Reactivity Order
HCHO > RCHO > ArCHO > RCOR' > ArCOR'
Formaldehyde is most reactive because it has no alkyl groups (no +I effect, no steric hindrance). Alkyl groups reduce electrophilicity of C=O. Aromatic aldehydes are less reactive due to resonance delocalization.
Key Tests to Distinguish
Silver mirror formed: ALDEHYDE (not ketone). [Ag(NH₃)₂]⁺ (Tollens' reagent) oxidizes aldehyde to carboxylate, Ag⁺ → Ag⁰ (silver mirror). Ketones do NOT give this test.
Cu₂O (brick-red ppt) formed: Aliphatic ALDEHYDE only. Aromatic aldehydes and ketones do NOT respond. Fehling's is less sensitive than Tollens'.
Yellow CHI₃ precipitate: Compounds with CH₃−CO− group OR CH₃−CHOH− group. Positive for: ethanal, acetone, ethanol, isopropanol. Negative for: formaldehyde, benzaldehyde, diethyl ketone.
Named Reactions You Must Know
Aldol Condensation
Two molecules of aldehyde/ketone with α-H combine (NaOH catalyst) to form β-hydroxy carbonyl compound (aldol). Dehydration gives α,β-unsaturated carbonyl compound.
Cannizzaro Reaction
Aldehydes with NO α-H (HCHO, C₆H₅CHO) undergo self-oxidation-reduction in conc. NaOH: one molecule oxidised to acid, another reduced to alcohol.
Clemmensen Reduction
Reduces C=O to −CH₂− using Zn-Hg/conc. HCl. Works in acidic conditions.
Wolf-Kishner Reduction
Also reduces C=O to −CH₂− but uses N₂H₄/KOH in ethylene glycol. Works in basic conditions. Complementary to Clemmensen.
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