HCN breaks and form CN- ion and act as a nucleophile .
CN- attacks at carbonyl part , so it's a nucleophilic addition reaction .

So , right answer is "Nucleophilic addition"...

$$Hope it's help... $$

Nucleophilic Addition Reaction of HCN with Carbonyl Compounds

Nucleophilic addition reactions involve the attack of a nucleophile on an electrophile, resulting in the formation of a new bond. In the case of carbonyl compounds, such as aldehydes and ketones, the carbon atom of the carbonyl group is electrophilic due to the presence of the highly polarized C=O bond.

HCN (hydrogen cyanide) is a nucleophile that contains a lone pair of electrons on the nitrogen atom. When HCN reacts with a carbonyl compound, such as an aldehyde or ketone, it acts as a nucleophile and adds to the electrophilic carbon atom of the carbonyl group.

The reaction proceeds through the following steps:

1. Nucleophilic attack: The nitrogen atom of HCN attacks the electrophilic carbon atom of the carbonyl group, forming a new bond between the carbon and the nitrogen.

2. Formation of an intermediate: The resulting intermediate is a cyanohydrin, which contains a hydroxyl group (-OH) and a cyano group (-CN) attached to the same carbon atom.

3. Protonation: The hydroxyl group of the cyanohydrin can be protonated by an acid, such as HCl, to form a more stable compound.

Overall, the addition of HCN to a carbonyl compound results in the formation of a cyanohydrin. This reaction is an example of nucleophilic addition because the nucleophile (HCN) adds to the carbonyl group, resulting in the addition of a new group to the molecule.

Importance of Nucleophilic Addition of HCN to Carbonyl Compounds:

- Cyanohydrins are versatile intermediates that can be further transformed into various functional groups.
- Cyanohydrins can be hydrolyzed to form carboxylic acids or their derivatives.
- Cyanohydrins can undergo nucleophilic substitution reactions to form amines or amides.
- Cyanohydrins can be reduced to form primary alcohols.
- Cyanohydrins can be used as building blocks in the synthesis of pharmaceuticals and agrochemicals.

In conclusion, the addition of HCN to a carbonyl compound is an example of nucleophilic addition, as the nucleophile (HCN) adds to the electrophilic carbon atom of the carbonyl group, resulting in the formation of a cyanohydrin.