Section 1: Lassaigne's Test (Sodium Fusion)
Principle: Organic compound fused with Na metal converts C/N → NaCN, S → Na2S, X → NaX. The ionic extract is tested systematically.
Nitrogen test (S absent): Extract + FeSO4 + NaOH (boil, cool) + H2SO4 → Prussian blue (Fe4[Fe(CN)6]3). Requires freshly prepared FeSO4 (Fe2+ must not be pre-oxidized).
Sulfur test: Extract + sodium nitroprusside → purple/violet; OR extract + lead acetate → black PbS.
N + S simultaneously (Critical): NaSCN forms. FeCl3 → blood-red Fe(SCN)3. Prussian blue test FAILS. This is the single highest-frequency NEET trap for this topic.
Halogen test: Boil with dilute HNO3 first (if N or S present) → add AgNO3. AgCl = white/NH3 soluble; AgBr = pale yellow/partially NH3 soluble; AgI = yellow/NH3 insoluble. Note: AgF is water-soluble, fluorine cannot be detected by this test.
Section 2: Functional Group Identification
Phenol: FeCl3 → violet/blue/green; Br2/H2O → white 2,4,6-tribromophenol.
Aldehyde: Tollens' → silver mirror (all aldehydes); Fehling's → brick-red Cu2O (aliphatic aldehydes only, NOT benzaldehyde); Schiff's → pink (all aldehydes).
Carbonyl (C=O): 2,4-DNP → orange/yellow precipitate (BOTH aldehydes and ketones — cannot distinguish).
Methyl ketone: I2/NaOH (iodoform test) → yellow CHI3. Positive for: CH3CO-R, CH3CHO, C2H5OH, CH3CHOH-R. Negative for: non-methyl ketones (e.g., 3-pentanone), benzaldehyde.
Carboxylic acid: NaHCO3 → CO2 effervescence. (Phenols do NOT give this reaction with NaHCO3.)
Primary amine: Carbylamine test (CHCl3 + KOH, heat) → foul isocyanide. Specific for 1° amines only.
Aromatic primary amine: Azo dye test (NaNO2/HCl at 0–5 °C, then β-naphthol/NaOH) → orange-red azo dye. Specific for aromatic 1° amines (aliphatic diazonium salts are unstable).
Section 3: Preparations
Acetanilide (CC(=O)Nc1ccccc1): Aniline + acetic anhydride → acetanilide + acetic acid. Used in synthesis as amino-protecting group, enables selective para-nitration.
p-Nitroacetanilide (CC(=O)Nc1ccc([N+](=O)[O-])cc1): Acetanilide + HNO3/H2SO4 (cold, nitrating mixture) → p-nitroacetanilide (major) + o-isomer (minor). Para preferred due to steric reasons.
Iodoform (C(I)(I)I): Acetone or ethanol + I2/NaOH → CHI3 (yellow crystals) + acetate ion. Formerly used as antiseptic.
Section 4: KMnO4/Oxalic Acid Titration
Equation:
Key facts: Self-indicating (no external indicator), temperature 60–70 °C, acidic medium (H2SO4), autocatalytic (Mn2+ catalyzes own reaction). Endpoint = first persistent pink. Mn: +7 → +2 (5e- per molecule). Oxalate: +3 → +4 (2e- per molecule).
Section 5: Salt Analysis
Systematic cation group separation (Groups I–VI) using progressive precipitation. Key confirmatory tests: Fe3+ (KSCN → blood-red), Cu2+ (excess NH3 → deep blue [Cu(NH3)4]2+), NH4+ (Nessler's → brown). Anion tests: SO42- (BaCl2 → white BaSO4 insoluble in HCl), S2- (lead acetate → black PbS), NO3- (brown ring test).