Protonation enables CN- nucleophilic attack on ethanol

Steps:

• Liquid HCN acts as acid, protonating oxygen of ethanol (CH3CH2OH) to form CH3CH2OH2+ intermediate.
• Protonation converts OH into good leaving group (H2O).
• CN- from KCN performs SN2 attack on carbon of protonated ethanol.
• Displacement of H2O yields ethyl cyanide (CH3CH2CN).

Why C is correct:

• Describes acid-base proton transfer from HCN to ethanol's oxygen, generating electrophilic intermediate for CN- nucleophilic attack, per SN2 mechanism requirements.

Why the others are wrong:

• A: HCN has lone pair on N (sp hybridized); CN- is nucleophile, not catalyst.
• B: No ethoxide (CH3CH2O-) forms; ethanol remains neutral and gets protonated, not deprotonated.
• D: True overall, but ignores key intermediate (protonated ethanol) and proton transfer step.

Final answer: C