Qualitative Analysis of Organic Compounds

Full Notes

Detection of Carbon and Hydrogen

The presence of carbon and hydrogen is confirmed by oxidising the organic compound in the presence of copper(II) oxide (CuO).

Reaction Principle:

• Carbon is oxidised to carbon dioxide: C + 2CuO → CO₂ + 2Cu
• Hydrogen is oxidised to water: 2H + CuO → H₂O + Cu

Identification Tests:

• CO₂ is passed through lime water. Formation of a white precipitate confirms carbon: CO₂ + Ca(OH)₂ → CaCO₃ ↓ + H₂O
• H₂O is passed through anhydrous copper(II) sulphate. Its colour change from white to blue confirms hydrogen.

Detection of Other Elements

These elements are covalently bonded and not readily detectable, so they are converted into ionic form by Lassaigne’s Test (fusion with sodium).

Lassaigne’s Test: The organic compound is fused with sodium metal to break covalent bonds and convert elements like N, S, and halogens into soluble ionic forms. The fused mass is extracted in water to get the Lassaigne’s Filtrate, which is tested further.

Test for Nitrogen

In sodium fusion, nitrogen forms sodium cyanide (NaCN):
Na + C + N → NaCN

The filtrate is treated with FeSO₄ and then acidified with H₂SO₄. Formation of Prussian blue confirms nitrogen:

• 6NaCN + FeSO₄ → Na₄[Fe(CN)₆] + Na₂SO₄
• Na₄[Fe(CN)₆] + Fe³⁺ → Fe₄[Fe(CN)₆]₃ (Prussian blue ppt)

Test for Sulphur

Sulphur forms sodium sulphide (Na₂S): 2Na + S → Na₂S

Detection Methods:

• Lead Acetate Test: Black ppt of lead sulphide confirms sulphur: Na₂S + Pb(CH₃COO)₂ → PbS ↓ (black) + 2CH₃COONa
• Sodium Nitroprusside Test: A purple/violet colour indicates sulphide ions: Na₂S + Na₂[Fe(CN)₅NO] → Na₄[Fe(CN)₅NOS]

Test for Halogens (Cl, Br, I)

Sodium fusion yields sodium halides (NaX, X = Cl, Br, I). Filtrate is acidified with HNO₃ and treated with AgNO₃:

• White ppt (AgCl) – soluble in NH₄OH → Chlorine
• Pale yellow ppt (AgBr) – slightly soluble in NH₄OH → Bromine
• Yellow ppt (AgI) – insoluble in NH₄OH → Iodine
• NaX + AgNO₃ → AgX ↓ + NaNO₃

Test for Phosphorus

Organic compound is heated with an oxidising agent (conc. HNO₃). Solution is boiled with ammonium molybdate. Yellow precipitate confirms phosphorus:

H₃PO₄ + 12(NH₄)₂MoO₄ + 21HNO₃ → (NH₄)₃PO₄·12MoO₃ ↓ (yellow ppt) + 21NH₄NO₃ + 12H₂O

Summary

• Carbon and hydrogen are detected via oxidation with CuO and identification of CO₂ and H₂O.
• Lassaigne’s test converts N, S and halogens into ionic forms for confirmatory reactions.
• Prussian blue confirms nitrogen and black PbS confirms sulphur.
• AgNO₃ gives characteristic precipitates with halides and ammonium molybdate confirms phosphorus.