C=O is sp2 and electrophilic ($C^{delta+}$): Reactivity order — HCHO > aldehydes > ketones (steric + +I effects of alkyl groups reduce ketone reactivity).

Distinction Tests: 2,4-DNP = all C=O (orange ppt); Tollens' = aldehydes only (silver mirror); Fehling's = aldehydes only (brick-red Cu2O); iodoform $\frac{I2}{NaOH}$ = CH3CO- compounds → yellow CHI3.

Reductions: NaBH4/LiAlH4 → C-OH (alcohol, O retained); Clemmensen (acidic, Zn-Hg/HCl) or Wolff-Kishner (basic, NH2NH2/KOH) → CH2 (O removed completely).

Alpha-H Rule (the most important rule): Alpha-H present → Aldol condensation (dil. NaOH) → beta-hydroxy carbonyl → (heat) → alpha,beta-unsaturated carbonyl. No alpha-H → Cannizzaro (conc. NaOH) → acid salt + alcohol.

Iodoform Positives vs Negatives: Positive: CH3CHO, CH3COCH3, any CH3COR, ethanol, isopropanol. Negative: HCHO, methanol, diethyl ketone, benzaldehyde, propan-1-ol.