Cyanohydrin formation via catalyzed nucleophilic addition

Steps:

• Propanone (CH₃COCH₃) has a polar carbonyl group, making carbon electrophilic.
• KCN provides CN⁻, which nucleophilically adds to the carbonyl carbon.
• This forms a tetrahedral alkoxide intermediate, R₂C(O⁻)CN.
• The intermediate deprotonates HCN to yield the cyanohydrin product and regenerate CN⁻.

Why A is correct:

• Catalysts speed reactions without net consumption; here, CN⁻ initiates addition but is regenerated after proton transfer from HCN.

Why the others are wrong:

• B: The reaction is nucleophilic addition to the carbonyl, not a general "addition reaction" like those for alkenes or alkynes.
• C: The tetrahedral intermediate (alkoxide) acts as a nucleophile in formation but as a base in protonation; the statement oversimplifies its dual role.
• D: Not enough information.

Final answer: A