LASSAIGNE'S TEST
| If compound contains | Fusion product | Test result |
|---|---|---|
| N only | NaCN | Prussian blue (FeSO4/NaOH/H2SO4) |
| S only | Na2S | Purple (nitroprusside); Black PbS (lead acetate) |
| N + S together | NaSCN | Blood-red (FeCl3) — NOT Prussian blue |
| Cl | NaCl | White AgCl (soluble in dilute NH3) |
| Br | NaBr | Pale yellow AgBr (partially soluble in conc. NH3) |
| I | NaI | Yellow AgI (insoluble in NH3) |
| F | NaF | No precipitate (AgF is water-soluble) |
Before AgNO3 test: If N or S present, BOIL with dilute HNO3 first.
FUNCTIONAL GROUP TESTS
| Group | Test | Positive Result |
|---|---|---|
| Phenol (-OH ar.) | Neutral FeCl3 | Violet/blue/green color |
| Phenol | Br2/H2O | White 2,4,6-tribromophenol ppt |
| Aldehyde (-CHO) | Tollens' | Silver mirror |
| Aldehyde (aliphatic) | Fehling's | Brick-red Cu2O ppt |
| Aldehyde | Schiff's | Pink color |
| Both C=O | 2,4-DNP | Orange/yellow ppt |
| Methyl ketone | I2/NaOH | Yellow CHI3 (iodoform) |
| -COOH | NaHCO3 | CO2 effervescence |
| 1° amine (-NH2) | CHCl3/KOH | Foul isocyanide smell |
| ArNH2 | NaNO2/HCl then β-naphthol | Orange-red azo dye |
KMNO4/OXALIC ACID TITRATION
- No indicator (self-indicating)
- Temperature: 60–70 °C
- Endpoint: First persistent faint pink
- Medium: Dilute H2SO4 (NOT HCl)
- Mn: +7 → +2 (gains 5e- per KMnO4)
- Oxalate: +3 → +4 (loses 2e- per C2O42-)
PREPARATIONS
| Product (SMILES) | From | Reagent |
|---|---|---|
Acetanilide CC(=O)Nc1ccccc1 | Aniline + Ac2O | Glacial AcOH, heat |
p-Nitroacetanilide CC(=O)Nc1ccc([N+](=O)[O-])cc1 | Acetanilide | HNO3/H2SO4 cold — para major |
Iodoform C(I)(I)I | Acetone/ethanol | I2/NaOH, RT |
CATION GROUPS
| Group | Reagent | Key precipitates |
|---|---|---|
| I | Dilute HCl | PbCl2 white (soluble hot water) |
| II | H2S/acidic | CuS black, PbS black |
| III | NH4Cl + NH4OH | Al(OH)3 white, Fe(OH)3 brown |
| IV | H2S/basic | ZnS white, NiS black |
| V | (NH4)2CO3 | CaCO3, BaCO3, SrCO3 white |
| VI | No group reagent | Mg(OH)2 white; NH4+ → Nessler's brown |
CRITICAL NEET TRAPS
- N + S → blood-red (NOT Prussian blue)
- KMnO4 titration → no indicator, 60–70 °C
- AgI → yellow, insoluble in NH3
- Iodoform → methyl ketones only (not all ketones)
- Fehling's → negative for benzaldehyde
- Carbylamine → primary amines only
- Boil with HNO3 before AgNO3 test if N or S present