The carbonyl carbon is electrophilic (sp2, delta+). Nucleophilic attack converts it from sp2 to sp3 (tetrahedral intermediate).

Base-catalyzed: Nu:- attacks C=O → alkoxide → protonation Acid-catalyzed: H+ protonates O first → activates C=O → weaker nucleophile can attack

Reactivity order: HCHO >> RCHO >> R2CO >> ArCOR >> Ar2CO Factors: (1) Steric — fewer/smaller R groups = less hindered attack, (2) Electronic — +I of R groups reduces electrophilicity of C, (3) Aromatic ketones have additional stabilization by resonance with ring.

JEE tip: Compare reactivity by combining both factors. p-NO2-C6H4-CHO is MORE reactive than C6H5CHO because -NO2 is -M, making C more electrophilic.