Detection Functional Groups

8 MCQs9-step worked example

Source: NCERT Practical and Analytical ChemistryOfficial key: NTA-verifiedLast updated: 27 Sep 2026

Detection Functional Groups, explained for NEET

Functional group detection in organic qualitative analysis is a high-frequency practical chemistry topic. NEET questions test whether you can match a specific chemical test (reagent + observable change) to the correct functional group — and the trap is confusing tests that produce similar observations for different groups.

Core principle: Each functional group reacts with a characteristic reagent to give a unique observable — a colour change, precipitate, gas evolution, or effervescence. Your job is to memorise the reagent–observation–inference triplet, not just the reagent name.

Key functional-group tests (NCERT Chemistry Lab Manual Class 12, Unit 8, pages 86–99; the Hinsberg test is in NCERT Class 12 Chemistry Chapter 9, page 271):

Unsaturation (C=C): Bromine water decolourises (addition across the double bond). Baeyer's test — dilute alkaline KMnO₄ decolourises (syn-hydroxylation).
Alcoholic –OH: Lucas test (ZnCl₂ + conc. HCl) — turbidity timing distinguishes 1°, 2°, 3° alcohols. Ceric ammonium nitrate gives red colour. Ethanol and secondary alcohols carrying a CH₃–CH(OH)R group give the iodoform test.
Phenolic –OH: Neutral FeCl₃ gives violet/blue/green colouration. Phthalein dye test — heated with phthalic anhydride and conc. H₂SO₄, then poured into dilute NaOH, a phenol gives a pink, blue, green or red colour.
Aldehydic –CHO: Tollens' test (silver mirror). Fehling's test (red Cu₂O precipitate). 2,4-DNP gives yellow/orange precipitate (carbonyl — shared with ketones).
Ketonic C=O: 2,4-DNP positive (yellow/orange ppt), but Tollens' negative, Fehling's negative. This distinction is the single most tested point.
Carboxylic –COOH: NaHCO₃ — effervescence (CO₂ evolution). Litmus turns red.
Primary amine –NH₂: Carbylamine test (isocyanide — foul smell with CHCl₃ + alc. KOH). Hinsberg test distinguishes 1°, 2°, 3° amines.

Common NEET trap: 2,4-DNP is positive for BOTH aldehydes and ketones (it detects the C=O bond). Students who see "2,4-DNP positive" and immediately conclude "aldehyde" lose marks. You must check Tollens'/Fehling's to distinguish.

Second trap: Bromine water decolourisation is shared between alkenes and phenols (phenols undergo electrophilic substitution, not addition). Context matters — if the compound is aromatic and gives FeCl₃ colour, the Br₂ decolourisation is phenolic substitution, not unsaturation.

Can you answer these Detection Functional Groups MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Which reagent specifically distinguishes an aldehyde from a ketone?

Show answer and why every option is right or wrong

Answer: B. Tollens' reagent gives a silver mirror only with aldehydes (they are oxidised to carboxylate). Ketones do not reduce Tollens' reagent under mild conditions (NCERT Chemistry Lab Manual Class 12, Unit 8, pages 93–94).

Why A is wrong: 2,4-DNP reacts with both aldehydes AND ketones (detects C=O bond generally) — it cannot distinguish between them.

Why C is wrong: Brady's reagent is simply another name for 2,4-DNP — same limitation as option A.

Why D is wrong: Sodium bisulphite forms addition products with both aldehydes and methyl ketones — not a distinguishing test for aldehyde vs all ketones.

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MCQ 2Direct ApplicationPractice

A compound decolourises bromine water AND gives a violet colour with neutral FeCl₃. The functional group present is:

Show answer and why every option is right or wrong

Answer: A. Phenols decolourise bromine water via electrophilic aromatic substitution AND give a characteristic violet colour with neutral FeCl₃. Alkenes decolourise Br₂ water but do not respond to FeCl₃ (ferric chloride test: NCERT Chemistry Lab Manual Class 12, Unit 8, page 91; bromine water: page 87).

Why B is wrong: Alkenes decolourise Br₂ water (addition reaction) but give no colour with FeCl₃ — the FeCl₃ positive rules out a simple alkene.

Why C is wrong: Alcohols do not decolourise bromine water under normal conditions and do not give a violet colour with FeCl₃ (ceric ammonium nitrate is used for alcohols instead).

Why D is wrong: Carboxylic acids do not decolourise bromine water and give no characteristic colour with FeCl₃ — they are detected by NaHCO₃ effervescence.

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MCQ 3Easy RecallPractice

The carbylamine test (isocyanide test) is specific for:

Show answer and why every option is right or wrong

Answer: C. The carbylamine reaction (CHCl₃ + alc. KOH + amine → isocyanide with foul smell) occurs only with primary amines (–NH₂). Secondary and tertiary amines do not form isocyanides under these conditions (NCERT Chemistry Lab Manual Class 12, Unit 8, page 97).

Why A is wrong: Secondary amines do not undergo the carbylamine reaction — they react differently with CHCl₃/KOH (no isocyanide formed).

Why B is wrong: Tertiary amines have no N–H bond available for the carbylamine mechanism — they give no reaction.

Why D is wrong: The test is NOT universal for all amines — it is specific to the primary class only, which is precisely what makes it a useful distinguishing test.

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MCQ 4CalculationPractice

A compound gives a positive 2,4-DNP test but negative Tollens' test. The functional group is:

Show answer and why every option is right or wrong

Answer: A. 2,4-DNP positive indicates a carbonyl group (C=O). Negative Tollens' eliminates aldehyde (which would give silver mirror). Therefore the carbonyl is a ketone. This is the standard two-test sequence for aldehyde/ketone distinction (NCERT Chemistry Lab Manual Class 12, Unit 8, pages 92–94).

Why B is wrong: Aldehydes give BOTH 2,4-DNP positive AND Tollens' positive — the negative Tollens' result rules out aldehyde.

Why C is wrong: Carboxylic acids generally do not give a positive 2,4-DNP test (the carbonyl in –COOH is not reactive with 2,4-DNP under standard conditions due to resonance stabilisation).

Why D is wrong: Esters do not react with 2,4-DNP under standard conditions — the carbonyl is deactivated by the –OR group's lone-pair donation.

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MCQ 5Easy RecallPractice

Fehling's test gives a red precipitate with:

Show answer and why every option is right or wrong

Answer: D. Fehling's solution (alkaline Cu²⁺ tartrate complex) is reduced by aldehydes to Cu₂O (brick-red precipitate). Acetaldehyde (CH₃CHO) is an aliphatic aldehyde and gives a positive result. Ketones (acetone, acetophenone) and acids do not reduce Fehling's (NCERT Chemistry Lab Manual Class 12, Unit 8, page 94).

Why A is wrong: Acetone is a ketone — ketones cannot reduce Fehling's solution (Cu²⁺ remains in solution, no precipitate forms).

Why B is wrong: Acetophenone is an aromatic ketone — doubly unable to reduce Fehling's (ketone + resonance stabilisation of the carbonyl).

Why C is wrong: Acetic acid is a carboxylic acid — the –COOH group is already at the highest oxidation state of a carbonyl carbon and cannot reduce Cu²⁺.

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MCQ 6Direct ApplicationPractice

In the Hinsberg test, a secondary amine reacts with benzenesulphonyl chloride to form a product that is:

Show answer and why every option is right or wrong

Answer: B. Secondary amines form N,N-disubstituted sulphonamides with no acidic N–H hydrogen. This product is insoluble in aqueous NaOH. Primary amines form mono-substituted sulphonamides with one N–H → soluble in NaOH (acidic proton). This solubility difference is the basis of the Hinsberg distinction (NCERT Class 12 Chemistry Chapter 9, page 271).

Why A is wrong: NaOH-soluble product indicates a PRIMARY amine sulphonamide (has acidic N–H that deprotonates in base) — not a secondary amine product.

Why C is wrong: Foul smell is the carbylamine test result (primary amines + CHCl₃/KOH) — completely different reaction.

Why D is wrong: KMnO₄ decolourisation tests for unsaturation or reducing groups — it is unrelated to the Hinsberg sulphonamide classification.

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MCQ 7Direct ApplicationPractice

A liquid organic compound gives effervescence with NaHCO₃ solution. The gas evolved turns lime water milky. The functional group present is:

Show answer and why every option is right or wrong

Answer: C. Only carboxylic acids are acidic enough to decompose NaHCO₃, releasing CO₂ (which turns lime water milky). Alcohols, amines, and aldehydes are not sufficiently acidic to react with NaHCO₃ (NCERT Chemistry Lab Manual Class 12, Unit 8, page 95).

Why A is wrong: Alcohols have very weak acidity (pKa ~16–18) — far too weak to protonate bicarbonate ion. No effervescence occurs.

Why B is wrong: Amines are basic, not acidic — they cannot donate a proton to bicarbonate. No CO₂ evolution.

Why D is wrong: Aldehydes are neutral compounds — they have no acidic proton to react with NaHCO₃.

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MCQ 8Concept TrapPractice

An organic compound gives a positive Baeyer's test (decolourises dilute alkaline KMnO₄) but does NOT decolourise bromine water. The most likely explanation is:

Show answer and why every option is right or wrong

Answer: B. Primary (and some secondary) alcohols are oxidised by cold dilute KMnO₄ (positive Baeyer's) but do not react with bromine water under standard conditions. Simple alkenes would decolourise BOTH reagents. This tests whether students recognise that Baeyer's test detects any oxidisable functionality, not only C=C unsaturation (Baeyer's test, run on an alcohol-free solution for this reason: NCERT Chemistry Lab Manual Class 12, Unit 8, page 87).

Why A is wrong: A simple alkene would decolourise BOTH dilute KMnO₄ AND bromine water — the negative Br₂ water test rules this out.

Why C is wrong: Aromatic ketones are not easily oxidised by dilute KMnO₄ at room temperature — they would typically give a negative Baeyer's test.

Why D is wrong: The results are perfectly consistent with an oxidisable group (like –OH) that does not undergo addition with Br₂ — there is no contradiction.

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How do you solve a Detection Functional Groups question? A worked example

1
Given
• 2,4-DNP positive → carbonyl group (C=O) present• Tollens' negative → NOT an aldehyde• Iodoform positive → CH₃CO– group present
2
Required
Identify the functional group and the simplest structure consistent with all three tests.
3
Concept
2,4-DNP detects any C=O (aldehyde or ketone). Tollens' distinguishes: aldehydes positive, ketones negative. Iodoform test is positive for methyl ketones (CH₃–CO–R) and acetaldehyde (CH₃CHO) — but since Tollens' is negative, acetaldehyde is excluded.
4
Reasoning chain
• C=O present (2,4-DNP ✓)• Not aldehyde (Tollens' ✗)• Therefore: ketone• CH₃CO– group present (iodoform ✓)• Simplest methyl ketone: CH₃COCH₃ (acetone)
5
Verification
Acetone: 2,4-DNP → positive (ketone C=O) ✓
Acetone: Tollens' → negative (ketone, not aldehyde) ✓
Acetone: Iodoform → positive (has CH₃CO– adjacent to a methyl/H) ✓

All three observations are consistent.
6
Final answer
Functional group: Ketone (C=O) with a methyl group adjacent to the carbonyl.
Simplest compound: Acetone (CH₃COCH₃)
7
Common trap
Students who see "iodoform positive" sometimes jump to "ethanol" (which also gives iodoform after oxidation). But ethanol has no C=O detectable by 2,4-DNP directly — you must read ALL test results together, not act on one alone.
8
Extension
If Tollens' had been POSITIVE (silver mirror) along with iodoform positive, the answer would be acetaldehyde (CH₃CHO) — the only aldehyde that gives iodoform.
9
Similar NEET-style question
"Compound Y gives a positive 2,4-DNP test, negative Fehling's test, and does NOT give iodoform. Identify the type of carbonyl compound." (Answer: a ketone without a CH₃CO– group, e.g., diethyl ketone.)

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What to remember before solving Detection Functional Groups questions

Exercise

Tests for aldehydes and ketones

Both aldehydes and ketones react with 2,4-dinitrophenylhydrazine (2,4-DNP) to form 2,4-dinitrophenylhydrazones. Schiff's test, Fehling's test and Tollen's test are given by aldehydes only: aldehydes give a pink colour with Schiff's reagent; Tollen's reagent is an alkaline solution of silver cation complexed with ammonia; Fehling's reagent is made by mixing equal amounts of Fehling's solution A (an aqueous copper sulphate solution) and Fehling's solution B (an alkaline solution of sodium potassium tartarate, Rochelle's salt), and aldehydes give a brick red precipitate of copper(I) oxide. Aromatic aldehydes do not give a positive Fehling's test.

-- NCERT Chemistry Lab Manual Class 12, Unit 8, p. 93

Exercise

Tests for alcohols

Alcohols give a red colouration with ceric ammonium nitrate (a complex). Primary, secondary and tertiary alcohols are distinguished by the iodoform test and the Lucas test: ethanol and secondary alcohols containing the CH3-CH(OH)R group give a positive iodoform test.

-- NCERT Chemistry Lab Manual Class 12, Unit 8, p. 88

Exercise

Tests for phenols

Phenols give a coloured complex with neutral ferric chloride solution; phenol gives a violet complex. Resorcinol and o-, m- and p-cresol give violet or blue colouration.

-- NCERT Chemistry Lab Manual Class 12, Unit 8, p. 90

Exercise

Tests for carboxylic acids

Sodium hydrogencarbonate test: evolution of brisk effervescence of CO2 on adding the compound to saturated aqueous sodium hydrogencarbonate indicates a carboxyl group. Ester test: heating with ethanol or methanol and a few drops of concentrated sulphuric acid.

-- NCERT Chemistry Lab Manual Class 12, Unit 8, p. 96

Exercise

Tests for amines

Carbylamine test: aliphatic as well as aromatic primary amines, heated with chloroform (and KOH), give the carbylamine (isocyanide). Azo dye test: aromatic primary amines such as aniline react with nitrous acid, generated in situ from sodium nitrite and HCl at 0-5 °C, to give a diazonium salt that couples with β-naphthol to give a scarlet red dye.

-- NCERT Chemistry Lab Manual Class 12, Unit 8, p. 97

Exercise

Tests for unsaturation

Unsaturated compounds add bromine (bromine water or bromine in CCl4, chloroform or glacial acetic acid): addition to an alkene gives a vicinal dibromide and the reddish orange colour of bromine is discharged. Baeyer's test: with 1% potassium permanganate containing 1% sodium carbonate, discharge of the colour of more than one drop of permanganate indicates unsaturation.

-- NCERT Chemistry Lab Manual Class 12, Unit 8, p. 86

More in Practical and Analytical Chemistry: 6 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.

Detection Functional Groups questions from past NEET papers

No question in our NEET 2020–2025 set targets this topic directly.

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Sources

NCERT refs: Class 12 Chemistry Chapter 9, p.271

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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