Nucleophilic Addition of HCN to Carbonyls

Steps:

• Identify reaction type: HCN adds to C=O forming cyanohydrin via CN⁻ nucleophile attacking carbonyl carbon.
• Evaluate catalysis: requires base like KCN to generate CN⁻, not acid.
• Assess product for pentan-2-one: CH₃C(O)CH₂CH₂CH₃ yields CH₃C(OH)(CN)CH₂CH₂CH₃ with chiral C bearing four different groups.
• Classify overall: addition reaction, not substitution or condensation.

Why B is correct:

• Pentan-2-one's carbonyl carbon attaches to CH₃ and propyl groups, so cyanohydrin has a tetrahedral chiral center per stereochemistry definition.

Why the others are wrong:

• A: Reaction uses base catalysis (e.g., KCN/H⁺), not concentrated H₂SO₄ which protonates carbonyl.
• C: No small molecule elimination; it's pure addition, not condensation.
• D: CN⁻ adds to C=O without displacing group, unlike substitution.

Final answer: B