Cyanohydrin Formation via Nucleophilic Addition

Steps:

• Recognize the reaction as cyanohydrin synthesis, where HCN (with KCN as catalyst) adds to a carbonyl group, treating ethanol as context for ethanal (acetaldehyde) analog.
• CN⁻ nucleophile attacks the electrophilic carbonyl carbon, breaking the C=O π bond.
• The carbon of CN bonds to the carbonyl carbon, forming a new C–C σ bond.
• Protonation of the alkoxide yields the cyanohydrin, with no breakage of C–H or C–N bonds.

Why B is correct:

• In cyanohydrin formation, the product structure RCH(OH)CN includes a new C–C bond between the original carbonyl carbon and the CN carbon, per nucleophilic addition mechanism.

Why the others are wrong:

• A: The C–H bond in HCN remains intact; H⁺ adds to oxygen without breaking substrate C–H.
• C: The C≡N triple bond in cyanide persists unbroken throughout the addition.
• D: Option incomplete; no bonding change specified.

Final answer: B