Practical & Purification of Organic Compounds

1. Detection of Elements — Lassaigne's Test

Organic compound is fused with sodium metal to convert covalent forms of N, S, and halogens into ionic forms (NaCN, Na₂S, NaX) that are water-soluble and can be tested.

Fusion: Heat organic compound + Na metal in ignition tube → plunge into distilled water → boil → filter. This filtrate is called the sodium fusion extract (Lassaigne's extract).

Nitrogen Detection: Na + C + N → NaCN Test: Add FeSO₄ to extract → boil → add FeCl₃ → acidify with H₂SO₄ → Prussian blue precipitate (Fe₄[Fe(CN)₆]₃) confirms N.

Sulfur Detection: Na + S → Na₂S Test 1: Add sodium nitroprusside [Na₂Fe(CN)₅NO] → purple/violet color confirms S. Test 2: Add lead acetate → black precipitate of PbS.

When both N and S are present: Na + C + N + S → NaCNS (sodium thiocyanate). Add FeCl₃ → blood-red color (not Prussian blue) due to Fe(CNS)₃. To test N separately, add excess Na to convert all S to Na₂S and all CN to NaCN (avoiding NaCNS formation).

Halogen Detection: Na + X → NaX (X = Cl, Br, I) Test: Acidify extract with HNO₃ → add AgNO₃.

• White precipitate soluble in NH₃ → Cl (AgCl)
• Pale yellow precipitate partially soluble → Br (AgBr)
• Yellow precipitate insoluble in NH₃ → I (AgI)

Important: If N or S is present, boil the extract with concentrated HNO₃ FIRST to decompose CN- and S₂- (which would interfere by forming AgCN or Ag₂S precipitates).

Beilstein test (qualitative halogen test): Heat Cu wire → dip in compound → heat in flame. Green/blue-green flame confirms halogen (Cu forms volatile CuX₂). Does not distinguish which halogen.

2. Detection of Functional Groups

3. Purification Methods

Crystallization: Dissolve impure solid in minimum hot solvent → filter hot to remove insoluble impurities → cool slowly → pure crystals form → filter (suction filtration). Choice of solvent: must dissolve compound well when hot, poorly when cold. Used for: benzoic acid (from hot water), naphthalene, sugar.

Simple Distillation: For liquids with boiling point difference > 25°C and no azeotrope. Liquid heated → vapor → condenser → pure distillate. Used for: separating ether (35°C) from toluene (110°C).

Fractional Distillation: For miscible liquids with bp difference < 25°C. Uses fractionating column (provides multiple vaporization-condensation cycles). Used for: acetone (56°C) and methanol (65°C), petroleum fractions.

Steam Distillation: For water-immiscible liquids with high bp that decompose before boiling. Pass steam through mixture → compound co-distills below its normal bp (total vapor pressure = $P_{water}$ + $P_{compound}$ = atmospheric P). Used for: aniline , essential oils, nitrobenzene .

Distillation under Reduced Pressure (Vacuum Distillation): For liquids that decompose at their normal bp. Reduced pressure lowers bp. Used for: glycerol (bp 290°C at 1 atm, distills at 180°C under vacuum).

Differential Extraction: Separate compound from aqueous solution using immiscible organic solvent (ether, CH₂Cl₂). Compound preferentially dissolves in organic layer. Multiple extractions with small volumes are more efficient than one extraction with the same total volume (Nernst distribution law: K = $\frac{C_{organic}}{C_{aqueous}}$).

Sublimation: Direct solid → vapor → solid (skipping liquid). For solids with high vapor pressure: camphor, naphthalene c1ccc2ccccc2c1, anthracene, iodine, benzoic acid .

Common lab molecules encountered in purification:

Aniline (purified by steam distillation)

Benzoic acid (purified by crystallization or sublimation)

Naphthalene (purified by sublimation)

4. Chromatography

Principle: Separation based on differential distribution between a stationary phase and a mobile phase. Components with greater affinity for stationary phase move slower.

Column Chromatography: Stationary = alumina or silica gel packed in glass column. Mobile = organic solvent (eluent). Most adsorbed component elutes last. Polarity order: more polar compound adsorbs more strongly. Used for separating plant pigments, drug purification.

Thin Layer Chromatography (TLC): Stationary = thin layer of silica gel on glass plate. Mobile = solvent mixture. Developed in closed chamber → visualized under UV or with I₂. Rf = distance traveled by solute / distance traveled by solvent front. Higher Rf = less polar compound (moves faster with non-polar eluent).

Paper Chromatography: Stationary = water trapped in cellulose fiber of paper. Mobile = organic solvent. Partition chromatography (not adsorption). Used for: separating amino acids, food dyes. Rf value identifies compounds.

5. Qualitative Salt Analysis (Inorganic)

Cation Analysis (Systematic): Add group reagents sequentially to separate cations into groups:

• Group I: Dilute HCl → PbCl₂, AgCl, Hg₂Cl₂ precipitate (white)
• Group II: H₂S in acidic medium → CuS (black), PbS (black), CdS (yellow), As₂S₃ (yellow)
• Group III: NH₄Cl + NH₄OH → Fe(OH)₃ (brown), Al(OH)₃ (white), Cr(OH)₃ (green)
• Group IV: H₂S in basic medium → NiS (black), CoS (black), MnS (pink), ZnS (white)
• Group V: (NH₄)₂CO₃ → BaCO₃, SrCO₃, CaCO₃ (white precipitates)
• Group VI: No group reagent — Mg₂+, Na+, K+, NH₄+ (identified by specific tests)

Anion Tests: $CO3^{2}$- (acid → CO₂ effervescence), $SO4^{2}$- (BaCl₂ → white BaSO₄ insoluble in HCl), Cl- (AgNO₃ → white AgCl soluble in NH₃), NO₃- (brown ring test with FeSO₄ + conc. H₂SO₄).

6. Titrimetric Analysis

Acid-Base Titration: Strong acid + strong base: phenolphthalein or methyl orange indicator. Equivalence point at pH 7. For weak acid + strong base: phenolphthalein (basic equivalence point). For strong acid + weak base: methyl orange (acidic equivalence point).

Redox Titration: KMnO₄ titrations (self-indicating — pink/purple color disappears in acidic medium as MnO₄- → Mn₂+). Oxalic acid is primary standard for KMnO₄ standardization: 2KMnO₄ + 5H₂C₂O₄ + 3H₂SO₄ → 2MnSO₄ + K₂SO₄ + 10CO₂ + 8H₂O.

The key problem-solving concept is selecting the correct purification technique based on physical properties (bp, solubility, volatility) and identifying elements/functional groups through systematic chemical tests.