Classifying mechanisms by nucleophile role in substitution vs. addition

Steps:

• First reaction (CH3Br + KCN): CN⁻ acts as nucleophile attacking carbon, displacing Br⁻, indicating nucleophilic substitution.
• Second reaction (CH3COCH3 + HCN): CN⁻ adds to electrophilic carbonyl carbon of ketone, forming cyanohydrin, indicating nucleophilic addition.
• Eliminate options with electrophilic mechanisms: A and C mislabel first reaction; A and B mislabel second.
• Select D as it matches both: nucleophilic substitution followed by nucleophilic addition.

Why D is correct:

• Nucleophilic substitution involves Nu⁻ replacing a leaving group (CN⁻ displaces Br⁻); nucleophilic addition involves Nu⁻ bonding to π system (CN⁻ adds to C=O per carbonyl addition rule).

Why the others are wrong:

• A: Labels both as electrophilic, ignoring CN⁻ as nucleophile in both cases.
• B: Correctly identifies first but errs on second as electrophilic addition, not nucleophilic.
• C: Correctly identifies second but errs on first as electrophilic substitution, not nucleophilic.

Final answer: D