JEE Mains Chemistry · Organic Reaction Mechanisms

Functional Group Identification Tests

Each laboratory test pairs one reagent with one functional group and one thing you see, a colour, a precipitate, a smell or a gas, and an unknown is identified by the tests it passes and fails.

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Why this matters

Eight PYQs, two of them asking for a number. Five match a test to what it detects or what you see: bromine water, Baeyer's reagent, ceric ammonium nitrate, neutral FeCl₃, the phthalein dye test, 2,4-DNP, Schiff's, Benedict's, carbylamine, Molisch and biuret. Three combine clues: compounds that pass two tests at once, a solubility screen with NaHCO₃ and NaOH, and a C₈H₉Br isomer identified from its oxidation product and alcoholic AgNO₃.

Concept 1 of 1: Functional-group tests: reagent, group and observation

A test is a fast reaction that only one kind of group gives, with a result you can see. Learn each as three linked facts: the reagent, the group it detects, and the observation. Questions give one of the three and ask for another, or list compounds and ask how many pass. When two tests are named together, a compound counts only if it passes both.

Definition

Unsaturation: bromine water loses its reddish-orange colour; Baeyer's reagent (cold, dilute, alkaline $\mathrm{KMnO_4}$ ) loses its pink colour. Phenol and aniline also decolourise bromine water, with a white precipitate.
Alcohol: ceric ammonium nitrate turns from yellow to red. Phenol: neutral $\mathrm{FeCl_3}$ gives a violet, blue, green or red colour; the phthalein dye test gives a pink dye in alkali.
Carbonyl: 2,4-DNP gives a yellow, orange or orange-red precipitate with any aldehyde or ketone. Aldehyde only: Tollens' silver mirror, Schiff's pink colour, Fehling's or Benedict's red-brown $\mathrm{Cu_2O}$ (aliphatic aldehydes only).
Iodoform ( $\mathrm{I_2}$ and NaOH, or KI and NaOCl): yellow $\mathrm{CHI_3}$ from $\mathrm{CH_3CO{-}}$ or $\mathrm{CH_3CH(OH){-}}$ . Ethanol and ethanal are the only primary alcohol and aldehyde that pass.
Amines: carbylamine ( $\mathrm{CHCl_3}$ , alcoholic KOH) gives a foul-smelling isocyanide with a primary amine only; Hinsberg's reagent sorts 1°, 2° and 3°. Molisch (α-naphthol, conc. $\mathrm{H_2SO_4}$ ) detects carbohydrates; biuret (alkaline $\mathrm{CuSO_4}$ ) detects the peptide bond.
Solubility screen: cold $\mathrm{NaHCO_3}$ dissolves a carboxylic acid (with $\mathrm{CO_2}$ ); cold NaOH also dissolves a phenol; an ester dissolves only in HOT NaOH, by hydrolysis; an ether or hydrocarbon dissolves in none.
Structure clues: hot $\mathrm{KMnO_4}$ turns every alkyl side chain with a benzylic H into COOH, so the acid formed shows which ring positions carry groups. Alcoholic $\mathrm{AgNO_3}$ gives AgX quickly only with a reactive halide (benzylic, allylic or tertiary), never with a halogen on the ring. From percentages: atoms of an element = molar mass × % ÷ atomic mass.

Test (reagent)	Detects	Positive result	Example that passes
Bromine water	C=C or C≡C; also phenol and aniline	Reddish-orange colour disappears; white precipitate with phenol or aniline	Cyclohexene
Baeyer's reagent: cold, dilute, alkaline $\mathrm{KMnO_4}$	C=C or C≡C	Pink colour disappears; brown $\mathrm{MnO_2}$	But-2-ene
Ceric ammonium nitrate	Alcoholic –OH	Yellow solution turns red	Ethanol
Neutral $\mathrm{FeCl_3}$	Phenol (and enols)	Violet, blue, green or red colour	Phenol gives violet
Phthalein dye: phthalic anhydride and conc. $\mathrm{H_2SO_4}$ , then NaOH	Phenol	Pink or red dye in alkali	Phenol gives phenolphthalein
Lucas reagent: conc. HCl and $\mathrm{ZnCl_2}$	Class of alcohol	Cloudy at once for 3°, in about 5 min for 2°, not at room temperature for 1°	2-Methylpropan-2-ol, at once
2,4-Dinitrophenylhydrazine (2,4-DNP)	C=O of aldehydes and ketones	Yellow, orange or orange-red precipitate	Propanone
Tollens' reagent: ammoniacal $\mathrm{AgNO_3}$	Aldehyde, aliphatic or aromatic	Silver mirror	Benzaldehyde
Fehling's or Benedict's solution	Aliphatic aldehyde	Red-brown precipitate of $\mathrm{Cu_2O}$	Ethanal
Schiff's reagent	Aldehyde	Pink or magenta colour returns	Methanal
Iodoform: $\mathrm{I_2}$ and NaOH, or KI and NaOCl	$\mathrm{CH_3CO{-}}$ or $\mathrm{CH_3CH(OH){-}}$	Yellow precipitate of $\mathrm{CHI_3}$	Propan-2-ol
$\mathrm{NaHCO_3}$ solution	Carboxylic acid	Brisk effervescence of $\mathrm{CO_2}$	Ethanoic acid
Carbylamine: $\mathrm{CHCl_3}$ and alcoholic KOH	Primary amine, aliphatic or aromatic	Foul-smelling isocyanide	Aniline
Hinsberg's reagent, $\mathrm{C_6H_5SO_2Cl}$	Primary, secondary or tertiary amine	1°: product dissolves in alkali; 2°: insoluble solid; 3°: no reaction	Ethanamine dissolves; N-methylaniline gives an insoluble solid
Azo dye: $\mathrm{NaNO_2/HCl}$ at 0–5 °C, then alkaline 2-naphthol	Aromatic primary amine	Orange-red dye	Aniline
Molisch's test: α-naphthol and conc. $\mathrm{H_2SO_4}$	Carbohydrate	Violet ring where the layers meet	Glucose
Biuret test: alkaline $\mathrm{CuSO_4}$	Peptide bond	Violet colour	Egg albumin
Alcoholic $\mathrm{AgNO_3}$ , warm	Reactive C–X: benzylic, allylic or tertiary	AgCl white, AgBr pale yellow precipitate	Benzyl chloride A halogen on the ring (chlorobenzene) gives no precipitate.

Every row is reagent, group and observation; a question gives one and asks for another.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 8 Apr 2023 · Q41Moderate

Example 1 · Organic Reaction Mechanisms · Functional Group Identification Tests

Match List-I with List-II: Choose the correct answer from the options given below:

Fehling's and Benedict's miss aromatic aldehydes

Benzaldehyde gives a silver mirror with Tollens' reagent but does not reduce Fehling's or Benedict's solution. Only aliphatic aldehydes give the red-brown

\mathrm{Cu_2O}

Iodoform needs CH₃CO or CH₃CH(OH)

A tertiary alcohol with a methyl group, such as 2-methylpropan-2-ol, fails: its carbinol carbon has no H, so it cannot be oxidised to a methyl ketone. Ethanol and ethanal pass; methanol, propan-1-ol and propanal fail.

Phenols dissolve in NaOH but not in NaHCO₃

A simple phenol is too weak an acid to release

\mathrm{CO_2}

from hydrogencarbonate. Only carboxylic acids (and strongly acidic phenols such as 2,4,6-trinitrophenol) dissolve in

\mathrm{NaHCO_3}

CAN is for alcohols, FeCl₃ is for phenols

Ceric ammonium nitrate gives a red colour with an alcoholic OH; neutral

\mathrm{FeCl_3}

gives a violet or similar colour with a phenol. A match list that swaps them is wrong.

Decolourised bromine water does not prove a C=C

Phenol and aniline also remove the colour of bromine water, by ring substitution, and give a white precipitate. Baeyer's reagent is the cleaner test for a C=C.

Drill 7 more on functional-group tests: reagent, group and observation

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Functional-group tests: reagent, group and observation18 rows

Test (reagent)	Detects	Positive result	Example that passes
Bromine water	C=C or C≡C; also phenol and aniline	Reddish-orange colour disappears; white precipitate with phenol or aniline	Cyclohexene
Baeyer's reagent: cold, dilute, alkaline $\mathrm{KMnO_4}$	C=C or C≡C	Pink colour disappears; brown $\mathrm{MnO_2}$	But-2-ene
Ceric ammonium nitrate	Alcoholic –OH	Yellow solution turns red	Ethanol
Neutral $\mathrm{FeCl_3}$	Phenol (and enols)	Violet, blue, green or red colour	Phenol gives violet
Phthalein dye: phthalic anhydride and conc. $\mathrm{H_2SO_4}$ , then NaOH	Phenol	Pink or red dye in alkali	Phenol gives phenolphthalein
Lucas reagent: conc. HCl and $\mathrm{ZnCl_2}$	Class of alcohol	Cloudy at once for 3°, in about 5 min for 2°, not at room temperature for 1°	2-Methylpropan-2-ol, at once
2,4-Dinitrophenylhydrazine (2,4-DNP)	C=O of aldehydes and ketones	Yellow, orange or orange-red precipitate	Propanone
Tollens' reagent: ammoniacal $\mathrm{AgNO_3}$	Aldehyde, aliphatic or aromatic	Silver mirror	Benzaldehyde
Fehling's or Benedict's solution	Aliphatic aldehyde	Red-brown precipitate of $\mathrm{Cu_2O}$	Ethanal
Schiff's reagent	Aldehyde	Pink or magenta colour returns	Methanal
Iodoform: $\mathrm{I_2}$ and NaOH, or KI and NaOCl	$\mathrm{CH_3CO{-}}$ or $\mathrm{CH_3CH(OH){-}}$	Yellow precipitate of $\mathrm{CHI_3}$	Propan-2-ol
$\mathrm{NaHCO_3}$ solution	Carboxylic acid	Brisk effervescence of $\mathrm{CO_2}$	Ethanoic acid
Carbylamine: $\mathrm{CHCl_3}$ and alcoholic KOH	Primary amine, aliphatic or aromatic	Foul-smelling isocyanide	Aniline
Hinsberg's reagent, $\mathrm{C_6H_5SO_2Cl}$	Primary, secondary or tertiary amine	1°: product dissolves in alkali; 2°: insoluble solid; 3°: no reaction	Ethanamine dissolves; N-methylaniline gives an insoluble solid
Azo dye: $\mathrm{NaNO_2/HCl}$ at 0–5 °C, then alkaline 2-naphthol	Aromatic primary amine	Orange-red dye	Aniline
Molisch's test: α-naphthol and conc. $\mathrm{H_2SO_4}$	Carbohydrate	Violet ring where the layers meet	Glucose
Biuret test: alkaline $\mathrm{CuSO_4}$	Peptide bond	Violet colour	Egg albumin
Alcoholic $\mathrm{AgNO_3}$ , warm	Reactive C–X: benzylic, allylic or tertiary	AgCl white, AgBr pale yellow precipitate	Benzyl chloride A halogen on the ring (chlorobenzene) gives no precipitate.

Every row is reagent, group and observation; a question gives one and asks for another.

Watch out for (5)

Fehling's and Benedict's miss aromatic aldehydes
→ Functional-group tests: reagent, group and observation
Iodoform needs CH₃CO or CH₃CH(OH)
→ Functional-group tests: reagent, group and observation
Phenols dissolve in NaOH but not in NaHCO₃
→ Functional-group tests: reagent, group and observation
CAN is for alcohols, FeCl₃ is for phenols
→ Functional-group tests: reagent, group and observation
Decolourised bromine water does not prove a C=C
→ Functional-group tests: reagent, group and observation

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