Detection of Functional Groups
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Direct answer
Iodoform's yellow crystals of CHI3 appear only when the system contains a methyl carbonyl, CH3CO-, or the CH3CH(OH)- group that oxidises into one — which is why the test fingers methyl ketones, acetaldehyde, ethanol and secondary alcohols of that shape while ignoring ethanol's homologues. The same specificity-by-mechanism logic runs the whole identification kit: Lassaigne's sodium fusion converts bonded N, S and halogens into NaCN, Na2S and NaX for detection (Prussian blue for nitrogen, violet with sodium nitroprusside for sulphur); Tollens reagent mirrors aldehydes but not ketones; Fehling solution accepts aliphatic aldehydes only; and the amine tier is separated by Hinsberg's reagent, the carbylamine stink test and the azo-dye coupling. Every test is a tiny mechanism statement, which is how the exam frames it.
What you must remember
- Lassaigne ledger: N gives Prussian blue Fe4[Fe(CN)6]3 with ferric chloride; S gives a violet colour with sodium nitroprusside; both together give blood-red ferric thiocyanate; halides precipitate as AgCl (white, soluble in dilute NH4OH), AgBr (pale yellow, soluble in concentrated NH4OH), AgI (yellow, insoluble).
- Carbonyl split: 2,4-DNP gives orange-red hydrazones with both aldehydes and ketones (not with carboxylic acids or esters); Tollens silver mirror flags aldehydes and formic acid; Fehling's red Cu2O flags aliphatic aldehydes — benzaldehyde refuses it.
- Iodoform club: CH3CO- compounds (acetone, acetophenone), CH3CHO, ethanol and CH3CH(OH)- secondary alcohols — the only primary alcohol in the club is ethanol.
- Acid fizz: carboxylic acids liberate CO2 briskly from sodium bicarbonate; phenols do not, separating the two acidities at a stroke.
- Phenol signatures: violet colouration with neutral FeCl3 and a white precipitate of 2,4,6-tribromophenol from bromine water.
- Amine tier tests: Hinsberg separates cleanly — primary amines form sulphonamides soluble in alkali, secondary form insoluble ones, tertiary refuse to react; carbylamine's foul isocyanide odour flags primary amines only; diazonium coupling with beta-naphthol gives a red dye for aromatic primary amines.
- Lucas ladder: anhydrous ZnCl2 in concentrated HCl clouds tertiary alcohols immediately, secondary within about five minutes, primary not at room temperature — reactivity riding on carbocation stability.
- Unsaturation pair: bromine in CCl4 decolourises and Baeyer's alkaline KMnO4 turns brown for alkenes and alkynes, with the caveat that sulphur dioxide and other reductants also decolourise permanganate.
How the exam frames it
Detection of elements and functional groups sits inside the Purification and Characterisation unit of the current JEE Main syllabus, delivered mostly as match-the-test or identify-the-compound questions — pure reward marks if the colour outcomes are banked. JEE Advanced occasionally asks why a test works, expecting the mechanism sentence: iodoform needs the methyl carbonyl for exhaustive halogenation, Tollens works because aldehydes oxidise to carboxylates while ketones cannot without C-C cleavage. The traps: expecting Fehling to answer for benzaldehyde, forgetting formic acid mirrors an aldehyde in both Tollens and Fehling, and reading a negative bromine-water test as proof of saturation when a phenol may simply have precipitated instead.
Frequently asked questions
Which test distinguishes aromatic from aliphatic aldehydes?
Fehling's solution — aliphatic aldehydes reduce the blue copper tartrate complex to red copper(I) oxide, while benzaldehyde and other aromatic aldehydes leave it unchanged.
Why does formic acid give a positive Tollens test?
Its structure carries an aldehydic hydrogen on the carboxyl carbon, so it oxidises to carbonic species while reducing silver just as an aldehyde does.
Why is iodoform positive with ethanol but not propanol?
Ethanol oxidises in situ to acetaldehyde, which has the methyl carbonyl motif the exhaustive iodination requires; propanol's oxidation product lacks it.
How does the Hinsberg test separate the three amine classes?
Primary amines give sulphonamides with an acidic N-H that dissolves in alkali, secondary amines give N,N-disubstituted sulphonamides that cannot, and tertiary amines simply do not react with the reagent.
What colour signals nitrogen in Lassaigne's test?
Prussian blue — the deep-blue ferric ferrocyanide Fe4[Fe(CN)6]3 precipitate formed when the fused sodium cyanide is first converted to sodium ferrocyanide with ferrous sulphate, then oxidised with ferric chloride.