The reaction of formaldehyde with HCN is:

Explanation

The reaction of formaldehyde (HCHO) with hydrogen cyanide (HCN) involves a nucleophilic addition rather than a nucleophilic substitution.In a nucleophilic addition reaction, a nucleophile adds to a molecule, typically at a carbon-carbon double bond (as in alkenes and carbonyl compounds), resulting in the formation of a new compound.In the case of formaldehyde and HCN, the oxygen atom of formaldehyde's carbonyl group (C=O) acts as the electrophilic site, while the cyanide ion (CN⁻) acts as the nucleophile. The CN⁻ attacks the carbon atom of the carbonyl group, and the reaction results in the formation of a hydroxynitrile compound (a cyanohydrin). This addition reaction leads to the creation of a new carbon-carbon bond and the replacement of the oxygen atom with the cyanide group.Nucleophilic substitution reactions, on the other hand, involve the replacement of a leaving group in a molecule by a nucleophile. In the case of formaldehyde and HCN, no leaving group is being substituted by the cyanide ion, so it is not a nucleophilic substitution reaction.In summary, the reaction of formaldehyde with HCN is an example of nucleophilic addition.