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ChemistryQualitative Analysis
Which of the following reactions is NOT used in Lassaigne's test (sodium fusion test) for detection of elements in organic compounds?
Options
1
$Na + C + N \to NaCN$
2
$Na + S \to Na_2S$
3
$2CuO + C \to 2Cu + CO_2$
4
$Na + Cl \to NaCl$
Correct Answer
$2CuO + C \to 2Cu + CO_2$
Solution
1

Lassaigne's test: organic compound + Na metal (fusion) → NaCN (N), Na₂S (S), NaCl/NaBr/NaI (halogen)

A: $Na + C + N \to NaCN$ ✓ (part of Lassaigne)

B: $Na + S \to Na_2S$ ✓ (part of Lassaigne)

2

C: $2CuO + C \to 2Cu + CO_2$ ✗ — This is the copper oxide method for detecting C. NOT Lassaigne!

D: $Na + Cl \to NaCl$ ✓ (part of Lassaigne)

Answer: C

Lassaigne uses Na fusion: converts N→NaCN, S→Na₂S, X→NaX
CuO+C→CO₂ is the carbon detection method — NOT Lassaigne
Theory: Qualitative Analysis
1. Lassaignes Test (Sodium Fusion Test)

Lassaigne's test detects N, S, and halogens (Cl, Br, I) in organic compounds. Principle: organic compound fused with sodium metal in an ignition tube → N converted to NaCN, S to Na₂S, halogens to NaX (NaCl, NaBr, NaI). The fusion product (Lassaigne extract) dissolved in water → tested for these ionic species. Why Na? Sodium is a highly reactive metal that converts covalently bonded elements (C-N, C-S, C-halogen) into ionic forms (CN⁻, S²⁻, X⁻) detectable by precipitation or colour reactions. The high temperature of fusion ensures complete conversion. Precautions: must cool the tube before adding water (Na reacts violently with water). Excess Na must be destroyed. P can also be detected: Na₃PO₄ formed, detected by ammonium molybdate yellow precipitate.

2. Detection of Nitrogen

Lassaigne extract (containing CN⁻ from NaCN) is tested: (1) Add few drops of freshly prepared FeSO₄ solution (source of Fe²⁺). (2) Heat briefly. (3) Add FeCl₃ solution (Fe³⁺). (4) Acidify with dilute H₂SO₄. Result: Prussian blue precipitate (Fe₄[Fe(CN)₆]₃) indicates N. Chemistry: CN⁻ + Fe²⁺ → [Fe(CN)₆]⁴⁻ (hexacyanoferrate II). Then 3[Fe(CN)₆]⁴⁻ + 4Fe³⁺ → Fe₄[Fe(CN)₆]₃↓ (Prussian blue, KFe[Fe(CN)₆] or Berlin blue). Note: if both N and S are present, NaSCN forms (not NaCN and Na₂S separately). Test for NaSCN: add FeCl₃ → blood red colour. To detect NaCN separately when SCN⁻ present: boil extract with conc. HNO₃ (oxidises SCN⁻ but not CN⁻, though CN⁻ also oxidised... test becomes unreliable).

3. Detection of Sulphur

Method A: Sodium nitroprusside test. Lassaigne extract (containing S²⁻ from Na₂S) + Na₂[Fe(CN)₅NO] (sodium nitroprusside) → violet/purple colour. $Na_2S + Na_2[Fe(CN)_5NO] \to Na_4[Fe(CN)_5NOS]$ (violet complex). Very sensitive test. Method B: Silver nitrate test. Lassaigne extract + AgNO₃ → black precipitate of Ag₂S ($K_{sp} = 6.7\times10^{-50}$). Black colour distinguishes from AgCl (white), AgBr (pale yellow), AgI (yellow). Method C: Lead acetate test. S²⁻ + Pb(CH₃COO)₂ → black PbS↓. Used directly on gases (like H₂S from acid + sulfide) — moist lead acetate paper turns black.

4. Detection of Halogens

Lassaigne extract acidified with dilute HNO₃ (to remove CN⁻ and S²⁻ which would interfere). Then add AgNO₃ solution. Results: Cl⁻: white curdy AgCl precipitate (soluble in NH₃). Br⁻: pale yellow AgBr (partially soluble in concentrated NH₃). I⁻: yellow AgI (insoluble in NH₃). This solubility in NH₃ distinguishes the three halogens. Additional confirmation: CCl₄ extraction test (Beilstein test not standard). Beilstein test (copper wire): halogen compounds on heating on Cu wire → green/blue-green flame (CuX₂ volatile, green). F detected differently: not as silver salt (AgF soluble). F detected as: voluminous white precipitate with CaCl₂ (CaF₂). Also: F etches glass (SiO₂ + HF → SiF₄).

5. Detection of Carbon and Hydrogen

Copper oxide method: organic compound mixed with CuO → heated in glass tube → CO₂ (turns limewater milky) and H₂O (condenses in cool part of tube, turns anhydrous CuSO₄ blue). Confirmation of CO₂: passes into Ca(OH)₂ solution → white milky CaCO₃. CaCO₃ + excess CO₂ → clear Ca(HCO₃)₂. Confirmation of H₂O: anhydrous CuSO₄ (white) → blue CuSO₄·5H₂O. Carbon and hydrogen are assumed to be present in organic compounds (by definition) — this test only needed for rigorous proof. Modern method: CHN analyser (combustion in O₂, CO₂ and H₂O measured by IR detector or thermal conductivity). Simultaneous C, H, N determination in minutes.

6. Organic Analysis — Qualitative Tests

Functional group tests: Alcohols: Lucas test (conc. HCl + ZnCl₂, warm) → turbidity (SN2 → alkyl chloride, insoluble). Primary: no reaction initially. Secondary: 5 min. Tertiary: immediate. Ferric chloride test: alcohols → no colour. Phenols → purple/violet (phenol-Fe complex). Aldehydes: Fehling's test (blue → brick red Cu₂O) and Tollens' test (silver mirror). Not ketones (except methyl ketones, which are oxidised by different pathway). Iodoform test (I₂/NaOH): CH₃CO-, CH₃CHOH- → CHI₃ (yellow ppt, antiseptic smell). Tests methyl ketones and secondary alcohols with adjacent CH₃. Carbonyl: 2,4-DNP (2,4-dinitrophenylhydrazine) → yellow/orange precipitate (DNP derivative). Both aldehydes and ketones. Carboxylic acids: react with NaHCO₃ (CO₂ evolution). Distinguish from phenols (phenol doesn't react with NaHCO₃).

7. Nitrogen-Containing Functional Groups

Amines: primary (1°) amines react with HNO₂ (formed from NaNO₂+HCl) → diazonium salt (ArN₂⁺X⁻ for aromatic; unstable for aliphatic → N₂ evolution). Coupling with β-naphthol → azo dye (red/orange colour). Secondary amines + HNO₂ → N-nitrosamines (yellow oily, carcinogenic). Tertiary → no reaction with HNO₂ (Hinsberg test separates 1°, 2°, 3°). Carbylamine test (isocyanide test): primary amine + CHCl₃ + KOH (warm) → foul-smelling isocyanide (distinctive). Specific for primary amines only. Amides: dilute acid hydrolysis → acid + amine. Differ from amines in reactions. Nitro compounds (ArNO₂): reduce with Fe/HCl → aniline (test with diazonium coupling). Nitriles: hydrolyse to amide then carboxylic acid (acid/base catalyst, heat).

8. Elemental Analysis — Modern Methods

Elemental microanalysis: Pregl method (modern CHN analyser). Small sample (1-2 mg) burned completely in pure O₂. CO₂ absorbed in KOH, H₂O in CaCl₂ or P₂O₅, N₂ measured by gas volume or thermal conductivity. Accuracy: ±0.3% for C, H, N. Halogens: by AgNO₃ precipitation (Carius method: seal with fuming HNO₃ + AgNO₃ in glass tube, heat to 200°C). Sulphur: Carius method (→ BaSO₄, weigh) or Schöniger flask. ICP-OES (Inductively Coupled Plasma): trace metals at ppb level. GC-MS: identifies volatile organic compounds by mass spectrum + retention time. NMR spectroscopy: most powerful structural tool (¹H, ¹³C, ³¹P, ¹⁹F NMR). FTIR: functional group identification by characteristic absorption frequencies. Mass spectrometry: molecular weight, fragmentation pattern → structure.

Frequently Asked Questions
1. What is the principle behind Lassaigne's test? ⌄
Sodium (Na) is a powerful reducing agent that reacts with non-metallic elements in organic compounds at high temperature. Organic compounds have covalent bonds (C-N, C-S, C-Cl etc.). These covalent bonds are converted to ionic compounds with sodium: C-N + Na → NaCN (ionic), C-S + Na → Na₂S (ionic), C-Cl + Na → NaCl (ionic). Ionic compounds are water-soluble and give characteristic ionic reactions. This conversion from covalent to ionic is the key: we cannot directly test for covalently-bonded Cl⁻ or N in an organic compound. But NaCl, NaCN, Na₂S dissolved in water give free ions that are easily detectable by precipitation or colour reactions. Sodium also reduces organic compounds to give Na₂CO₃ (tests for C by CO₂ evolution when acidified) and NaOH (if O present).
2. Why must HNO₃ be added before testing for halogens? ⌄
The Lassaigne extract contains: NaCN (if N present), Na₂S (if S present), NaCl/NaBr/NaI (if halogen present). If we add AgNO₃ directly without removing CN⁻ and S²⁻: AgCN (white, $K_{sp} = 6\times10^{-16}$) would precipitate — mimicking AgCl. Ag₂S (black, $K_{sp} = 6.7\times10^{-50}$) would precipitate with S²⁻ — masking halide precipitates. Adding dilute HNO₃ first: CN⁻ + H⁺ → HCN (gas, volatile, removed). S²⁻ + 2H⁺ → H₂S (gas, removed). Now only X⁻ remains → add AgNO₃ → clean AgX precipitation without interference. Always remember: acidify with HNO₃ (not HCl — that would add Cl⁻ and give false positive for chlorine!).
3. How does the Prussian blue test for nitrogen work? ⌄
Step by step: (1) CN⁻ (from NaCN in Lassaigne extract) + FeSO₄ → $6CN^- + Fe^{2+} \to [Fe(CN)_6]^{4-}$. The six CN⁻ coordinate to Fe²⁺. (2) On heating: ensures complete complex formation. (3) Add FeCl₃: $3[Fe(CN)_6]^{4-} + 4Fe^{3+} \to Fe_4[Fe(CN)_6]_3$ (Prussian blue). Also written as KFe[Fe(CN)₆]. Deep blue-green precipitate. (4) Acidify with H₂SO₄: removes excess Fe(OH)₃ colloid that can give false colour. The intense blue arises from charge transfer between Fe²⁺ and Fe³⁺ in the complex lattice (mixed-valence compound). Very sensitive — can detect trace amounts of N.
4. How would you detect phosphorus if present in an organic compound? ⌄
By Lassaigne's test: P is converted to sodium phosphate (Na₃PO₄). The Lassaigne extract is boiled with concentrated HNO₃ (oxidises P-containing ions to phosphate). Then add ammonium molybdate solution: $HPO_4^{2-} + 12MoO_4^{2-} + 23H^+ \to [P(Mo_3O_{10})_4]^{3-} + 12H_2O$ → yellow crystalline precipitate of ammonium phosphomolybdate confirms P. Alternative: sodium extract + Mg mixture (Mg, MgO, Na₂CO₃) → magnesium ammonium phosphate (white crystalline) precipitate with NH₄Cl + NH₃. Modern: ICP-OES detects P at ppb level routinely. ³¹P NMR is also a powerful tool for P-containing organic compounds (phospholipids, phosphate esters, organophosphorus compounds).
5. What happens when both N and S are present in the same compound? ⌄
When an organic compound contains both N and S, the fusion with Na may give NaSCN (sodium thiocyanate) instead of NaCN and Na₂S separately, depending on the relative amounts. NaSCN detection: Lassaigne extract + FeCl₃ → blood-red colour [Fe(SCN)]²⁺ (confirms N+S together). If NaSCN is present: boiling with HNO₃ is supposed to destroy SCN⁻ and allow separate detection of N (as CN⁻ after proper handling) and S. In practice: when both N and S are detected by the overall tests (positive Prussian blue and positive S test), both elements are reported as present even if the intermediate was NaSCN. The thiocyanate can also be specifically identified by its blood-red colour with Fe³⁺, distinguishing it from pure cyanide (which gives Prussian blue only).
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