Concept:The mechanism of a reaction is defined by the first attacking species and the final connectivity of the product.
Solution:- Formaldehyde (\( HCHO \)) has a highly polarized \( C=O \) bond, making the carbon electrophilic (electron deficient).
- The reaction with \( HCN \) requires a base catalyst to generate the Cyanide nucleophile (\( CN^- \)).
- This nucleophile attacks the carbonyl carbon, breaking the \( \pi \) bond without kicking out any leaving groups.
- Because an electron-rich species initiates the attack and the whole molecule adds across the double bond, it is a Nucleophilic addition reaction.
Why other options are incorrect:Electrophilic additions are typical for \( C=C \) bonds. Substitutions require a leaving group to be displaced, which does not occur during cyanohydrin formation.
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