Nucleophilic addition reactions
Aldehydes and Ketones · Chemistry
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Why does HCN add to carbonyls but not to alkenes? Step 1: C=O is polarized: Cδ⁺ (electrophile), Oδ⁻. Step 2: CN⁻ (nucleophile) attacks the δ⁺ carbon → tetrahedral alkoxide. Step 3: Protonation → cyanohydrin (new C–C bond!). Step 4: Alkenes aren't polarized δ⁺ — no electrophilic carbon. Polarity drives addition!