: 250
The C=O group is polarized (C delta+, O delta-), making the carbon electrophilic. Nucleophilic addition follows: Nu:- attacks C → tetrahedral intermediate → protonation or further reaction. Reactivity decreases with steric bulk and electron donation: HCHO > RCHO > R2CO > ArCOR > Ar2CO. Key nucleophilic additions: HCN → cyanohydrin (chain extension), RMgX → alcohols (HCHO→1°, RCHO→2°, R2CO→3°), NaHSO3 → bisulfite adduct (purification tool, works with aldehydes + methyl ketones), Wittig ylide → alkene (only method for precise C=C placement). The Grignard reaction is the most versatile — RMgX also reacts with CO2 (→ RCOOH), ethylene oxide (→ primary alcohol +2C), esters (→ tertiary alcohol), and nitriles (→ ketone). IMPORTANT: RMgX is destroyed by any protic source (H2O, ROH, RCOOH, terminal alkynes, NH3). All Grignard reactions must be anhydrous. For acid-catalyzed additions (acetal formation, imine/oxime), the mechanism adds a protonation step that activates the C=O before nucleophilic attack. Acetals (RCH(OR')2) are stable in base but cleaved by acid — used as protecting groups for C=O during base-catalyzed reactions.