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 will follow these steps:

### Step 1: Identify the Reactants
The reactants in this reaction are an aldehyde (RCHO) and potassium cyanide (KCN). KCN dissociates into potassium ions (K⁺) and cyanide ions (CN⁻) in solution.

**Hint:** Remember that KCN is a salt that dissociates into K⁺ and CN⁻ ions in aqueous solution.

### Step 2: Understand the Role of the Cyanide Ion
The cyanide ion (CN⁻) acts as a nucleophile. A nucleophile is a species that donates an electron pair to form a chemical bond. In this case, the CN⁻ ion will attack the electrophilic carbon atom of the carbonyl group (C=O) in the aldehyde.

**Hint:** Identify nucleophiles as electron-rich species that seek electron-deficient centers (electrophiles).

### Step 3: Nucleophilic Attack
The nucleophilic cyanide ion attacks the carbon atom of the carbonyl group in the aldehyde. This results in the formation of a tetrahedral intermediate. The double bond between carbon and oxygen breaks, and the oxygen atom becomes negatively charged.

**Hint:** Visualize the mechanism: the nucleophile attacks the electrophilic carbon, leading to a change in bonding.

### Step 4: Protonation of the Intermediate
The negatively charged oxygen in the tetrahedral intermediate can be protonated by an acid (H⁺) or by water in a hydrolysis step. This results in the formation of the hydroxyl group (-OH) in the final product.

**Hint:** Protonation is a common step in reactions involving nucleophilic addition to stabilize the intermediate.

### Step 5: Formation of Cyanohydrin
The final product of this reaction is a cyanohydrin (R-CH(OH)-CN), which contains both a hydroxyl group and a cyanide group. This product is formed through the nucleophilic addition of the cyanide ion to the aldehyde.

**Hint:** The name "cyanohydrin" reflects the presence of both the -OH group and the -CN group in the product.

### Conclusion
The correct description of the reaction is that the cyanide ion acts as a nucleophile and performs a nucleophilic addition to the aldehyde, leading to the formation of cyanohydrin.

**Final Answer:** The correct option is that the cyanide ion acts as a nucleophile and does nucleophilic addition.