The most reactive compound towards cyanohydrin Formation is

Options

A. Ph - CHO
B.
C.
D. `Ph-CH_(2)-CHO`

Answer

To determine which compound is the most reactive towards cyanohydrin formation, we need to analyze the electrophilicity of the carbonyl carbon in each compound. The more electrophilic the carbonyl carbon, the more reactive it will be towards nucleophilic attack by cyanide (HCN).

### Step-by-Step Solution:

1. **Identify the Compounds**: We are given several compounds to analyze for their reactivity towards cyanohydrin formation. These compounds include:
- Benzaldehyde
- A compound with a nitro group
- A compound with a hydroxyl (OH) group
- A compound with a benzene ring and a CH2 group attached to a carbonyl (aldehyde)

2. **Analyze Benzaldehyde**:
- Benzaldehyde has a carbonyl group (C=O) attached to a benzene ring. The benzene ring is electron-donating due to resonance, which increases the electron density on the carbonyl carbon.
- **Conclusion**: The increased electron density makes the carbonyl carbon less electrophilic, thus less reactive towards nucleophiles like cyanide.

3. **Analyze the Compound with Nitro Group**:
- The nitro group (–NO2) is a strong electron-withdrawing group. It pulls electron density away from the carbonyl carbon, making it more electrophilic.
- **Conclusion**: The reduced electron density on the carbonyl carbon increases its electrophilicity, making it more reactive towards cyanide.

4. **Analyze the Compound with Hydroxyl Group**:
- The hydroxyl group (–OH) is an electron-donating group through resonance. It increases the electron density on the carbonyl carbon, making it less electrophilic.
- **Conclusion**: The increased electron density decreases the reactivity of the carbonyl carbon towards cyanide.

5. **Analyze the Compound with CH2 Group**:
- The CH2 group attached to the carbonyl can exhibit a +I (inductive) effect, which increases the electron density on the carbonyl carbon.
- **Conclusion**: This also makes the carbonyl carbon less electrophilic and less reactive towards cyanide.

6. **Final Comparison**:
- Among the compounds analyzed, the one with the nitro group is the most reactive towards cyanohydrin formation due to the strong electron-withdrawing effect of the nitro group, which enhances the electrophilicity of the carbonyl carbon.

### Final Answer:
The most reactive compound towards cyanohydrin formation is the compound with the **nitro group**.