An aldehyde reacts with KCN to form cyanohydrin. In this reaction

Options

A. `CN^-` acts as nucleophile and does nucleophilic addition
B. `CN^-` acts as nucleophile and does electrophilic addition
C. `CN^-` acts as an electrophilic and does electrophilic addition
D. `CN^-` acts as nuclcophile and does nucleophile substitution

Answer

To solve the question regarding the reaction of an aldehyde with KCN to form cyanohydrin, we can break down the process step by step.

### Step-by-Step Solution:

1. **Identify the Reactants**:
- We have an aldehyde (for example, acetaldehyde, CH₃CHO) and potassium cyanide (KCN).

2. **Dissociation of KCN**:
- In solution, KCN dissociates into potassium ions (K⁺) and cyanide ions (CN⁻). The cyanide ion (CN⁻) is the active species in this reaction.

3. **Nucleophilic Attack**:
- The carbon atom of the carbonyl group (C=O) in the aldehyde is electrophilic due to the partial positive charge on it. The cyanide ion (CN⁻), being electron-rich, acts as a nucleophile and attacks the electrophilic carbon atom.

4. **Formation of Tetrahedral Intermediate**:
- Upon the nucleophilic attack, the double bond between carbon and oxygen breaks, leading to the formation of a tetrahedral intermediate. The oxygen atom now carries a negative charge (alkoxide ion).

5. **Protonation of the Alkoxide Ion**:
- In the presence of water (during hydrolysis), the alkoxide ion (O⁻) can be protonated by a water molecule, leading to the formation of the hydroxyl group (–OH).

6. **Formation of Cyanohydrin**:
- The final product of this reaction is a cyanohydrin, which contains both a hydroxyl group (–OH) and a cyano group (–CN) attached to the same carbon atom.

### Conclusion:
The reaction of an aldehyde with KCN results in the formation of cyanohydrin through a nucleophilic addition mechanism.