Cyanohydrin Formation via Nucleophilic Addition

Steps:

• KCN dissociates to provide CN⁻ nucleophile, while HCN maintains equilibrium.
• CN⁻ attacks the electrophilic carbonyl carbon of the aldehyde (ethanal, likely intended over ethanol).
• This forms a tetrahedral alkoxide intermediate.
• Protonation of the alkoxide yields the hydroxynitrile product.

Why B is correct:

• B depicts CN⁻ addition to the carbonyl, the defining first step in base-catalyzed nucleophilic addition per organic mechanism principles.

Why the others are wrong:

• A: Involves H⁺ addition, which is acid-catalyzed path, not initial with KCN base.
• C: Shows deprotonation of HCN last, ignoring prior CN⁻ generation.
• D: Describes product formation directly, skipping nucleophilic attack.

Final answer: B