The most reactive compound towards cyanohydrin Formation is

Options

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

Answer

To determine the most reactive compound towards cyanohydrin formation, we need to analyze the electrophilicity of the carbonyl carbon in different aldehydes and ketones. Here’s a step-by-step solution:

### Step 1: Understand the Reaction
Cyanohydrin formation involves the nucleophilic addition of cyanide ion (CN⁻) to the carbonyl group (C=O) of aldehydes or ketones. The reactivity of these compounds towards this reaction depends on the electrophilicity of the carbonyl carbon.

### Step 2: Identify the Compounds
We are given four compounds to compare:
1. pH-CHO (Benzaldehyde)
2. NO2-CHO (Nitrobenzaldehyde)
3. OH-CHO (Hydroxybenzaldehyde)
4. pH-CH2-CHO (Benzylaldehyde)

### Step 3: Analyze Electrophilicity
- **Electrophilicity** refers to the ability of the carbonyl carbon to attract nucleophiles. A more electrophilic carbon will react more readily with nucleophiles like CN⁻.
- **Electron-withdrawing groups** (like NO2) increase the electrophilicity of the carbonyl carbon by pulling electron density away from it, making it more positive and more attractive to nucleophiles.
- **Electron-donating groups** (like OH or alkyl groups) decrease the electrophilicity by pushing electron density towards the carbonyl carbon, making it less positive and less reactive.

### Step 4: Compare the Compounds
1. **Benzaldehyde (pH-CHO)**: Has a phenyl group which is weakly electron-donating, thus it has moderate electrophilicity.
2. **Nitrobenzaldehyde (NO2-CHO)**: The nitro group is a strong electron-withdrawing group, significantly increasing the electrophilicity of the carbonyl carbon.
3. **Hydroxybenzaldehyde (OH-CHO)**: The hydroxyl group is electron-donating, which decreases the electrophilicity of the carbonyl carbon.
4. **Benzylaldehyde (pH-CH2-CHO)**: The benzyl group (CH2 attached to a phenyl) is also slightly electron-donating, reducing electrophilicity.

### Step 5: Conclusion
Among the four compounds, **Nitrobenzaldehyde (NO2-CHO)** 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 **Nitrobenzaldehyde (NO2-CHO)**.

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