Organic Functional Group Tests for H2 Chemistry Paper 4: Tollens, Fehling, Bromine Water, KMnO₄

> **Q:** Why do organic functional group tests have their own section in Paper 4?\
> **A:** Unlike inorganic qualitative analysis, where SEAB supplies a full table of reagents and observations in the exam, organic functional group tests must be recalled from memory. You need to know the reagent, the exact observation, and the structural inference it supports — and you must distinguish tests that look similar but target different functional groups.

> **TL;DR**\
> Eight reagent systems cover the full range of functional groups examinable in H2 Chemistry Paper 4: 2,4-DNPH for carbonyls, Tollens' and Fehling's to distinguish aldehydes from ketones, bromine water and acidified KMnO₄ for alkenes and oxidisable groups, Lucas reagent for alcohol class, sodium carbonate and sodium bicarbonate for carboxylic acid vs phenol, and neutral FeCl₃ for phenols. Observations must be precise — "silver mirror" not "silver colour", "brick-red precipitate" not "red solution". This guide covers all eight tests, a quick-reference matrix, a worked identification scenario, and ACE evaluation points.\
> Read this alongside the [H2 Chemistry Experiments hub](https://eclatinstitute.sg/blog/h2-chemistry-experiments) and the [Qualitative Analysis Workflow](https://eclatinstitute.sg/blog/h2-chemistry-experiments/H2-Chemistry-Qualitative-Analysis-Workflow) for inorganic counterparts.

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## 1 | Why organic functional group tests are different from inorganic QA

In the inorganic section of H2 Chemistry Paper 4, candidates receive a data booklet containing reagents, ions, and their characteristic observations. That resource is not provided for organic functional group tests.

This distinction matters practically. For organic tests you must recall:

1. Which reagent or combination of reagents to use.
2. The precise observation — colour, physical state of precipitate, whether warming is required.
3. What the observation does and does not confirm — specifically, which structural features are consistent with the result and which are ruled out.

A common examiner comment is that candidates describe an observation vaguely ("colour change") without specifying what colour or whether a precipitate forms. In organic functional group testing, the physical form of the product — silver mirror on glass, brick-red solid settling through solution, white crystalline precipitate — is part of the mark-scoring observation.

The 2026 SEAB H2 Chemistry syllabus (9476) includes organic chemistry practical work within Paper 4, and functional group identification is a recurring element of the planning and practical sections. [1]

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## 2 | Quick-reference matrix

The table below gives the expected outcome (positive or negative) for each combination of functional group and reagent. "Positive" means the named observation occurs; "negative" means no visible reaction. One or two words describe the key positive observation where space allows.

| Functional group | 2,4-DNPH | Tollens' | Fehling's | Br₂ water | Acid. KMnO₄ | Lucas | NaHCO₃ | FeCl₃ |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Aldehyde (aliphatic) | yellow/orange ppt | silver mirror | brick-red ppt | decolourises | decolourises | no reaction (cold) | negative | negative |
| Aldehyde (aromatic) | yellow/orange ppt | silver mirror | negative | decolourises | decolourises | no reaction | negative | negative |
| Ketone | yellow/orange ppt | negative | negative | negative | negative (cold) | no reaction | negative | negative |
| Primary alcohol | negative | negative | negative | negative | decolourises (warm) | negative (cold) | negative | negative |
| Secondary alcohol | negative | negative | negative | negative | decolourises (warm) | slow cloudiness | negative | negative |
| Tertiary alcohol | negative | negative | negative | negative | negative | immediate cloudiness | negative | negative |
| Alkene | negative | negative | negative | decolourises | decolourises | — | negative | negative |
| Carboxylic acid | negative | negative | negative | negative | negative | — | effervescence | negative |
| Phenol | negative | negative | negative | white ppt | negative | — | negative | purple/violet |

Notes on the matrix: Lucas reagent applies only to alcohols. Acidified KMnO₄ requires heat for primary and secondary alcohol oxidation; tertiary alcohols are resistant. Aromatic aldehydes (e.g., benzaldehyde) give a positive Tollens' result but a negative Fehling's result — this distinction is a frequent exam point. The bromine water result for phenol includes both decolourisation and a white precipitate (2,4,6-tribromophenol).

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## 3 | Test 1: 2,4-DNPH (Brady's reagent) — detecting carbonyls

**What it detects:** The carbonyl group ($\text{C=O}$) in both aldehydes and ketones.

**Reagent:** 2,4-dinitrophenylhydrazine dissolved in dilute sulfuric acid. This is Brady's reagent and is usually supplied as a ready-made orange solution in school labs.

**Procedure:** Add a few drops of the unknown compound to approximately 1 cm³ of Brady's reagent in a test tube. Shake gently.

**Positive observation:** A yellow, orange, or red crystalline precipitate forms. The colour of the precipitate corresponds loosely to the degree of conjugation in the product (2,4-dinitrophenylhydrazone), though for exam purposes "yellow/orange precipitate" is the standard safe answer for any carbonyl compound.

**What it confirms:** The presence of a carbonyl functional group ($\text{C=O}$). It does not distinguish between an aldehyde and a ketone.

**What it rules out:** If Brady's reagent gives no precipitate, neither an aldehyde nor a ketone is present.

**Common pitfall:** Esters and carboxylic acids can occasionally give faint or false positives under prolonged contact, because trace hydrolysis products may form a carbonyl species. In Paper 4, if the result is faint or develops only after several minutes, report it with that qualification. A true positive with an aldehyde or ketone is rapid and gives a distinct crystalline solid.

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## 4 | Test 2: Tollens' reagent — the silver mirror test

**What it detects:** Aldehydes specifically. Ketones give no reaction.

**Reagent preparation:** This is the most technically demanding of the eight tests to prepare correctly, and incorrect preparation is a frequent source of error.

1. Dissolve a small quantity of silver nitrate ($\text{AgNO}_3$) in water to give a clear solution.
2. Add a few drops of sodium hydroxide solution. A dark brown precipitate of silver(I) oxide forms.
3. Add aqueous ammonia ($\text{NH}_3$(aq)) dropwise, swirling between drops, until the precipitate just dissolves and the solution becomes clear again. This produces the diamminesilver(I) complex, $[\text{Ag(NH}_3)_2]^+$.

The reagent must be prepared fresh immediately before use. **Do not store Tollens' reagent.** If left to stand — particularly if any silver has deposited — explosive silver nitride ($\text{Ag}_3\text{N}$) can form. Dispose of any unused reagent immediately by washing down the sink with plenty of water.

**Procedure:** Add a few drops of the unknown compound to the freshly prepared Tollens' reagent. Place the test tube in a water bath at approximately 60 °C. Do not heat directly over a flame.

**Positive observation:** A bright silver mirror forms on the interior of the glass test tube. The solution may also become grey if the silver deposits as a dispersed precipitate rather than a coherent mirror, but the mirror is the canonical Paper 4 observation.

**The half-equation for the oxidation of an aldehyde:**

$$\text{RCHO} + 2[\text{Ag(NH}_3)_2]^+ + 3\text{OH}^- \rightarrow \text{RCOO}^- + 2\text{Ag}(s) + 4\text{NH}_3 + 2\text{H}_2\text{O}$$

The aldehyde is oxidised to a carboxylate; the silver(I) ion is reduced to silver metal.

**What it confirms:** An aldehyde is present.

**What it rules out:** Ketones do not reduce Tollens' reagent under normal lab conditions. A negative result with Tollens' — after confirming a positive with 2,4-DNPH — points to a ketone.

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## 5 | Test 3: Fehling's solution — detecting aliphatic aldehydes

**What it detects:** Aliphatic aldehydes. Aromatic aldehydes such as benzaldehyde give a negative result. Ketones give a negative result.

**Reagent preparation:** Fehling's solution is made by mixing equal volumes of two separate solutions immediately before use.

- **Fehling's A:** a blue solution of copper(II) sulfate ($\text{CuSO}_4$).
- **Fehling's B:** a colourless alkaline solution of sodium potassium tartrate (Rochelle salt) in sodium hydroxide. The tartrate acts as a chelating ligand to keep copper(II) in solution under alkaline conditions.

When mixed, the two solutions give a deep blue solution containing the copper(II)-tartrate complex.

**Procedure:** Add the unknown compound to freshly mixed Fehling's solution. Warm gently in a water bath at approximately 60 °C. Do not boil.

**Positive observation:** A brick-red precipitate of copper(I) oxide ($\text{Cu}_2\text{O}$) forms. The solution changes from blue to colourless (or pale blue) as the copper(II) is reduced.

**Why aromatic aldehydes do not react:** Benzaldehyde fails to reduce Fehling's solution because the electron-withdrawing effect of the benzene ring through resonance lowers the reducing power of the aldehyde group. The copper(II) complex, which is a weaker oxidising agent than the diamminesilver(I) complex in Tollens', cannot oxidise benzaldehyde under these mild conditions.

This is the key distinction: if a compound gives a positive Tollens' result but a negative Fehling's result, the aldehyde is aromatic.

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## 6 | Test 4: Bromine water — alkenes, phenol, and activated aromatics

**What it detects:** Carbon-carbon double bonds (alkenes), phenol, and activated aromatic rings.

**Reagent:** Aqueous bromine solution (orange-brown).

**Procedure:** Add the unknown compound to bromine water in a test tube. Observe immediately and after gentle shaking. No heat is required for a true alkene or phenol; warming may be needed for some activated aromatic compounds.

**Positive observations — three distinct cases:**

- **Alkene:** The orange-brown bromine water is decolourised rapidly. No precipitate forms. The reaction is electrophilic addition of bromine across the double bond.
- **Phenol:** The orange-brown colour is decolourised and a white precipitate forms. The precipitate is 2,4,6-tribromophenol, formed by electrophilic substitution at the three positions activated by the hydroxyl group.
- **Simple benzene ring (not activated):** No reaction at room temperature without a catalyst.

**Common student mistake:** Any decolourisation of bromine water is not automatically evidence of an alkene. Aldehydes can also decolourise bromine water by an oxidation mechanism (not addition). The presence of a white precipitate alongside decolourisation points specifically to phenol. If decolourisation occurs without precipitate in a compound already shown to contain no alkene by other methods, consider an aldehyde.

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## 7 | Test 5: Acidified KMnO₄ — a broad oxidising test

**What it detects:** A wide range of oxidisable functional groups: alkenes, primary and secondary alcohols, aldehydes, and some aromatic side-chains with benzylic hydrogen atoms.

**Reagent:** Dilute potassium permanganate solution acidified with dilute sulfuric acid. The solution is purple.

**Procedure:** Add the unknown compound to acidified KMnO₄. Shake. For alcohols and some other groups, warming may be required before the colour change is observed.

**Positive observation:** The purple colour of permanganate is discharged. The solution becomes colourless (under acidic conditions, $\text{Mn}^{2+}$ is the reduced product). Under neutral or alkaline conditions, a brown precipitate of $\text{MnO}_2$ would form instead — but the acidified version is standard for Paper 4.

**What it confirms:** An oxidisable functional group is present. Acidified KMnO₄ is less selective than the other tests. A positive result narrows the field significantly but does not identify the group on its own.

**Selectivity note:** Tertiary alcohols do not react with acidified KMnO₄ at room temperature because they have no hydrogen on the carbon bearing the $\text{-OH}$ group. This selectivity is exploited in multi-test identification: if bromine water and acidified KMnO₄ both give negative results for a compound known to be an alcohol, it is likely tertiary.

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## 8 | Test 6: Lucas reagent — distinguishing alcohol classes

**What it detects:** The class of an alcohol (primary, secondary, or tertiary) by its rate of reaction with a halogenating agent.

**Reagent:** Zinc chloride ($\text{ZnCl}_2$) dissolved in concentrated hydrochloric acid. The reagent is a Lewis acid catalyst system that converts the $\text{-OH}$ group to $\text{-Cl}$ via an SN1 mechanism (for secondary and tertiary alcohols) or SN2 (for primary).

**Procedure:** Add the unknown alcohol (a few drops or a small amount) to Lucas reagent in a test tube at room temperature. Observe for cloudiness (the alkyl chloride product is insoluble in the Lucas reagent).

**Positive observations by alcohol class:**

- **Tertiary alcohol:** Immediate cloudiness (within seconds to 1 minute) due to rapid SN1 reaction stabilised by a tertiary carbocation.
- **Secondary alcohol:** Cloudiness appears after approximately 5 to 10 minutes at room temperature.
- **Primary alcohol:** No cloudiness at room temperature. A positive reaction requires heating.

**What it confirms:** The class of the alcohol. It does not identify the specific compound or the chain length.

**Important limitation:** Lucas reagent is only applicable to alcohols with fewer than approximately six carbon atoms. Longer-chain alcohols are not sufficiently soluble in the aqueous reagent for the test to work reliably.

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## 9 | Test 7: Sodium carbonate and sodium bicarbonate — carboxylic acid vs phenol

**What it detects:** Acidic functional groups: carboxylic acids and phenols. The two reagents allow them to be distinguished.

**Reagents and outcomes:**

- $\text{Na}_2\text{CO}_3$ (sodium carbonate): reacts with both carboxylic acids and phenols to produce effervescence ($\text{CO}_2$) from carboxylic acids; phenols react to form sodium phenoxide but do not produce a gas.
- $\text{NaHCO}_3$ (sodium bicarbonate): reacts with carboxylic acids to produce brisk effervescence ($\text{CO}_2$); does not react with phenols.

**Key distinction:** If the compound effervesces with $\text{NaHCO}_3$, it is a carboxylic acid. If it does not effervesce with $\text{NaHCO}_3$ but dissolves in $\text{Na}_2\text{CO}_3$ solution, phenol is more likely.

**What this rules out:** Alcohols and esters do not react with either sodium carbonate or sodium bicarbonate.

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## 10 | Test 8: Neutral FeCl₃ — identifying phenol

**What it detects:** Phenols specifically, through formation of a coloured complex with iron(III) ions.

**Reagent:** Aqueous iron(III) chloride solution, neutral (not acidified). Acidic conditions suppress the complex formation.

**Procedure:** Add a few drops of neutral FeCl₃ solution to the unknown compound in aqueous solution or dissolved in a small volume of water.

**Positive observation:** A purple or violet complex forms immediately. The colour arises from coordination of the phenoxide oxygen to the iron(III) centre.

**What it confirms:** A phenol group is present.

**What it rules out:** Simple aliphatic alcohols, carboxylic acids, and aldehydes do not give a purple complex with neutral FeCl₃. This test in combination with the NaHCO₃ test gives high confidence in distinguishing phenol from carboxylic acid: phenol gives purple with FeCl₃ and no effervescence with NaHCO₃; carboxylic acid gives neither purple complex nor the FeCl₃ colour, but does give effervescence with NaHCO₃.

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## 11 | Worked scenario: identifying an unknown compound

An unknown compound **X** gives the following test results:

- **2,4-DNPH:** yellow-orange crystalline precipitate forms within 1 minute.
- **Tollens' reagent:** no silver mirror after 5 minutes in a 60 °C water bath.
- **Fehling's solution:** no brick-red precipitate after warming.
- **Acidified KMnO₄:** purple colour is discharged on warming.
- **Lucas reagent:** slow cloudiness after approximately 8 minutes at room temperature.

**Interpreting the results step by step:**

**Step 1 — 2,4-DNPH positive** confirms a carbonyl group ($\text{C=O}$). X is an aldehyde or a ketone.

**Step 2 — Tollens' negative** rules out an aldehyde (both aliphatic and aromatic). X is therefore a ketone.

**Step 3 — Acidified KMnO₄ positive (on warming)** seems initially inconsistent: ketones are not normally oxidised by KMnO₄. However, if X is a mixture or if the structure contains an additional secondary alcohol group elsewhere in the molecule, the KMnO₄ result is explained by that secondary alcohol.

**Step 4 — Lucas test gives slow cloudiness (approximately 8 minutes)** is consistent with a secondary alcohol, confirming that X contains a secondary $\text{-OH}$ group.

**Conclusion:** X is most likely a hydroxy-ketone — a compound containing both a ketone carbonyl and a secondary alcohol group. A structure such as $\text{CH}_3\text{CO-CH(OH)-CH}_3$ (3-hydroxybutanone) would be consistent with all four results: positive 2,4-DNPH (ketone carbonyl), negative Tollens'/Fehling's (no aldehyde), positive acidified KMnO₄ on warming (secondary alcohol), and secondary Lucas rate.

This scenario illustrates the core strategy: each test result eliminates functional groups from the list of candidates. The conclusion is the intersection of what each positive confirms and each negative rules out.

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## 12 | Common student errors

**Using Fehling's on an aromatic aldehyde.** Benzaldehyde gives a positive Tollens' result (silver mirror) but a negative Fehling's result (no brick-red precipitate). Students who treat these tests as interchangeable miss the aromaticity distinction. If the question gives a compound with a benzene ring and an aldehyde group, do not expect a Fehling's positive.

**Assuming bromine water decolourisation equals alkene.** Aldehydes also decolourise bromine water by oxidation, not addition. If the compound has already been shown to contain an aldehyde (positive 2,4-DNPH and Tollens'), bromine water decolourisation does not add evidence for a separate alkene group — it may simply be due to the aldehyde.

**Skipping the water bath for Tollens' and Fehling's.** Both tests require gentle heating (approximately 60 °C water bath) for a reliable result. Cold conditions with short contact time give false negatives. Never heat over a direct flame — bumping and spattering make the result uninterpretable and create safety issues.

**Forgetting to acidify KMnO₄.** Using neutral or alkaline KMnO₄ gives a brown precipitate ($\text{MnO}_2$) as the reduced product, not a colourless solution. The observation must specify "acidified" and the endpoint must be reported as "purple discharged / colourless" — not "brown ppt forms".

**Preparing Tollens' reagent in advance.** Stale Tollens' reagent loses activity and may form explosive silver nitride. Prepare it immediately before use and dispose of the remainder before the end of the practical session.

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## 13 | ACE evaluation: what examiners want

ACE (Accuracy, Completeness, Evaluation) marks in Paper 4 require more than stating a limitation — they require a quantified or mechanistic improvement strategy.

**Tollens' reagent freshness.** Tollens' reagent prepared more than a few minutes before use begins to deposit silver, reducing the concentration of the active $[\text{Ag(NH}_3)_2]^+$ complex. This leads to a fainter silver mirror or a longer time to develop, potentially causing a false negative for a weakly reducing aldehyde. The qualitative limitation becomes quantifiable as follows: prepare Tollens' reagent in batches of no more than 2 cm³, use within 5 minutes, and standardise by checking that freshly prepared reagent gives a full silver mirror with a known 1% ethanal solution within 3 minutes at 60 °C before applying it to the unknown. This converts qualitative limitation into a quantitative fix — a target time and a reference compound set a measurable performance standard.

**Temperature control in Fehling's and Tollens'.** A water bath at 60 °C gives consistent mild heating across all replicates. Heating directly on a hot plate produces variable temperatures — the solution near the bottom of the tube may exceed 80 °C while the top remains cooler. This introduces variability in reaction rate and may cause charring or spattering that obscures the observation. Specification: use a water bath thermostatted to $60 \pm 2$ °C and verify with a thermometer in the bath (not the tube) before beginning the series.

**Contamination between tests.** Rinsing test tubes with distilled water between tests can leave trace amounts of previous reagents, causing carry-over. For example, trace Tollens' reagent left in a tube can give a faint silver deposit with the next compound even if it is a ketone, leading to a false positive. Fix: rinse each tube three times with distilled water, then once with the new reagent before adding the sample; alternatively, use a fresh tube for each test.

**Lucas reagent temperature sensitivity.** The rate of cloudiness in the Lucas test is strongly temperature-dependent — a tertiary alcohol at 20 °C gives an immediate result, but at 15 °C the same reaction may take 2-3 minutes, potentially being misclassified as secondary. Control the ambient temperature by conducting all Lucas tests in a water bath at a fixed temperature of $25 \pm 1$ °C, and compare results within a single session.

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## 14 | Next steps

Work through these tests in combination with your broader Paper 4 preparation:

- [H2 Chemistry Qualitative Analysis Workflow](https://eclatinstitute.sg/blog/h2-chemistry-experiments/H2-Chemistry-Qualitative-Analysis-Workflow) — the inorganic counterpart, covering cation and anion identification with the data booklet as reference.
- [Advanced Organic Synthesis and Purification Labs for H2 Chemistry Paper 4](https://eclatinstitute.sg/blog/h2-chemistry-experiments/H2-Chemistry-Organic-Synthesis-Purification-Labs) — reflux, distillation, extraction, and recrystallisation workflows that precede functional group confirmation in many practical scenarios.
- [H2 Chemistry Experiments hub](https://eclatinstitute.sg/blog/h2-chemistry-experiments) — full index of Paper 4 technique guides.

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> **Need a supervised lab session?**\
> We run H2 Chemistry Paper 4 practical workshops with full equipment access, trained supervisors, and SEAB-aligned marking rubrics. [Contact us to find out more →](https://eclatinstitute.sg/blog/general/Science-Lab-Access-Private-Schools-Homeschool-Centres-Singapore)

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## References

[1] SEAB. (2024). _Chemistry (Syllabus 9476) GCE A-Level 2026._ Singapore Examinations and Assessment Board. (Paper 4 practical scope; organic chemistry practical skills; functional group identification.)