Qualitative Analysis
Qualitative analysis in organic chemistry refers to the systematic identification of elements and functional groups present in an unknown organic compound. Unlike quantitative analysis, which measures how much of each element is present, qualitative analysis simply determines what is there. Because most organic compounds contain carbon and hydrogen as default, chemists focus on detecting heteroatoms — nitrogen (N), sulfur (S), halogens (X = F, Cl, Br, I) — and then use specific reagent tests to identify functional groups such as alcohols, aldehydes, ketones, carboxylic acids, and amines. The foundation of elemental qualitative analysis is the Lassaigne test (also called the sodium fusion test), developed by Jean-Louis Lassaigne in the early 19th century. The organic compound is fused with sodium metal at high temperature, converting any heteroatoms into water-soluble ionic forms: nitrogen becomes cyanide (CN⁻), sulfur becomes sulfide (S²⁻), and halogens become halide ions (X⁻). The resulting sodium fusion extract (SFE) is then treated with specific reagents to precipitate or produce colored products that confirm the presence of each element. Once the elemental composition is established, chemists perform functional group tests. These are chemical reactions designed to give a clear positive result — usually a color change, precipitate formation, or gas evolution — only in the presence of a specific functional group. Together, elemental detection and functional group tests allow a chemist to narrow down the identity of an unknown compound before resorting to spectroscopic methods such as IR or NMR spectroscopy.
- Sodium Fusion (Lassaigne's Test) — Preparation of SFE
- Test for Nitrogen — Prussian Blue Test
- Test for Sulfur — Sodium Nitroprusside Test
- Test for Halogens — Silver Nitrate Test (Beilstein Test)
- Functional Group Detection Tests
Sodium Fusion (Lassaigne's Test) — Preparation of SFE
A small piece of freshly cut sodium metal (~0.5 g) is placed in an ignition tube. A tiny amount (~50 mg) of the organic compound is added carefully on top. The tube is heated strongly until it glows red-hot, then plunged immediately into 15–20 mL of distilled water in a mortar. The tube shatters, and the melt dissolves. The solution is filtered; the clear filtrate is the Sodium Fusion Extract (SFE). During fusion, carbon–nitrogen bonds are converted to NaCN, carbon–sulfur bonds to Na₂S, and carbon–halogen bonds to NaX.
Test for Nitrogen — Prussian Blue Test
To a portion of the SFE, add freshly prepared FeSO₄ solution and a small amount of FeCl₃ solution, then acidify with dilute H₂SO₄. A Prussian blue precipitate (Fe₄[Fe(CN)₆]₃) confirms nitrogen. Mechanism: NaCN + FeSO₄ → Na₂[Fe(CN)₆] (sodium ferrocyanide), which reacts with Fe³⁺ to give the intense blue iron(III) hexacyanoferrate(II) complex. A blood-red coloration with FeCl₃ alone (without FeSO₄) indicates both N and S are present, forming iron(III) thiocyanate, Fe(SCN)₃.
Test for Sulfur — Sodium Nitroprusside Test
To a fresh portion of SFE, add a few drops of sodium nitroprusside solution, Na₂[Fe(CN)₅NO]. A violet or purple color confirms the presence of sulfide ions (S²⁻), indicating sulfur in the original compound. Alternatively, acidifying the SFE with dilute HCl and testing the gas evolved over lead acetate paper: blackening of the paper (forming PbS) also confirms sulfur. If both N and S are present, NaCNS (sodium thiocyanate) forms instead of NaCN and Na₂S individually.
Test for Halogens — Silver Nitrate Test (Beilstein Test)
Acidify the SFE with dilute HNO₃ (to destroy CN⁻ and S²⁻ which could interfere). Then add excess AgNO₃ solution. Precipitate color indicates the halogen: white precipitate soluble in dilute NH₃ → Cl⁻ (AgCl); pale yellow precipitate soluble only in conc. NH₃ → Br⁻ (AgBr); yellow precipitate insoluble in NH₃ → I⁻ (AgI). The Beilstein test provides a quick preliminary check: a copper wire tip coated with compound burned in a flame turns green/blue-green if any halogen is present.
Functional Group Detection Tests
After elemental analysis, functional group tests are applied to the compound directly. Key tests include: (a) Litmus/pH — carboxylic acids turn blue litmus red; amines turn red litmus blue. (b) Lucas test — treat with ZnCl₂/conc. HCl; primary alcohols show no turbidity in cold, secondary alcohols turn turbid in ~5 min, tertiary alcohols turn turbid immediately — distinguishing 1°, 2°, 3° alcohols. (c) Tollens' test — treat with Ag(NH₃)₂⁺ (ammoniacal silver nitrate); aldehydes produce a silver mirror while ketones do not. (d) Fehling's test — treat with Fehling's A+B; aldehydes reduce Cu²⁺ to form a brick-red Cu₂O precipitate; ketones (except α-hydroxy ketones) do not react. (e) Bromine water test — decolorization of Br₂/H₂O without HBr gas evolution indicates an alkene or alkyne (addition); decolorization with HBr gas indicates a phenol or active methylene group (substitution). (f) FeCl₃ test — phenols give an intense violet/purple complex with neutral FeCl₃ solution.
The Key Equation
- \ce{NaCN} — Sodium cyanide (—): Formed when N-containing organic compounds are fused with sodium metal.
- \ce{Na4[Fe(CN)6]} — Sodium hexacyanoferrate(II) (sodium ferrocyanide) (—): Intermediate complex formed between cyanide ions and Fe²⁺.
- \ce{Fe4[Fe(CN)6]3} — Iron(III) hexacyanoferrate(II) — Prussian Blue (—): Intensely blue precipitate; positive result for nitrogen in the Lassaigne test.
- \ce{FeCl3} — Iron(III) chloride (—): Added to the SFE to react with ferrocyanide and produce Prussian Blue.
Worked Example
Problem. An unknown liquid organic compound X is subjected to qualitative analysis. (i) Sodium fusion extract gives a Prussian blue precipitate with FeSO₄/FeCl₃/H₂SO₄. (ii) The SFE acidified with HNO₃ gives a pale yellow precipitate with AgNO₃ that dissolves in concentrated NH₃. (iii) Direct addition of Tollens' reagent to compound X gives no silver mirror. (iv) Addition of 2,4-dinitrophenylhydrazine (2,4-DNP) to X gives an orange precipitate. Identify which elements are present and which functional group is most likely.
Answer. Elements present: Nitrogen and Bromine. Functional group: Ketone (>C=O). The compound is likely a nitrogen- and bromine-containing ketone.
Origins · 1 minJean-Louis Lassaigne and how this was discovered
Lassaigne was a French chemist who developed the sodium fusion test in 1843 for the detection of nitrogen, sulfur, and halogens in organic compounds. His method remained the standard technique for elemental qualitative analysis of organic substances for over a century and is still widely taught and used in undergraduate laboratories today. He also made contributions to forensic chemistry and pharmaceutical analysis. Lassaigne's fusion test was so reliable that versions of it were used in 19th-century criminal investigations to detect poisons in biological samples, making him one of the early pioneers of forensic analytical chemistry.
Related Concepts
- Purification: Recrystallisation & Melting Point — After identifying functional groups qualitatively, a compound is often purified by recrystallisation and its purity confirmed by a sharp melting point — the next step after qualitative analysis.
- Organic Functional Groups — Functional group tests in qualitative analysis depend on understanding the chemical reactivity specific to each functional group, such as –OH, –CHO, –COOH, and –NH₂.
- Infrared Spectroscopy — IR spectroscopy is the instrumental follow-up to qualitative chemical tests; it confirms functional groups by identifying characteristic bond-stretching frequencies.
- Precipitation Reactions & Ionic Equations — The Lassaigne halogen and nitrogen tests rely on precipitation reactions (e.g., AgX, Prussian blue); understanding ionic equilibria explains why these precipitates form and their solubility in NH₃.