Functional Group Tests – Class 12 Chemistry
This table summarizes characteristic tests used to identify common organic functional groups with two to three types of tests for each.
| Functional Group | General Formula / Example | Tests & Reagents | Observations | Inference | Chemical Reaction / Notes |
|---|---|---|---|---|---|
| Alcohol (-OH) | R–OH |
1. Lucas Test (ZnCl₂ + HCl) 2. Reaction with Sodium Metal 3. Oxidation (Potassium Permanganate Test) |
1. Turbidity or cloudiness; 3° fastest, 1° slow 2. Effervescence of H₂ gas 3. Purple KMnO₄ decolorizes (primary and secondary) |
Confirms presence of alcohol group |
R–OH + HCl (ZnCl₂) → R–Cl + H₂O 2R–OH + 2Na → 2R–ONa + H₂↑ R–CH₂OH + KMnO₄ → R–COOH (oxidation) |
| Aldehyde (-CHO) | R–CHO |
1. Tollens’ Test (AgNO₃ + NH₄OH) 2. Fehling’s Test 3. Schiff’s Test |
1. Silver mirror formed 2. Brick-red ppt. formed 3. Pink colouration appears |
Presence of aldehyde group |
R–CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → R–COO⁻ + 2Ag↓ + 4NH₃ + 2H₂O R–CHO + 2Cu²⁺ + 5OH⁻ → R–COO⁻ + Cu₂O↓ + 3H₂O R–CHO + Schiff’s reagent → Pink colour due to reduction |
| Ketone (=CO) | R–CO–R’ |
1. 2,4-Dinitrophenylhydrazine (2,4-DNP) 2. Iodoform Test (for methyl ketones) |
1. Yellow/orange precipitate 2. Yellow ppt. of iodoform with methyl ketones |
Presence of ketone functional group |
R–CO–R’ + 2,4-DNP → Hydrazone derivative (yellow/orange ppt.) CH₃–CO–R + I₂ + NaOH → CHI₃↓ + R–COONa (iodoform) |
| Carbon-Carbon Double Bond (C=C) | Alkenes, e.g. R–CH=CH–R’ |
1. Bromine Water Test 2. Baeyer’s Test (Dilute KMnO₄) 3. Addition of Hydrogen (Catalytic hydrogenation) |
1. Decolorization of bromine water 2. Purple KMnO₄ solution turns brown 3. Alkene converts to alkane (no unsaturation) |
Confirms presence of C=C double bond |
R–CH=CH–R’ + Br₂ → Dibromo derivative R–CH=CH–R’ + KMnO₄ → Glycols + MnO₂↓ R–CH=CH–R’ + H₂ (catalyst) → R–CH₂–CH₂–R’ |
| Carboxylic Acid (-COOH) | R–COOH |
1. Reaction with Sodium Bicarbonate (NaHCO₃) 2. Esterification Test (with Alcohol + H₂SO₄) 3. Ferric Chloride Test (for some acids) |
1. Effervescence due to CO₂ 2. Fruity smell of ester 3. Violet coloration (for phenolic acids) |
Confirms presence of carboxylic acid group |
R–COOH + NaHCO₃ → R–COONa + CO₂↑ + H₂O R–COOH + R’–OH + H₂SO₄ → RCOOR’ + H₂O (esterification) Phenolic acids give violet color with FeCl₃ |
| Phenolic -OH | Ar–OH (Phenol) |
1. Ferric Chloride Test 2. Bromine Water Test 3. Reaction with Sodium Hydroxide (NaOH) |
1. Violet or green coloration 2. White ppt. of bromophenol formed (decolorization) 3. Formation of phenolate salt (soluble) |
Presence of phenolic hydroxyl group |
Ar–OH + FeCl₃ → Violet complex Ar–OH + Br₂ → Ar–Br + HBr (white ppt.) Ar–OH + NaOH → Ar–ONa + H₂O |
| Amino Group (-NH₂) | R–NH₂ (Primary amines) |
1. Hinsberg Test (with Benzenesulfonyl Chloride) 2. Reaction with Nitrous Acid (NaNO₂ + HCl) 3. Reaction with Acetyl Chloride |
1. Soluble sulfonamide formed for 1°, insoluble for 2° 2. Evolution of N₂ gas with 1° amines 3. Formation of acetanilide derivative |
Presence of –NH₂ group and amine type |
R–NH₂ + C₆H₅SO₂Cl + NaOH → R–NHSO₂C₆H₅ (soluble) + NaCl R–NH₂ + HNO₂ + HCl → R–OH + N₂↑ + HCl R–NH₂ + CH₃COCl → R–NHCOCH₃ + HCl |