3CHO`
D. `C_2H_5COCH_3`

Answer

To determine which compound has the maximum reactivity towards hydrogen cyanide (HCN), we need to analyze the given options based on their structure and steric hindrance. The reactivity of carbonyl compounds (aldehydes and ketones) towards nucleophiles like HCN is influenced by steric hindrance and the electronic nature of the substituents.

### Step-by-step Solution:

1. **Identify the Compounds**: The given options are:
- A) Formaldehyde (HCHO)
- B) Acetone (CH3COCH3)
- C) Acetaldehyde (CH3CHO)
- D) Butan-2-one (C2H5COCH3)

2. **Understand the Reactivity Principle**: The reactivity of carbonyl compounds towards nucleophiles like HCN is inversely proportional to steric hindrance. This means that the less hindered a carbonyl carbon is, the more reactive it will be.

3. **Analyze Steric Hindrance**:
- **Formaldehyde (HCHO)**: Has no bulky groups attached to the carbonyl carbon. It is the least hindered.
- **Acetone (CH3COCH3)**: Has two bulky methyl groups attached to the carbonyl carbon, leading to increased steric hindrance.
- **Acetaldehyde (CH3CHO)**: Has one bulky methyl group, which provides some steric hindrance but is less than acetone.
- **Butan-2-one (C2H5COCH3)**: Has an ethyl and a methyl group attached, creating significant steric hindrance.

4. **Rank the Compounds**: Based on the analysis of steric hindrance:
- Least hindered: **Formaldehyde (HCHO)**
- Moderate hindrance: **Acetaldehyde (CH3CHO)**
- More hindered: **Butan-2-one (C2H5COCH3)**
- Most hindered: **Acetone (CH3COCH3)**

5. **Conclusion**: Since formaldehyde is the least hindered carbonyl compound, it will have the maximum reactivity towards HCN.

### Final Answer:
**Formaldehyde (HCHO) has the maximum reactivity towards HCN.**