Qualitative Analysis of Organic Compounds
Secure your Practical Organic Chemistry marks. Master Lassaigne's Extract, the chemical formulas of complex precipitates, and the high-yield exceptions for N, S, P, and Halogens.
Module Focus: From Covalent to Ionic
Organic compounds contain elements like Nitrogen, Sulfur, Halogens, and Phosphorus bonded covalently. Typical inorganic tests use aqueous reagents that require ions to react. Therefore, the core principle of qualitative organic analysis is converting these covalent heteroatoms into soluble ionic salts before testing. This is achieved via Lassaigne's test.
1. Detection of Carbon and Hydrogen
Carbon and Hydrogen are detected by heating the compound with dry Copper(II) oxide ($CuO$). Carbon is oxidized to $CO_2$, and Hydrogen to $H_2O$.
The evolved $CO_2$ gas is passed through lime water ($Ca(OH)_2$), turning it milky due to the formation of insoluble Calcium Carbonate ($CaCO_3$).
The water vapor is tested with anhydrous Copper(II) sulfate. It turns from white to blue, forming $CuSO_4 \cdot 5H_2O$.
2. Lassaigne's Test (Sodium Fusion Extract)
The organic compound is fused with a small piece of Sodium metal in an ignition tube. Sodium is highly electropositive and violently reduces the covalent heteroatoms into water-soluble ionic sodium salts. The fused mass is extracted with boiling distilled water to form Sodium Fusion Extract (SFE).
- Hydrazine ($NH_2NH_2$) and Hydroxylamine ($NH_2OH$): These compounds contain Nitrogen but do NOT contain Carbon. Lassaigne's test for Nitrogen requires Carbon to form the Cyanide ion ($CN^-$). Thus, they give a negative test.
- Diazonium Salts ($R-N_2^+ Cl^-$): Upon heating during fusion, they violently decompose to release $N_2$ gas before it can react with Sodium. Thus, they also give a negative test.
3. Chemical Tests and Precipitates
The SFE ($NaCN$) is boiled with $FeSO_4$, then acidified with concentrated $HCl$ and a few drops of $FeCl_3$.
$3[Fe(CN)_6]^{4-} + 4Fe^{3+} \xrightarrow{xH_2O} Fe_4[Fe(CN)_6]_3 \cdot xH_2O$
Ferriferrocyanide (Prussian Blue)
Note: Oxidation state of Fe outside the bracket is +3, inside is +2.
SFE ($Na_2S$) is treated with Sodium nitroprusside. A deep violet color indicates Sulfur.
Alternatively, adding Acetic acid followed by Lead Acetate yields a black precipitate of Lead Sulfide ($PbS$).
During fusion, instead of forming cyanide and sulfide separately, sodium forms Sodium Thiocyanate ($NaSCN$).
TRAP: If excess Sodium is used during fusion, the thiocyanate decomposes back to $NaCN$ and $Na_2S$, and the blood-red color is NOT observed!
The SFE is first boiled with concentrated $HNO_3$ (to expel $HCN$ and $H_2S$ gasses which interfere with the test). Then $AgNO_3$ is added.
- White ppt soluble in $NH_4OH$ $\rightarrow$ Chlorine ($AgCl$).
- Pale Yellow ppt sparingly soluble in $NH_4OH$ $\rightarrow$ Bromine ($AgBr$).
- Yellow ppt completely insoluble in $NH_4OH$ $\rightarrow$ Iodine ($AgI$).
Compound is heated with an oxidizing agent ($Na_2O_2$) to convert P to Phosphate ($PO_4^{3-}$). The solution is boiled with $HNO_3$ and treated with Ammonium Molybdate.
$(NH_4)_3PO_4 \cdot 12MoO_3$
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