Cyanohydrin formation mechanism

Steps:

• Propanone's carbonyl carbon is electrophilic and attacked by nucleophilic CN⁻ from KCN.
• CN⁻ addition forms tetrahedral alkoxide intermediate, [ (CH₃)₂C(O⁻)CN ].
• Intermediate deprotonates HCN, yielding cyanohydrin product and regenerating CN⁻.
• Overall, HCN adds across C=O bond.

Why A is correct:

• A catalyst speeds reaction without net consumption; CN⁻ adds to carbonyl then regenerates by abstracting H⁺ from HCN.

Why the others are wrong:

• B: Reaction is nucleophilic addition, but statement omits specificity, making it incomplete.
• C: Intermediate is nucleophilic anion that accepts proton, but it primarily acts as nucleophile in addition step.
• D: Not enough information (option incomplete).

Final answer: A