Read the following passage and answer the questions based on it.

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two large groups in ketones hinders the attack of nucleophile to carbonyl carbon than in aldehydes. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in the former.

Which among the following compound is formed, when aldehyde reacts with HCN in presence of base?

Cyanohydrin

The correct answer is Option (3) → Cyanohydrin.

In carbonyl compounds (aldehydes), the electron density is higher near the oxygen atom as it is more electronegative. This leads to the generation of a partial negative charge on the oxygen atom and a partial positive charge on the carbon atom.

Since the carbonyl carbon holds a partial positive charge, it behaves as an electrophile. And the partial negative charge on the oxygen atom is neutralized via the proton of an acidic group.

The reaction of HCN with carbonyl compounds is an example of the nucleophilic addition reaction. The mechanism involved is discussed below:

Step 1: Attack of nucleophile from HCN to the slightly positive carbon atom.

The base is used as a catalyst and it deprotonates the HCN molecule to produce a stronger nucleophile – cyanide ion.

Step 2: The negative charge in the tetrahedral intermediate is then neutralized by accepting a proton from a water molecule. This results in the formation of cyanohydrin – an additional product and hydroxyl ion. This hydroxyl ion can further be used up in the formation of the nucleophile.