MCQ Detailed View

The reaction between aldehydes and hydrogen cyanide (HCN) is a classic example of a nucleophilic addition reaction. Aldehydes contain a polar carbonyl group (C=O), where the carbon atom is electrophilic due to the higher electronegativity of oxygen. This makes the carbonyl carbon highly susceptible to attack by nucleophiles.

In the presence of HCN, the cyanide ion (CN⁻) acts as the attacking nucleophile. The mechanism proceeds in three steps:

1. 
   

   Nucleophilic attack:
   The cyanide ion (CN⁻) attacks the electrophilic carbonyl carbon of the aldehyde. This breaks the π bond of the C=O group and generates a tetrahedral alkoxide ion intermediate.

   
   


2. 
   

   Intermediate formation:
   The oxygen atom now bears a negative charge, forming an alkoxide ion. This step is key to stabilizing the molecule before final protonation.

   
   


3. 
   

   Protonation:
   The alkoxide ion is protonated by HCN (or another proton donor) to give the final product — a cyanohydrin. In this structure, both an –OH group and a –CN group are bonded to the same carbon atom.

   
   

This reaction is classified as nucleophilic addition because:

• 
  

  A nucleophile (CN⁻) adds to the electrophilic carbonyl carbon.

  
  


• 
  

  The carbonyl double bond (C=O) is converted into a single bond (C–OH).

  
  


• 
  

  No group is replaced, which rules out substitution.

  
  


• 
  

  The attacking species is a nucleophile, not an electrophile, which rules out electrophilic addition.

  
  

Cyanohydrins are important intermediates in organic synthesis. They can be further converted into carboxylic acids, α-hydroxy acids, or amino acids, making this reaction highly valuable in synthetic chemistry.

Therefore, the process by which aldehydes react with HCN to form cyanohydrins is correctly described as a nucleophilic addition reaction — a fundamental transformation in organic chemistry.