The carbonyl group (C=O) in aldehydes and ketones features an sp2-hybridized, electrophilic carbon ($C^{delta+}$) due to oxygen's high electronegativity.

The order of reactivity in nucleophilic addition is HCHO > other aldehydes > ketones, because alkyl groups donate electrons (+I effect) and create steric hindrance.

Nucleophilic addition proceeds: nucleophile attacks $C^{delta+}$, forming a tetrahedral alkoxide intermediate, which is protonated to give the addition product.

NH3 derivatives (hydroxylamine, phenylhydrazine, 2,4-DNP, semicarbazide) react via condensation (addition + loss of water) to give C=N compounds (oximes, hydrazones, semicarbazones).

2,4-DNP detects the C=O group in BOTH aldehydes and ketones; Tollens' (silver mirror) and Fehling's (red Cu2O) distinguish aldehydes from ketones — only aldehydes give positive results.

NaBH4 and LiAlH4 reduce C=O to C-OH (alcohol); Clemmensen (Zn-Hg/HCl, acidic) and Wolff-Kishner (NH2NH2/KOH, basic) reduce C=O completely to CH2.

Aldol condensation requires alpha-hydrogen; dilute NaOH forms an enolate that attacks another carbonyl molecule to give a beta-hydroxy carbonyl compound (aldol product).

Heating the aldol product causes dehydration to give an alpha,beta-unsaturated carbonyl compound (e.g., 2-butenal from acetaldehyde aldol).

Cannizzaro reaction occurs with NO alpha-hydrogen (HCHO, C6H5CHO): concentrated NaOH causes disproportionation — one molecule oxidized to carboxylate, one reduced to alcohol.

The iodoform test $\frac{I2}{NaOH}$ gives a yellow CHI3 precipitate with methyl ketones and their oxidizable precursors (ethanol, isopropanol) but not with methanol, propan-1-ol, or diethyl ketone.