The carbonyl compound which is most reactive in addition of HCN is:

Question

Accepted Answer

The carbonyl compound which is most reactive in the addition of HCN is formaldehyde (HCHO), which is given as Option 4. Here's a detailed explanation supporting why this is the correct answer:

1. **Reactivity of Carbonyl Compounds:**
   - The reactivity of carbonyl compounds towards nucleophilic addition reactions, such as the addition of HCN, is influenced by both steric and electronic factors.
   - Sterically, smaller groups attached to the carbonyl carbon allow easier access for the nucleophile. Electronically, less substituted carbonyl carbons are more electrophilic, making them more reactive towards nucleophiles.

2. **Explanation for Option 4 (HCHO):**
   - Formaldehyde (HCHO) is the simplest aldehyde with no alkyl groups attached to the carbonyl carbon. This lack of steric hindrance makes it highly accessible to nucleophiles.
   - Additionally, the carbonyl carbon in formaldehyde is highly electrophilic due to the absence of electron-donating alkyl groups, which would otherwise reduce its positive character.
   - These factors make formaldehyde the most reactive carbonyl compound in the addition of HCN.

3. **Explanation for Other Options:**
   - **Option 1 ($CH_{3}-C(=O)-CH_{3}$):** Acetone is a ketone with two methyl groups attached to the carbonyl carbon. These groups provide steric hindrance and donate electron density, reducing the electrophilicity of the carbonyl carbon, making it less reactive than formaldehyde.
   - **Option 2 ($CH_{3}CH_{2}-CHO$):** Propanal is an aldehyde, but it has an ethyl group attached to the carbonyl carbon. This group provides some steric hindrance and electron donation, making it less reactive than formaldehyde.
   - **Option 3 ($CH_{3}CHO$):** Acetaldehyde has one methyl group attached to the carbonyl carbon. While it is more reactive than ketones due to being an aldehyde, the presence of the methyl group still provides some steric hindrance and electron donation, making it less reactive than formaldehyde.

In conclusion, formaldehyde (HCHO) is the most reactive carbonyl compound in the addition of HCN due to its minimal steric hindrance and high electrophilicity, justifying why Option 4 is the correct answer.