Cyanide nucleophilic addition to aldehyde carbonyl

Steps:

• Cyanide ion (CN⁻) attacks the electrophilic carbonyl carbon of the aldehyde (likely ethanal, not ethanol as stated).
• This forms a tetrahedral alkoxide intermediate by breaking the C=O π bond and placing negative charge on oxygen.
• The intermediate ion is R-CH(O⁻)CH₂CN for ethanal.
• Next step: protonation of the alkoxide oxygen by HCN or solvent to yield the neutral cyanohydrin.

Why D is correct:

• D shows the tetrahedral alkoxide ion from CN⁻ addition, matching the standard nucleophilic addition mechanism for cyanohydrin formation.

Why the others are wrong:

• A: Incorrect carbocation structure; addition forms alkoxide, not carbocation.
• B: Wrong regiochemistry; CN attaches to carbonyl carbon, not elsewhere.
• C: Depicts protonated intermediate prematurely; charge is on oxygen post-addition.

Final answer: D