Chapter 8 / 8.9 Qualitative Analysis Of Organic Compounds

Partition Chromatography (Paper Chromatography)

How it works: Separates mixtures by letting components partition (distribute) between two phases:

• 📄 Stationary phase: Water trapped in chromatography paper.
• 🧪 Mobile phase: Solvent (e.g., ethanol/water mix).

Steps:

1. Spot the mixture near the bottom of the paper.
2. Dip the paper in solvent (mobile phase).
3. Solvent rises by capillary action 🌊, carrying components upward.
4. Components separate based on how much they “like” water vs. solvent.

Result: A chromatogram with colored spots at different heights. Colorless spots need UV light or spray reagents 🔍.

Qualitative Analysis of Organic Compounds

Organic compounds contain C and H, and sometimes O, N, S, halogens (Cl, Br, I), or P.

1. Detecting Carbon & Hydrogen 🔥

Heat compound with CuO:

• Carbon → CO₂ gas (turns limewater milky): \[ \ce{C + 2CuO ->[\Delta] 2Cu + CO2} \] \[ \ce{CO2 + Ca(OH)2 -> CaCO3 \downarrow (white) + H2O} \]
• Hydrogen → H₂O (turns anhydrous CuSO₄ blue): \[ \ce{2H + CuO ->[\Delta] Cu + H2O} \] \[ \ce{CuSO4 + 5H2O -> CuSO4.5H2O} \quad \text{(white → blue)} \]

2. Lassaigne’s Test (for N, S, Halogens, P)

Step 1: Fuse organic compound with sodium metal (Na) to convert elements to ions:

• \[ \ce{Na + C + N ->[\Delta] NaCN} \]
• \[ \ce{2Na + S ->[\Delta] Na2S} \]
• \[ \ce{Na + X ->[\Delta] NaX} \] (X = Cl, Br, I)

Boil the fused mass in water → Sodium Fusion Extract.

A. Test for Nitrogen (N) 💙

1. Boil extract with FeSO₄.
2. Acidify with concentrated H₂SO₄.
3. Prussian blue color = N present! \[ \ce{6CN- + Fe^{2+} -> [Fe(CN)6]^4-} \] \[ \ce{4Fe^{3+} + 3[Fe(CN)6]^4- -> Fe4[Fe(CN)6]3} \quad \text{(Prussian blue)} \]

B. Test for Sulphur (S) 🖤💜

• Option 1: Add lead acetate to acetic acid-acidified extract → black PbS precipitate. \[ \ce{S^{2-} + Pb^{2+} -> PbS \downarrow} \]
• Option 2: Add sodium nitroprusside → violet color. \[ \ce{S^{2-} + [Fe(CN)5NO]^{2-} -> [Fe(CN)5NOS]^{4-}} \]

C. Test for Halogens (Cl, Br, I) ⚪🟡

1. Acidify extract with HNO₃.
2. Add AgNO₃ solution:

• Cl: White ppt (soluble in NH₄OH)
• Br: Yellowish ppt (sparingly soluble in NH₄OH)
• I: Yellow ppt (insoluble in NH₄OH)

\[ \ce{X- + Ag+ -> AgX \downarrow} \]

Note: If N/S are present, boil extract with HNO₃ first to destroy CN⁻/S²⁻ ions.

D. Test for Phosphorus (P) 💛

1. Heat compound with oxidizing agent (e.g., Na₂O₂).
2. Boil with HNO₃.
3. Add ammonium molybdate → yellow color/precipitate = P present! \[ \ce{PO4^{3-} + 12MoO4^{2-} + 24H+ -> (NH4)3PO4.12MoO3 \downarrow} \]

⚠️ Special Case: Nitrogen + Sulphur Together

Forms NaSCN (sodium thiocyanate) → gives blood red color with Fe³⁺, not Prussian blue.

\[ \ce{Fe^{3+} + SCN- -> [Fe(SCN)]^{2+}} \quad \text{(blood red)} \]

Important Concepts for NEET 🔥

1. Lassaigne’s Test Steps: Sodium fusion → extract → specific tests for N, S, halogens.
2. Nitrogen Test: Prussian blue color after adding FeSO₄ + conc. H₂SO₄.
3. Halogen Test: AgNO₃ gives Cl (white), Br (pale yellow), I (yellow) precipitates.
4. Sulphur Tests: Black PbS with lead acetate or violet color with sodium nitroprusside.
5. N+S Together: Blood red color (no Prussian blue) due to thiocyanate formation.

Quick Summary

Element	Test	Positive Result
Carbon (C)	Heat with CuO → CO2	Limewater milky
Hydrogen (H)	Heat with CuO → H2O	Anhydrous CuSO4 turns blue
Nitrogen (N)	Lassaigne’s + FeSO4/H2SO4	Prussian blue
Sulphur (S)	Lassaigne’s + Pb(CH3COO)2	Black precipitate
Halogens (X)	Lassaigne’s + AgNO3	AgCl (white), AgBr (yellowish), AgI (yellow)

Keep practicing these tests – you’ll ace them! 🎉