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 Class 11 Chemistry, Chapter 12, practical section):
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.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which reagent specifically distinguishes an aldehyde from a ketone?
A compound decolourises bromine water AND gives a violet colour with neutral FeCl₃. The functional group present is:
The carbylamine test (isocyanide test) is specific for:
A compound gives a positive 2,4-DNP test but negative Tollens' test. The functional group is:
Fehling's test gives a red precipitate with:
In the Hinsberg test, a secondary amine reacts with benzenesulphonyl chloride to form a product that is:
A liquid organic compound gives effervescence with NaHCO₃ solution. The gas evolved turns lime water milky. The functional group present is:
An organic compound gives a positive Baeyer's test (decolourises dilute alkaline KMnO₄) but does NOT decolourise bromine water. The most likely explanation is:
Given
- 2,4-DNP positive → carbonyl group (C=O) present - Tollens' negative → NOT an aldehyde - Iodoform positive → CH₃CO– group present
Required
Identify the functional group and the simplest structure consistent with all three tests.
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.
Reasoning chain
- C=O present (2,4-DNP ✓) - Not aldehyde (Tollens' ✗) - Therefore: ketone - CH₃CO– group present (iodoform ✓) - Simplest methyl ketone: CH₃COCH₃ (acetone)
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.
Final answer
Functional group: **Ketone (C=O)** with a methyl group adjacent to the carbonyl. Simplest compound: **Acetone (CH₃COCH₃)**
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.
Extension
If Tollens' had been POSITIVE (silver mirror) along with iodoform positive, the answer would be acetaldehyde (CH₃CHO) — the only aldehyde that gives iodoform.
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.) ---
Standard tests: (a) -OH alcohol: Na metal → H2 evolved. (b) Phenol: neutral FeCl3 → violet colour. (c) Aldehyde: Tollens reagent → silver mirror; Fehling → red Cu2O ppt. (d) Ketone: 2,4-DNP → orange/yellow ppt; iodoform if methyl ketone. (e) -COOH: NaHCO3 → effervescence (CO2). (f) -NH2: HNO2 → diazotisation (1° aromatic); carbylamine test.
-- NCERT, p. 290Use when normality is awkward (e.g., diprotic acids). Stoichiometric coefficients from balanced equation.
| Symbol | Quantity | SI Unit |
|---|---|---|
| M | molarity | mol/L |
| V | volume | L |
| n | coefficient | - |
Equivalents of acid = equivalents of base at end-point. Or for redox: equivalents of oxidant = equivalents of reductant.
| Symbol | Quantity | SI Unit |
|---|---|---|
| N | normality | eq/L |
| V | volume | mL or L |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Cations like Pb²⁺ precipitate in BOTH Group I (with HCl) and Group II (with H2S) — assigning to only one group misses the redundancy.
Cation that appears in two analytical groups, e.g. Pb²⁺ (Group I + Group II) or Hg²⁺ vs Hg2²⁺.
Apply confirmatory tests for each candidate group; do not assume mutual exclusivity.
Category: Overthinking
Continuing to add titrant past the first persistent colour change because the colour seemed to fade after a swirl.
Question describes 'colour faded after swirling' or 'persistent colour' — distinguishes transient vs end-point.
End-point = first PERSISTENT colour change (lasts ≥30 s). Transient fades back to original on swirling.
Category: Similar Terms
Phenolphthalein (pH 8.2–10) and methyl orange (pH 3.1–4.4) only mark equivalence when the eq-pt pH falls within their range; using the wrong indicator gives an end-point that disagrees with the actual equivalence point.
Titration prompt mentions a specific weak/strong combination but asks which indicator is suitable.
Match the indicator's pH-change range to the equivalence-point pH: phenolphthalein for eq-pt > 7, methyl orange for eq-pt < 7.
Root cause: concept gap
Pb²⁺ appears in both Group I (PbCl2 white ppt, soluble in hot water) and Group II (PbS black ppt). Confirm with hot-water solubility test.
Root cause: concept gap
Strong-acid + strong-base: any indicator (eq pt = 7). Weak-acid + strong-base: phenolphthalein (eq pt > 7). Strong-acid + weak-base: methyl orange (eq pt < 7).
Root cause: rushed under time pressure
Slow drop-wise addition near the end-point; first persistent colour change is the end-point. Re-do if overshot.
3 questions from NEET 2024, 2025. Answers verified against NTA official keys.
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
wrong zero error direction
Sign convention is easily flipped.
Test yourself on this topic with real past-paper questions:
Practice this topic →Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.